2010 lake assessment of elk lake (15-0010), clearwater ... · elk lake (15-0010) clearwater county,...

Sentinel Lake Assessment Report Elk Lake (15-0010)

Clearwater County, Minnesota

Minnesota Pollution Control Agency Water Monitoring Section

Lakes and Streams Monitoring Unit &

Minnesota Department of Natural Resources Section of Fisheries

June 2011

Minnesota Pollution Control Agency 520 Lafayette Road North

Saint Paul, Minnesota 55155-4194651-296-6300 or 800-657-3864 toll free

TTY 651-282-5332 or 800-657-3864 toll free

This report is available in alternative formats upon request, and online athttp://www.pca.state.mn.us

Contributing Authors Kelly O’Hara, Steve Heiskary, Minnesota Pollution Control Agency

Ray Valley, Mike Habrat, Minnesota Department of Natural Resources

Editing Dana Vanderbosch, Minnesota Pollution Control Agency

Peter Jacobson, Minnesota Department of Natural Resources

Sampling Kelly O’Hara, Pam Anderson, Dereck Richter, Nicole Gabriel, Ann Zajak, Annie Haws,

Minnesota Pollution Control Agency Don Klick, Mike Habrat, Kimberly Strand, Donna Dustin, Brian Herwig, Jerry Younk,

Minnesota Department of Natural ResourcesJen Keville, Jessie Lepore, Patrick Tweedy,

University of Minnesota Ruth Rasmussen, Danielle Grunzke,

Master Naturalists

2010 Lake Assessment of Elk Lake (15-0010) Clearwater County, Minnesota Minnesota Pollution Control Agency

Water Monitoring Section Lakes and Streams Monitoring Unit

& Minnesota Department of Natural Resources

Section of Fisheries

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Document number: wq-2slice15-0010

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Contents List of Tables .................................................................................................................................................. i List of Figures ................................................................................................................................................ ii Executive Summary....................................................................................................................................... 1 Introduction .................................................................................................................................................. 3 History........................................................................................................................................................... 3 Background ................................................................................................................................................... 4

Lake morphometric and watershed characteristics ................................................................................. 4 Lake mixing and stratification ................................................................................................................... 6 Ecoregion and land use characteristics .................................................................................................... 6 Lake level and ice on/off ........................................................................................................................... 8 Precipitation and climate summary ......................................................................................................... 9

Methods...................................................................................................................................................... 11 Fisheries and aquatic plants ................................................................................................................... 11 Water quality .......................................................................................................................................... 11 Zooplankton ............................................................................................................................................ 11

Results and Discussion ................................................................................................................................ 12 Fisheries assessment .............................................................................................................................. 12 Fish management and population assessments .................................................................................... 14 Aquatic plant assessment ....................................................................................................................... 25 Water quality .......................................................................................................................................... 31

Dissolved oxygen profiles ................................................................................................................... 32 Temperature profiles ......................................................................................................................... 32 Total phosphorus ............................................................................................................................... 34 Chlorophyll-a ...................................................................................................................................... 35 Secchi disk transparency .................................................................................................................... 35 Dissolved minerals and organic carbon ............................................................................................. 37

Zooplankton ............................................................................................................................................ 38 National Lake Assessment Summary for Elk Lake....................................................................................... 42 Trophic State Index ..................................................................................................................................... 43 Trophic Status Trends ................................................................................................................................. 43 Modeling ..................................................................................................................................................... 47 303(d) Assessment and Goal Setting .......................................................................................................... 48 References .................................................................................................................................................. 49

Appendix A Ice-on and Ice-off Records for Elk Lake .................................................................................. 52 Appendix B Lake surface water quality data for Elk Lake for 2008-2010 .................................................. 53

List of Tables Table 1. Elk Lake and watershed morphometric characteristics ............................................................ 4 Table 2. Elk Lake ecoregion land use comparison .................................................................................. 7 Table 3. Trophic position, thermal guild, and tolerance to nutrient pollution of all fish species historically sampled in Elk Lake .................................................................................. 13 Table 4. Temperatures at 3 mg O2 interpolated from the profiles ....................................................... 17 Table 5. Common aquatic plant species sampled during past MDNR Fisheries lake surveys. ............. 26 Table 6. Plant species observed on June 6, 2007, along the Elk Lake north shore by the

Minnesota County Biological Survey....................................................................................... 27 Table 7. Percent frequency of occurrence of aquatic plant species at depths ≤ 15 feet sampled during point-intercept surveys on July 28, 2008, July 6, 2009, and

July 7, 2010, at Elk Lake, Clearwater County, Minnesota ....................................................... 28 Table 8. Elk Lake 2008-2010 summer mean water quality. .................................................................. 31 Table 9. Annual mean values for cations, anions, and organic carbon................................................. 31 Table 10. Elk Lake cation, anion, and total organic carbon measurements ........................................... 38 Table 11. Comparison of National Lake Assessment sediment core observed water chemistry

and diatom inferred chemistry................................................................................................ 42 Table 12. Minnesota Lake Eutrophication Analysis Procedures model results for Elk Lake................... 47 Table 13. Eutrophication standards by ecoregion and lake type............................................................ 48

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List of Figures Figure 1. Minnesota Department of Natural Resources map of Sentinel lakes and major land types ........................................................................................................................ 2 Figure 2. Elk Lake three dimensional depth contour map ....................................................................... 5 Figure 3. Lake Stratification ...................................................................................................................... 6 Figure 4. Minnesota ecoregions as mapped by the U.S. Environmental Protection Agency ................... 7 Figure 5. Elk Lake water chemistry monitoring location, watershed area, and

land use composition ................................................................................................................ 8 Figure 6. Elk Lake water level record ....................................................................................................... 9 Figure 7. Summer 2010 rainfall based on records for Itasca State Park .................................................. 9 Figure 8. 2010 Minnesota water year precipitation and departure from normal ................................. 10 Figure 9. Historical summer precipitation trends based on records for Itasca State Park .................... 10 Figure 10. Summer gill net catch rates, long-term lake average and lake class quartiles of

walleye, northern pike and yellow perch in Elk Lake, 1973 – 2008. ....................................... 18 Figure 12. Length distributions of female northern pike captured during ice-out trap netting

in Elk Lake, 2008–2010 ............................................................................................................ 20 Figure 13. Mean lengths of male and female muskellunge, captured during spring, in large-framed trap nets in Elk Lake, 1998– 2010 ...................................................................... 20 Figure 14. Summer trap net catch rates, long-term lake average and lake class quartiles of

bluegill in Elk Lake, 1973 – 2008 .............................................................................................. 21 Figure 15. Length distributions of bluegill captured by summer trap nets in Elk Lake, 1973–2010 ........ 21 Figure 16. Largemouth bass catch rates from trap nets (bars, left axis) and boat electrofishing

(right axis) in Elk Lake, 1973– 2010 ......................................................................................... 22 Figure 17. Summer trap net catch rates, long-term lake average and lake class quartiles of

brown bullhead in Elk Lake, 1973– 2010 ................................................................................. 22 Figure 18. Summer gill net catch rates, long-term lake average and lake class quartiles of white sucker in Elk Lake, 1973– 2010 ...................................................................................... 23 Figure 19. Coldwater habitat suitability for select cold-water species found in Minnesota ................... 24 Figure 20. Summer gill net catch rates, long-term lake average and lake class quartiles of cisco in Elk Lake, 1973 – 2008 .............................................................................................................. 25 Figure 21. Percent of water column occupied by vegetation (biovolume) in Elk Lake in summer 2008. ......................................................................................................................... 29 Figure 22. Percent of water column occupied by vegetation (biovolume) as a function of depth. ........ 30 Figure 23. Elk Lake 2010 dissolved oxygen and temperature profiles ..................................................... 33 Figure 24. Elk Lake July 1985 and 2010 dissolved oxygen and temperature profile comparison ............ 34 Figure 25. Elk Lake 2010 total phosphorus (surface) and chlorophyll-a concentrations, and Secchi depth ............................................................................................................................ 35 Figure 26. Elk Lake 2010 surface and depth total phosphorous comparison .......................................... 36 Figure 27. Elk Lake surface and depth total phosphorous three year comparison ................................. 36 Figure 28. Mean annual zooplankton densities for Northern Lakes and Forests Sentinel lakes ............. 39 Figure 29. Mean annual zooplankton biomass for Northern Lakes and Forests Sentinel lakes .............. 39 Figure 30. Mean monthly 2010 zooplankton densities for Elk Lake ........................................................ 40 Figure 31. Mean monthly 2010 zooplankton biomass for Elk Lake ......................................................... 40 Figure 32. Percent composition by organism count ................................................................................ 41 Figure 33. Elk Lake three year summer-mean total phosphorus and chlorophyll-a ................................ 44 Figure 34. Elk Lake long-term summer-mean Secchi disk depth. ............................................................ 45 Figure 35. Elk Lake historical Secchi transparency and precipitation comparison .................................. 45 Figure 36. Carlson’s Trophic State Index for Elk Lake............................................................................... 46

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Executive Summary The Minnesota Pollution Control Agency (MPCA) is working in partnership with the Minnesota Department of Natural Resources (MDNR) on the Sustaining Lakes in a Changing Environment (SLICE) Sentinel Lakes Program. The focus of this interdisciplinary effort is to improve understanding of how major drivers of change such as development, agriculture, climate change, and invasive species can affect lake habitats and fish populations, and to develop a long-term strategy to collect the necessary information to detect undesirable changes in Minnesota lakes (Valley, 2009). To increase our ability to predict the consequences of land cover and climate change on lake habitats, SLICE utilizes intensive lake monitoring strategies on a wide range of representative Minnesota lakes. This includes analyzing relevant land cover and land use, identifying climate stressors, and monitoring the effects on the lake’s habitat and biological communities.

The Sentinel Lakes Program has selected 24 lakes for long-term intensive lake monitoring (Figure 1). The “Deep” lakes typically stratify during the summer months only. “Shallow” lakes are defined as mixing continuously throughout the summer. “Cold Water” lakes are defined as lakes that harbor either cisco, lake whitefish, or lake trout and are the focus of research funded by the Environmental Trust Fund (ETF). “Super Sentinel” lakes also harbor cold-water fish populations and research on these lakes is also funded by the ETF.

Elk Lake was selected to represent a deep, mesotrophic lake in the Northern Lakes and Forests (NLF) ecoregion. With the exception of a group campsite and a paved nature trail, there is no development on Elk Lake. Additionally, Elk Lake was selected as a “Super Sentinel” lake where intensive habitat monitoring and mechanistic forecast models are being built by the U.S. Geological Survey (USGS) to predict future consequences of climate change on water quality and fish habitat. Elk Lake is a 109 hectare (271 acre lake) and is located within Itasca State Park in Clearwater County. It represents the headwaters of the Mississippi River (Headwaters-Lake Winnibigoshish) major watershed. Elk Lake has a maximum depth of 28.4 meters (93 feet) and a mean depth of 6.6 meters (21.8 feet). The lake is 24 percent littoral with one public access on the northern shore. The total contributing watershed for Elk Lake is 802 hectares (1,984 acres).

Elk Lake fully mixes in spring and fall, and generally remains stratified from May through September. Based on recent water quality data (2008-2010), Elk Lake is considered to be mesotrophic, with total phosphorus (TP), chlorophyll-a (chl-a), and Secchi values of: 18 micrograms per liter (µg/L), 5 µg/L, and 2.8 meters (9.2 feet), respectively. TP levels for Elk Lake were within the assessment standard for the NLF ecoregion. Chl-a levels were also within the assessment standard for the NLF ecoregion with no observed nuisance algal blooms and normal transparency values for all three monitoring seasons.

Secchi transparency was better than the assessment standard for NLF lakes; however, long-term Secchi data, collected since 1997, suggest a decline in the lakes transparency over time. Despite the decreasing level of water clarity, Elk Lake has been determined to be fully supporting of recreational activities in the 2010 assessment cycle and was not placed on the 303(d) (or Impaired Waters) List.

An ecoregion-based eutrophication model was used to predict in-lake TP based on Elk Lake’s size, depth, and watershed area. Using inputs for the NLF ecoregion, the model predicted in-lake TP for Elk Lake in 2010 to be 18 µg/L, which is fairly close to the observed 15 µg/L. A separate subroutine within the model estimated “background” TP for the lake at 22 µg/L. The model predictions, along with the overall assessment of Elk Lake’s water quality data, indicate the lake’s water quality meets the expectations for a lake of this size in this portion of the state.

2010 Sentinel Lake Assessment of Elk Lake in Clearwater County Minnesota Pollution Control Agency June 2011 Minnesota Department of Natural Resources

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The aquatic plant community in Elk Lake is diverse, which, together with modest water quality and an undisturbed forested watershed, supports a diverse fish community. Elk Lake supports several species intolerant to high levels of nutrients including a state-listed species of greatest conservation need (pugnose shiners [Notropis anogenus]). Furthermore, Elk Lake has sufficient deep, coldwater habitat to support cisco (Coregonus artedi), a forage species important for supporting large predator game fish. However, cisco has declined throughout the state in recent decades, presumably due to climate warming. Nevertheless, recent work by Fang et al. in review demonstrates that cold-water habitat in Elk Lake should remain suitable for cisco under most future climate change scenarios. Current work by USGS (lead investigator Dr. Richard Kiesling) will build on this work by incorporating land use and groundwater interactions and carbon dynamics to result in more robust predictions of habitat viability.

Muskellunge (Esox masquinongy) were introduced via stocking of Mississippi strain fish in 1982 and have been stocked intensively ever since. Given the success of stocking, Elk Lake is now used as a brood stock lake for supplying other stocked lakes throughout Minnesota, and the lake supports a trophy catch-andrelease fishery. Since the mid 1980s, regular walleye (Sander vitreus) stocking has occurred as well. Elk Lake also supports a quality northern pike (Esox lucius) fishery and, as of recently, a yellow perch (Perca flavescens) fishery. Long-term declines have been observed in net catches of brown bullhead (Ameiurus nebulosus), white sucker (Catostomus commersonii), and largemouth bass (Micropterus salmoides). Continued SLICE monitoring of the fishery will help evaluate the current management actions in Elk Lake.

Figure 1. MDNR map of Sentinel lakes and major land types


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Introduction This report provides a relatively comprehensive analysis of physical, water quality, and ecological characteristics of Elk Lake in Clearwater County, Minnesota (MN). This assessment was compiled based on Minnesota Department of Natural Resources (MDNR) surveys of the lake’s fish and aquatic plant communities, Minnesota Pollution Control Agency (MPCA) and volunteer water quality monitoring, and analysis of various other sources of data for the lake. The water quality assessment focuses on data collected during the 2008-2010 monitoring seasons; however, some historical data was used to provide perspective on variability and trends in water quality. Water quality data analyzed will include all available data in STORET, the national repository for water quality data. Further detail on water quality and limnological concepts and terms in this report can be found in the Guide to Lake Protection and Management: http://www.pca.state.mn.us/water/lakeprotection.html.

History Chronology provided by MDNR Fisheries

Late 1800s – Presumably, the channel between the lakes was excavated during the late 1800s to facilitate log drives.

1891 – Thanks, in part, to efforts by historian, anthropologist, and land surveyor Jacob Brower to preserve the virgin pine forests surrounding Lake Itasca, the Minnesota State Legislature establishes Itasca State Park, which includes the entire watershed of Elk Lake. Data from recent forest classification and stand aging demonstrate that approximately two thirds of Elk Lake’s watershed was never logged.

1909 – University of Minnesota establishes the Itasca Biological Station. Numerous resident and visiting scientists conduct numerous studies on Elk Lake. Most notably, paleolimnological studies by geologists W.E. Dean, J.P. Bradbury, and colleagues on Elk Lake during the 1980s – 1990s document several periods of climate change impacts to Elk Lake’s limnology over the lake’s 10,400 year history.

1911 – Initial stocking records for walleye (Sander vitreus), lake trout (Salvelinus namaycush), crappies (Pomoxis spp.), and northern pike (Esox lucius).

c 1930s – The Civilian Conservation Corps installed a stop log dam at Elk Lake’s outlet into Lake Itasca, bringing lake levels up approximately 0.8 meters (2.5 feet).

1973 – Whitefish (Coregonus clupeaformis) from Ten Mile Lake (Cass County) introduced.

1982 – Initial stocking of Mississippi strain of muskellunge (Esox masquinongy) to develop an alternative brood stock for stocking other Minnesota Lakes; four stocking events occurred from 1982-1988 at a relatively high density of 1.5 fingerlings per surface acre.

1985 – Elk Lake was used as an ecoregion reference lake by MPCA and water quality samples were collected.

1990 – Muskellunge minimum size limit increased from 40 inches to 48 inches to protect established brood population; a trophy fishery was developing.

1992-2007 – Muskellunge stocking density reduced to one fingerling per surface acre and set at an annual frequency for stable recruitment to the brood population. 1994 – The original stop log dam is partially removed by MDNR Parks.


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1998 – After an extensive environmental review process, the original stop log dam was replaced with sheet piling, creating a fixed-crest dam and buried under rock restoring water level to elevations maintained since the 1930s. Lake levels on both Elk Lake and Itasca presumably remain higher today than prior to European colonization, but retain a difference in elevation with gradient between basins similar to pre-settlement conditions.

2002 – Paleolimnological study of sediment cores infer significantly higher nutrient levels in Elk Lake during the late 1800s (Bradbury et al. 2002). These authors implicate that log driving activities around and in Elk Lake may have been largely responsible for the increase in nutrient levels. Inferred nutrient levels in recent decades appear similar to levels over 1,000 years ago.

2003 – Initiate PIT tagging of all muskellunge fingerlings stocked to evaluate recruitment relative to current and future adjustments to stocking densities.

2004 – Catch and release regulation for muskellunge implemented.

2007 – Minimum length limit of 40 inches for northern pike implemented.

2008 – Muskellunge stocking density reduced again from one fish per acre per year to 0.5 fish per acre per year.

Background

Lake morphometric and watershed characteristics Elk Lake is located in Clearwater County within the Mississippi River (Headwaters-Lake Winnibigoshish) watershed. Elk Lake is approximately 20 miles north of Park Rapids, MN and is within Itasca State Park. A public access is located on the northern shore. Elk Lake is a deep, dimictic lake that forms layers in the early spring and late fall, but remained stratified for a majority of the monitoring season.

Elk Lake’s morphometric characteristics are summarized in Table 1. Also, a three dimensional representation of the lake’s depth contour is presented in Figure 2. Percent littoral area refers to that portion of the lake that is 4.5 meters (15 feet [ft]) or less in depth, which often represents the depth to which rooted plants may grow in the lake. Lakes with a high percentage of littoral area often have extensive rooted plant (macrophyte) beds. These plant beds are a natural part of the ecology of these lakes and are important to maintain and protect. About 24 percent of Elk Lake is considered littoral.

Table 1. Elk Lake and watershed morphometric characteristics

Total Lake Lake Littoral Watershed Watershed: Max. Mean Name Lake ID Basin Area Area Lake Depth Depth

Hectares Hectares Meters Meters (Acres) % (Acres) Ratio (Feet) (Feet) 109 802 28.4 6.6

Elk Lake 15-0010 (271) 24 (1,984) 7:1 (93) (21.8) Lake bathymetry based on MDNR 2008 acoustic survey.


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Figure 2. Elk Lake three dimensional depth contour map

The Elk Lake contributing watershed lies within the Mississippi River (Headwaters-Lake Winnibigoshish) major watershed. The contributing watershed drains from the northern shore of Elk Lake. The contributing watershed has a total area of 802 hectares (1,984 acres) resulting in a watershed-to-lake area ratio of approximately 7:1. Watersheds for the state of Minnesota are delineated by MDNR Division of Ecological and Water Resources.

Elk Lake soils are defined as coarse- to medium-textured forest soils formed from glacial outwash from the Menahga-Marquette series. The area is level to rolling and the soils are light-colored and droughty. Agriculture is minimal and Jack pine is the most common tree species (Arneman, 1963). Elk Lake was likely formed by glacial deposition within the pitted outwash plain (Zumberge, 1952).


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Lake mixing and stratification Lake depth and mixing has a significant influence on lake processes and water quality. Thermal stratification (formation of distinct temperature layers), in which deep lakes (maximum depths of 9 meters or more) often stratify (form layers) during the summer months and are referred to as dimictic (Figure 3). These lakes fully mix or turn over twice per year, typically in spring and fall. Shallow lakes (maximum depths of 5 meters or less) in contrast, typically do not stratify and are often referred to as polymictic. Lakes with moderate depths may stratify intermittently during calm periods, but mix during heavy winds and during spring and fall. Measurement of temperature throughout the water column (surface to bottom) at selected intervals (e.g., every meter) can be used to determine whether the lake is well-mixed or stratified. The depth of the thermocline (zone of maximum change in temperature over the depth interval) can also be determined. In general, dimictic lakes have an upper, well-mixed layer (epilimnion) that is warm and has high oxygen concentrations. In contrast, the lower layer (hypolimnion) is much cooler and often has little or no oxygen. This low oxygen environment in the hypolimnion are conducive to TP being released from the lake sediments. During stratification, dense colder hypolimnion waters are separated from the nutrient hungry algae in the epilimnion. Intermittently (weakly) stratified polymictic lakes are mixed in high winds and during spring and fall. Mixing events allow the nutrient rich sediments to be re-suspended and are available to algae.

Figure 3. Lake stratification

Polymictic Lake Shallow, no layers, mixes continuously spring, summer, and Fall

Dimictic Lake Deep, form layers, Mixes Spring/fall

Intermittently Stratified Moderately deep. mixes during high winds spring, summer, and fall

Ecoregion and land use characteristics Minnesota is divided into seven regions, referred to as ecoregions, as defined by soils, land surface form, natural vegetation and current land use. Data gathered from representative, minimally impacted (reference) lakes within each ecoregion serve as a basis for comparing the water quality and characteristics of other lakes. Elk Lake lies within the Northern Lakes and Forest (NLF) ecoregion (Figure 4) and was one of the reference lakes for this ecoregion. NLF ecoregion values will be used for land use (Table 2) and summer-mean water quality comparisons (Table 8). Additionally, the NLF ecoregion will be used for the model application.


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Land use within the watershed is relatively typical for the NLF ecoregion with the exception of a slightly higher percentage of forested land use. When compared to historical records from 1969 and 1991, the land use for Elk Lake’s watershed shows little change with forest being the dominant land use (Figure 5 and Table 2). Since Elk Lake lies within Itasca State Park, the contributing watershed has maintained its natural status free from development or agricultural activities

Figure 4. Minnesota ecoregions as mapped by the U.S. Environmental Protection Agency

Table 2. Elk Lake ecoregion land use comparison. Typical (interquartile) range based on Northern Lakes and Forest ecoregion reference lakes noted for comparison (Heiskary and Wilson 2005)

Land Use (%) Elk Lake (1969)2

Elk Lake (1991)3

Elk Lake (2001)1

NLF Ecoregion

Developed <1 <1 <1 0 - 7 Cultivated (Ag) <1 <1 <1 <1 Pasture & Open <1 <1 <1 0 - 6 Forest 90 87 86 54 - 81 Water & Wetland 10 13 14 14 - 31

1National Land Cover Database www.mrlc.gov/index.php 2Minnesota Land Management Information Center www.lmic.state.mn.us/chouse/metadata/luse69.html 3Minnesota Land Cover 1991-1992: MAP www.lmic.state.mn.us/chouse/land_use_DNRmap.html


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Figure 5. Elk Lake water chemistry monitoring location, watershed area, and land use composition

Lake level and ice on/off The MDNR Division of Waters has been measuring water levels sporadically on Elk Lake since 1938; however, data are very sparse over the past 10 years (Figure 6). During the period of record (1938 - 2010), the lake has varied by 2.5 ft, based on 107 readings. The highest and lowest recorded elevations are 1,469.99 ft on May 6, 2010, and 1,467.54 ft on September 30, 1958. The ordinary high water mark for Elk Lake is 1,470.5 ft (red horizontal line within Figure 6). Based on recorded lake levels, the lake has typically remained over 1.5 feet below the ordinary high water mark, with a long-term mean of 1,468.9 (green horizontal line within Figure 6). The complete water level record may be obtained from the MDNR website at: http://www.dnr.state.mn.us/lakefind/showlevel.html?id=29025000.

Ice-on records for Elk Lake, dating back to 1987, indicate that ice typically forms within the last week of November. November 24, 1989, is the earliest recorded ice-on date and December 10, 1999, is the latest ice-on date. The ice is typically off of Elk Lake by mid-April. April 29, 1989, is the latest ice-off date on record (Appendix A). The earliest ice off date of March 20-24, 2010, was inferred from temperature logger data on the lake first established in 2008.


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1470.5 1470.0 1469.5 1469.0 1468.5 1468.0 1467.5 1467.0 1466.5 1466.0

Figure 6. Elk Lake water level record

Precipitation and climate summary Rain gage records from Itasca State Park show six one-inch plus rain events during summer 2010 (Figure 7). Large rain events will increase runoff into the lake and may influence in-lake water quality and lake levels. This will be considered in the discussion of lake water quality for 2010. Precipitation records for the 2010 water year (October 2009 through September 2010) showed that the Elk Lake area received 6-10 inches above normal precipitation amounts.

Figure 7. Summer 2010 rainfall based on records for Itasca State Park

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Figure 8. 2010 Minnesota water year precipitation and departure from normal

Figure 9. Historical summer precipitation trends based on records for Itasca State Park

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Methods

Fisheries and aquatic plants Frequency of occurrence of aquatic plant species were assessed using the point-intercept method (Madsen, 1999). This method entailed visiting sampling points on a grid within the vegetated zone of the lake, throwing a two-sided rake over one side of the boat at each point, raking the bottom approximately 1 meter (m), then retrieving the rake and identifying all species present, and recording the depth. Survey points were spaced approximately 80-m (0.7 points per littoral acre). Hydroacoustics were used to survey vegetation biovolume (percent of water column occupied by vegetation) along 40-m transects using methods and equipment described by Valley et al. (2005). Local kriging with VESPER 1.6 was used to create 15-m raster grids of biovolume (Walter et al. 2001; Minasny et al. 2002).

Most recent fisheries surveys follow guidelines outlined by MDNR Special Publication 147 (1993, Manual of Instructions for Lake Survey). Fish community integrity surveys were also completed on each Sentinel lake following methods described by Drake and Pereira (2002).

Water quality Water quality data for Elk Lake has been collected monthly from May through October since 2008 by MPCA staff. Itasca State Park staff and volunteers collected periodic dissolved oxygen (or DO) and temperature profiles and Secchi disk measurements. Lake surface samples were collected by MPCA staff with an integrated sampler, a poly vinyl chloride (PVC) tube 2 m (6.6 ft) in length, with an inside diameter of 3.2 centimeters (1.24 inches). Zooplankton samples were collected with an 80 µm mesh Wisconsin zooplankton net. Depth total phosphorous (TP) samples were collected with a Kemmerer sampler. Temperature and DO profiles and Secchi disk transparency measurements were also taken. Samples were collected at site 101 (Figure 5). Sampling procedures were employed as described in the MPCA Standard Operating Procedure for Lake Water Quality document, which can be found here: http://www.pca.state.mn.us/publications/wq-s1-16.pdf.

Laboratory analysis was performed by the Minnesota Department of Health Environmental Laboratory using U.S. Environmental Protection Agency (EPA)-approved methods. Samples were analyzed for nutrients, color, solids, pH, alkalinity, conductivity, chloride (Cl), metals, and chlorophyll-a (chl-a). Phytoplankton samples were analyzed at the MPCA using a rapid assessment technique.

Zooplankton Zooplankton samples were collected monthly from ice-out (April/May) through October 2008-2010. Two replicate vertical tows were taken at each sampling event. The net was lowered to within 0.5 m of the bottom and withdrawn at a rate of approximately 0.5 m per second. Contents were rinsed into sample bottles and preserved with 100 percent reagent alcohol. Analysis was conducted by MDNR personnel.

Each zooplankton sample was adjusted to a known volume by filtering through 80 µg/L mesh netting and rinsing specimens into a graduated beaker. Water was added to the beaker to a volume that provided at least 150 organisms per 5-milliliter aliquot. A 5-milliliter aliquot was withdrawn from each sample using a bulb pipette and transferred to a counting wheel. Specimens from each aliquot were counted, identified to the lowest taxonomic level possible (most to species level), and measured to the


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nearest .01 millimeter using a dissecting microscope and an image analysis system. Densities (#/liter), biomass (µg/L), percent composition by number and weight, mean length (millimeter), mean weight (µg) and total counts for each taxonomic group identified were calculated with the zooplankton counting program ZCOUNT (Charpentier and Jamnick, 1994 in Hirsch, 2009).

Results and Discussion

Fisheries assessment MDNR fisheries managers utilize netting survey information to assess the status of fish communities and measure the efficacy of management programs. Presence, absence, abundance, physical condition of captured fishes, and community relationships among fish species within survey catch information also provide good indicators of current habitat conditions and trophic state of a lake (Schupp and Wilson, 1993). This data is stored in a long-term fisheries survey database, which has proven valuable in qualifying and quantifying changes in environmental and fisheries characteristics over time.

Elk Lake’s fish community is diverse compared with other lakes of similar productivity (Table 3) with an assortment of cold-, cool-, and warm-water species. In 2008-2010, survey crews assessed the “biotic integrity” of the fish community in Elk Lake (Drake and Pereira, 2002). Indices of biotic integrity (IBI) have been used for decades across North America to assess status of aquatic communities and to classify biotic impairments (Angermeier and Karr, 1994). Although formal criteria have yet to be developed for classifying biotic impairments in Minnesota Lakes, IBI survey data from over 400 lakes across the state provide a good assessment of the range of conditions we might expect in lakes of differing productivity.

IBI surveys conducted in Elk Lake during 2008-2010 exceeded the 90th percentile when compared with other lakes of similar productivity (score range = 128-132). The pugnose shiner (Notropis anogenus) is a state-listed species of greatest conservation need. Several of the intolerant minnow species found in Elk Lake have disappeared from many Twin City metropolitan lakes whose watersheds have been developed or hydrologically altered (Dodd, 2009). Furthermore, cisco (Coregonus artedi) is in decline throughout the state as a result of eutrophication and climate change (Jacobson et al. 2010). High water clarity with dense Muskgrass (Chara sp.) is present and important for several intolerant littoral fish species (Valley et al. 2010). Despite a diverse fish community, some native species have declined over recent decades (largemouth bass (Micropterus salmoides), brown bullhead (Ameiurus nebulosus), and white sucker (Catostomus commersonii), while muskellunge and walleye have become prominent since the mid 1980s through stocking. Due to the remote nature of Elk Lake and special regulations that limit harvest of muskellunge, overall harvest of game fish is low to modest.


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Table 3. Trophic position, thermal guild, and tolerance to nutrient pollution of all fish species historically sampled in Elk Lake

Common name Species name Trophic guild Thermal Guilda Environmental toleranceb First Documented Cisco Coregonus artedi Zooplanktivore Cold Intolerant 1973 Mottled sculpin Cottus bairdii Insectivore Cold Intolerant 2008 Lake Whitefish Coregonus clupeaformis Zooplanktivore Cold Intolerant 1977c

Northern red-belly dace Phoxinus eos Herbivore Cold-cool Neutral 2009 Brook stickleback Culaea inconstans Insectivore Cool Neutral 2010 Iowa darter Etheostoma exile Insectivore Cool Intolerant 2008 Northern pike Esox lucius Predator Cool Neutral 1973 Muskellunge Esox masquinongy Predator Cool Intolerant 1982c

Central mudminnow Umbra limi Insectivore Cool-warm Neutral 2008 Creek chub Semotilus atromaculatus Insectivore Cool-warm Tolerant 2009 Banded killifish Fundulus diaphanous Insectivore Cool-warm Intolerant 1973 Johnny darter Etheostoma nigrum Insectivore Cool-warm Neutral 1973 Yellow perch Perca flavescens Insectivore Cool-warm Neutral 1973 White sucker Catostomus commersonii Omnivore Cool-warm Tolerant 2010 Rock bass Ambloplites rupestris Predator Cool-warm Intolerant 2008 Walleye Sander vitreus Predator Cool-warm Neutral 1961 Brown bullhead Ameiurus nebulosus Omnivore Warm Neutral 1973 Mimic shiner Notropis volucellus Insectivore Warm Intolerant 1995 Emerald shiner Notropis atherinoides Insectivore Warm Neutral 1995 Spottail shiner Notropis hudsonius Insectivore Warm Neutral 1973 Golden shiner Notemigonus crysoleucas Insectivore Warm Neutral 1973 Blackchin shiner Notropis heterodon Insectivore Warm Intolerant 2008 Pugnose shinerd Notropis anogenus Insectivore Warm Intolerant 2008 Pumpkinseed sunfish Lepomis gibbosus Insectivore Warm Neutral 1973 Bluegill sunfish Lepomis macrochirus Insectivore Warm Neutral 1973 Blacknose shiner Notropis heterolepis Insectivore Warm Intolerant 2010 Tadpole madtom Noturus gyrinus Insectivore Warm Neutral 2008 Bluntnose minnow Pimephales notatus Omnivore Warm Neutral 2008 Fathead minnow Pimephales promelas Omnivore Warm Tolerant 2010 Largemouth bass Micropterus salmoides Predator Warm Neutral 1973 aThermal guilds classified by Lyons et al. (2009) bEnvironmental tolerance classified by Drake and Pereira (2002) cInitial introduction through stocking dState designated as a species of greatest conservation need


13

Fish management and population assessments Early stocking records dating back to 1911 include pike (walleye, lake trout, crappies, and northern pike). Some of these, such as lake trout and possibly crappie, may have been attempted introductions. Walleye and northern pike are assumed to have been present prior to stocking. The initial survey in 1973 showed a typical species assemblage for lake class 23. No crappie or trout were sampled or have been sampled in any assessment since. Species introduced since 1973 are lake whitefish (1977, 1978) and muskellunge (1982).

A creel survey conducted in 2001-2003 on Elk Lake, along with three other lakes within Lake Itasca State Park, documented moderate levels of fishing pressure when compared to similar lakes (MDNR unpublished creel survey data). Seventy-one percent of the anglers interviewed on Elk Lake indicated they were fishing for muskellunge. Most of the rest of the angling pressure on Elk Lake was targeted at walleye, northern pike, or no particular species. Estimated annual pressure and harvest on Elk Lake, adding summer and winter creel data, was 7,244 angler hours or 26.73 angler hours per acre of pressure, with an estimated harvest of 962 fish, or 3.55 fish per acre, weighing 670 pounds or 2.47 pounds per acre. Angling pressure on Elk Lake was probably lower than average during the winter of 2001-2002 because of late ice-up and no snow to run a snowmobile to Elk Lake from Lake Itasca. The wilderness loop road access to Elk Lake is closed in the winter. During the winter creel, there was no fishing pressure recorded on Elk Lake after the end of the gamefish season in mid-February. Yellow perch (Perca flavescens) were caught in the summer at a per acre rate within the interquartile range, but harvested at a rate above the 75th percentile value. All other species were caught and harvested during the summer and winter at per acre rates within their respective interquartile ranges, or slightly below their respective 25th percentile values. Winter anglers caught and harvested yellow perch at per acre rates far exceeding the 75th percentile. Yield per acre was within the interquartile range, or below the 25th percentile, for summer and winter anglers for all species except yellow perch. They were harvested at a per acre yield above the 75th percentile value. All quartile values were derived from the creel database MNCREEL for lake class 23 (MDNR unpublished data).

Walleye fry were stocked periodically for 11 years between 1948 and 1973. Walleye abundance in the 1973 survey was high at 13.7 per gill net (Figure 10). No walleye were stocked between 1973 and 1984, and walleye abundance dropped to less than one per net, suggesting the population is strongly dependent on maintenance stocking. Walleye stocking resumed using fingerlings in 1984, 1985, and every third year thereafter until the present. By 1990, walleye abundance was again up to 9.7 per gill net and remained above the third quartile of 5 per gill net in the 1995 (8) and 2000 (8.7) surveys (Figure A). Although some unstocked year classes have been present in the samples, walleye stocking has increased abundance. In the 1995 assessment, for example, six year classes were sampled but 62 percent of the walleyes were from two stocked years. This pattern continued in the 2008 assessment where 64 percent of sampled fish originated from stocked year classes. The most recent assessment in 2008 indicated walleye abundance was 4.2 per gill net, below the management goal catch rate of > 5.0 per gill net. Catch rates were above management goal in four of the seven assessments. The long-term average catch rate of 6.5 per gill net is above the upper quartile for similar lakes despite the negative influence of low catch rates observed in 1984 and 2005 (Figure 10). Walleye size distribution has remained similar with >50 percent of sampled fish exceeding 15 inches in past surveys (Figure 11). Walleye growth is good in Elk Lake with fish exceeding 15 inches by age 4.


14

Northern pike abundance has ranged from 0 – 6.33 per gill net with a long-term average of 3.9 per gill net (Figure 10). These abundances fall between the lower quartile and median values for similar lake classes. Early lake management plans for Elk Lake recommended sustaining the northern pike population at low abundance to limit competition with stocked muskellunge. Northern pike recruitment has historically been low, but consistent year classes have been produced. Growth rates are good, with fish exceeding 20 inches by age 3. These factors contribute to the quality-sized individuals observed in Elk Lake (Figure 11). A special regulation (40-inch minimum size limit; possession limit of 1 fish) was implemented in 2007 to restrict harvest of large fish and promote a continued quality population of northern pike. Annual ice-out netting for northern pike began in 2008. Length frequencies indicate female size structure has been consistent since 2008 for fish up to 27 inches (Figure 12); however, female northern pike exceeding 27 inches were absent from the 2010 sample. Data from netting during spring of 2011 will better determine if the decline in fish over 27 inches observed in 2010 is a population trend or an artifact of sampling error.

Muskellunge were introduced in 1982, and additional stockings were made in 1984, 1987, 1988, 1990, and annually from 1992 to the present. All stockings have been progeny of Mississippi Strain muskellunge from Leech Lake, making the Elk Lake population a pure genetic source of that strain. Since muskellunge have matured in 1987, Elk Lake has been used as an egg source for the statewide muskellunge production program. Elk Lake has demonstrated true trophy potential with reports of fish exceeding 50 inches in length being caught by anglers. A special regulation mandating total catch and release was implemented in 2004. Muskellunge stocking since the initial introduction may be the most intensive rate and frequency used in any Minnesota Lake. The rate of 271 fingerlings is one fish per surface acre and the frequency has been annual since 1992, when a steady supply of Leech Lake strain became available. Initially, this aggressive stocking rate was used to quickly establish an alternate brood source for Mississippi strain eggs, since the parent lake (Leech Lake) is difficult to trap and is not a dependable source to meet statewide production demands. That plan was successful and now the established brood lakes provide an ample egg supply for statewide needs. The aggressive stocking rate and frequency was maintained in Elk Lake due to the small lake acreage and to maintain consistency for evaluation. Beginning in 2008, the stocking rate was reduced in half to 135 fingerlings annually. Fingerlings stocked into Elk Lake have been internally tagged using Passive Integrated Transponder tags beginning in 2003 as part of research into muskellunge abundance, recruitment, growth, and survival (Younk 2003). Adult population density estimates in 1998 and 2003-2005 have ranged from 0.33 – 0.39 fish per surface-acre and averaged 0.36 fish per surface acre. A 2003 creel survey estimated open-water anglers caught and released 110 muskellunge, a number which exceeded the population estimate of 90 adult muskellunge performed in spring of that year. Mean lengths of sampled female and male muskellunge during targeted spring muskellunge assessments have been increasing since 1998, averaging 45.6 and 41.8 inches, respectively, over the long term (Figure 13). The increase in mean length for muskellunge in Elk Lake is likely the result of changing angler attitudes towards increased voluntary release and the mandatory release regulation implemented in 2004. Netting for muskellunge prior to 1998 was focused on obtaining adult fish for propagation purposes. Timing and duration of these nettings were not equivalent to assessments after 1998, so comparisons cannot be made for these time periods.

Yellow perch abundance has ranged 3.7 to 45.7 per gill net with a long-term average of 23.5 per gill net (Figure 10). Abundance nearly doubled in each successive assessment from 1984 to 2005, resulting in the long-term average catch rate above the upper quartile for similar lakes. Yellow perch size has followed a similar trend where the percent of sampled fish >8 inches in length increased since 1995 (Figure 11). Fish exceeding 10 inches in length first appeared during the 2005 assessment. The increase


15

in both numbers and size of yellow perch is difficult to explain since age and growth data does not exist. There is also no evidence of decreased predation upon yellow perch as other prey (cisco) and predator (walleye, northern pike) fish abundances appear unchanged over time. Although the introduction of muskellunge in the early 1980s added another predator to the fish community of Elk Lake, yellow perch population indices have improved throughout this time. Research is needed to understand population dynamics of this important forage fish and its ability to support predator stocking in various types of lakes.

Bluegill (Lepomis macrochirus) abundance in Elk Lake has been highly variable, but size has remained consistent. The long-term average abundance of 44.2 per trap net is just above the upper quartile for similar lakes; however, this value is influenced by the historical high catch observed in 2005. Trap net catches have ranged 8.2 to 111.4 fish per trap net (Figure 14). Bluegill size in Elk Lake has been stable, with similar numbers of individuals exceeding 8 inches (Figure 15). Forty-five percent of sampled bluegill was larger than 6 inches in length in seven of the nine trap net surveys in Elk Lake, but the number of fish larger than 8 inches has exceeded 10 percent in only two surveys. The stability observed in bluegill size structure may be a result of consistent recruitment and low angler exploitation. Contribution of ages 3-7 has generally been even over all surveys in Elk Lake, with fish up to 10 years of age sampled. A creel survey in 2003 estimated 80 percent of angler caught bluegill were voluntarily released. Bluegill growth in Elk Lake is slow with fish exceeding 6 inches in length at age six. Bluegill trap net data is being further analyzed by MDNR Fisheries Research staff. Preliminary analyses suggest parameters such as catch rate, age at maturity, length at age and sex structure, as indexed by trap nets, is dependent upon timing of spawning as male and female fish vulnerabilities to trap nets differ throughout the spawning season (Valley R.D. Federal Sportfish Restoration Act Study 605 F-26-R-36 2010 progress report). Therefore, the precision of bluegill population parameters derived from trap nets may be low.

Largemouth bass abundance indexed by trap nets has averaged 1.4 fish per trap net, which is near the upper quartile for similar lakes; however, catches by trap net and boat electrofishing have been declining since 2000 (Figure 16). This trend is counter to the hypothesis that largemouth bass populations are expanding in northern Minnesota. The population in Elk Lake has been composed of small individuals. Mean length of sampled fish has ranged from 9.7–11.0 inches via boat electrofishing and 4.0–10.4 inches via trap nets. Possible mechanisms into declines of largemouth bass populations need further investigation.

Several fish species have been sampled in Elk Lake that have received little management attention. Pumpkinseed (Lepomis gibbosus) and rock bass (Ambloplites rupestris) have been routinely sampled by trap nets with long-term average catch rates of 8.5 and 3.2 fish per trap net and mean lengths of 4.4 and 6.4 inches, respectively. Brown bullhead have been the only documented species of bullhead in Elk Lake. Catches via trap nets declined from 1973–1995 and none have been documented since 1995 (Figure 17). Brown bullhead were only captured in gill nets in 1990. White sucker have been sampled in low numbers and their abundance may be on the decline. White sucker were absent from the 1984 assessment, but were sampled at their historic high in 1990, after which catches declined to zero in 2008 (Figure 18).

Experimental introductions of whitefish occurred in the late 1970s as part of a University of Minnesota study of the Ten Mile Lake strain of whitefish. The objective of this study was to see if growth patterns exhibited by Ten Mile Lake whitefish were genetic or habitat induced. No published report is available. Since the whitefish introduction in 1977–1978, none have been sampled in standard fisheries assessments; however, whitefish have been observed during electrofishing surveys and may still persist in relatively low numbers. Habitat suitability for whitefish in Elk Lake is marginal (Figure 19).


16

Cisco are an important cold-water forage fish that sustain predator communities in many Minnesota lakes. These species are highly sensitive to dissolved oxygen depletion and temperature increases in deep hypoliminetic water and are declining throughout the state, presumably as a result of climate warming in recent decades. Elk Lake is a deep lake that strongly stratifies (Figure 19). Oxygen concentrations usually remain sufficient in the metalimnion, but the hypolimnion becomes anoxic during the period of greatest oxythermal stress (July 28 through August 27 for stratified lakes).

The benchmark measure of coldwater habitat (temperature at 3 mg O2 – temperature at the depth where dissolved oxygen equals 3 mg/L (TDO3); Jacobson et al. 2010) suggests that coldwater resources in Elk are excellent (Table 4). The mean TDO3 was 11.8°C during the period of greatest oxythermal stress. However, the anoxic hypolimnion limits suitable habitat to a relatively narrow band in the metalimnion (Figure 19). On a scale of 0 to 100 (with 0 being worst and 100 best), Elk Lake has a Cisco Habitat Suitability Index of 91. Profile data replotted as temperature vs. oxygen (Figure 19c) illustrate how close oxythermal habitat approached lethal conditions (Jacobson et al. 2008). All profiles contained conditions where cisco could survive and were well away from the lethal niche boundary. Recent work by Fang et al. (in review) suggest cisco habitat in Elk Lake should remain suitable under most climate change scenarios; however, additional work by USGS (Richard Kiesling) will be looking closer at these model forecasts and accounting for other watershed and groundwater variables that could influence the validity of these predictions.

Table 4. Temperatures at 3 mg O2 interpolated from the profiles taken by MPCA and MDNR during the period of greatest oxythermal stress (July 28 through August 27) from Elk Lake

Date TDO3

7/30/2007 9.5

8/9/2007 9.3

8/26/2008 12.7

8/12/2009 15.9

8/2/2010 11.8

Mean 11.8

Historic cisco abundance has ranged from 2.7–52.6 fish per gill net and averaged 15.5 per gill net over the long term (Figure 20). Although long-term cisco abundance has been above the upper quartile for similar lakes, the basin morphometry of Elk Lake limits habitat for cisco. Only the southern third of Elk Lake contains enough volume of thermal habitat necessary for cisco to survive warm water temperatures typical in late summer (Figure 20). Current and planned research projects involving cisco in Elk Lake include vertically-detailed water quality monitoring, acoustic biomass assessments of cisco and acoustic thermal-use tracking of cisco and their predators.


17

Figure 10. Summer gill net catch rates, long-term lake average and lake class quartiles of walleye, northern pike and yellow perch in Elk Lake, Clearwater County, MN 1973 – 2008. Lake class quartiles

were derived from Schupp (1992) for lake class 23.

1973 1984 1990 1995 2000 2005 2008

Year

0

3

6

9

12

15

18 Walleye

Long term average Lake Class Quartiles

0

2

4

6

8

10

12

Num

ber/

Gill

Net

Northern Pike

0

10

20

30

40

50

60 Yellow Perch


18

Figure 11. Length distributions of walleye, northern pike and yellow perch captured by summer gill nets in Elk Lake, Clearwater County, MN 1973 – 2008

Freq

uenc

y

100%

80%

60%

40%

20%

0%

100%

80%

60%

40%

20%

0%

100%

80%

60%

40%

20%

0%

Walleye

% > 15"

% > 20"

Yellow Perch

% > 8"

% > 10"

Northern Pike

% > 24"

% > 30"

1973 1984 1990 1995 2000 2005 2008

Year


19

Figure 12. Length distributions of female northern pike captured during ice-out trap netting inElk Lake, Clearwater County, MN 2008– 2010

40%

30%

2008 2009 2010

Freq

uenc

y

% >24"

% > 30"

20%

10%

0%

Year

Figure 13. Mean lengths of male and female muskellunge, captured during spring, in large-framed trap nets in Elk Lake, Clearwater County, MN 1998– 2010. Error bars represent quantile values of sampled lengths.

50.0

Mea

n Le

ngth

(in) 48.0

46.0

44.0

42.0

40.0

38.0

Female

Male

1998 2000 2002 2004 2006 2008 2010

Year


20

Figure 14. Summer trap net catch rates, long-term lake average and lake class quartiles of bluegill in Elk Lake, Clearwater County, MN 1973 – 2008. Lake class quartiles were derived from Schupp (1992) for lake class 23.

120

Num

ber/

trap

net 100

80

60

40

20

0

Long term average

Lake Class Quartiles

1973 1984 1990 1995 2000 2005 2008 2009 2010

Year

Figure 15. Length distributions of bluegill captured by summer trap nets in Elk Lake, Clearwater County, MN 1973–2010

100%

Freq

uenc

y

80%

60%

40%

20%

0%

1973 1984 1990 1995 2000 2005 2008 2009 2010

% > 6" % > 7" % > 8"

Year


21

Figure 16. Largemouth bass catch rates from trap nets (bars, left axis) and boat electrofishing (right axis) in Elk Lake, Clearwater County, MN 1973– 2010. Long-term average and lake class quartiles apply to trap net data. Lake class quartiles

were derived from Schupp (1992) for Class 23 lakes.

Figure 17. Summer trap net catch rates, long-term lake average and lake class quartiles of brown bullhead in Elk Lake, Clearwater County, MN 1973– 2010. Lake class quartiles were derived from Schupp (1992) for Class 23 lakes.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Num

ber/

trap

net

Brown Bullhead

Long Term Average Lake Class Quartiles

1973 1984 1990 1995 2000 2005 2008 2009 2010

Year


22

Figure 18. Summer gill net catch rates, long-term lake average and lake class quartiles of white sucker in Elk Lake, Clearwater County, MN 1973– 2010. Lake class quartiles were derived from Schupp (1992) for Class 23 lakes.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Num

ber/

gill

net

White Sucker

Long term average

Lake Class Quartiles

1973 1984 1990 1995 2000 2005 2008

Year


23

Figure 19. Coldwater habitat suitability for select cold-water species found in Minnesota as described by Jacobson et al. (2010). Only cisco and lake whitefish are found in Elk Lake.

A) Temperature Profiles B) Oxygen Profiles

0 0

-5 -5

Dep

th (m

) -10

Hab

itat S

uita

bilit

y In

dex

Dep

th (m

) -10

-15 -15

-20 -20

-25 -25

-30 -30

5 10 15 20 25 0 2 4 6 8

Temperature (°C) Oxygen concentration (mg/l)

C) Proximity to Cisco Lethal Niche Boundary

Oxy

gen

conc

entra

tion

(mg/

l) 10 Profile Date

8 2007 07/30 2007 08/09

6 2008 08/26 2009 08/12 2010 08/02 4

2

Temperature (°C)

D) Coldwater Habitat Suitability

5 10 15 20 25

100

80

60

40

20

5 10 15 20

Temperature at 3 mg/l of O2 (°C)

Species HSI Burbot - 43 Cisco - 91 Lake trout - 5 Lake whitefish - 56


24

Figure 20. Summer gill net catch rates, long-term lake average and lake class quartiles of cisco in Elk Lake, Clearwater County, MN 1973 – 2008. Lake class quartiles were derived from Schupp (1992) for lake class 23.

60

40

50

Num

ber/

Gill

Net

Long term average Lake Class Quartiles

30

20

10

0

1973 1984 1990 1995 2000 2005 2008

Year

Aquatic plant assessment Aquatic plants have been qualitatively assessed as part of most fisheries surveys since 1948 (Table 5). In June 2007, the Minnesota County Biological Survey compiled a list of aquatic plant species observed, including the relatively rare and commonly misidentified blunt-tipped sago pondweed (Stuckenia filiformis, Table 6). In 2008-2010, detailed quantitative surveys of vegetation cover were conducted as part of the SLICE suite of lake surveys. In August 2008, hydroacoustic mapping of vegetation biovolume (percent of water column occupied by vegetation) was conducted.

Throughout its surveyed history, emergent species bulrush (Scirpus spp.) and cattail (Typha spp.) have been prevalent (Table 5). With regards to submersed plant communities, species have varied in their frequency and detection by surveyors, but muskgrass, coontail (Ceratophyllum demersum), sago pondweed (Stuckenia pectinata), and clasping-leaf pondweed (Potamogeton richardsonii) appear as common species across all historic surveys. The aquatic plant community in Elk Lake is quite diverse, with 11–14 species occurring at frequencies greater than or equal to 10 percent in depths less than 15 feet (Table 7). Elk Lake’s high plant diversity, low frequency of tolerant species, and high frequency of intolerant species produces high IBI scores as described by Beck et al. (2010; 2008-2010 scores ranging from 76-79 out of 100).

A narrow band of modestly dense aquatic plants rimmed most of the west shore of Elk Lake when it was mapped in 2008 (Figure 21). In 2008, plants covered approximately 43 percent of bottom areas, but most growth was sparse beyond 10 ft. On average, aquatic plants occupied 25 percent of the water column in depths less than 20 ft (i.e., the “vegetated zone”) plus or minus 15 percent (i.e., standard deviation of grid cell estimates, Figure 21). Vegetation abundance varied considerably near shore to depths of approximately 6 ft, vegetation biovolume declined rapidly as depth increased (Figure 22). Patterns of aquatic plant growth in Elk Lake resembles other Minnesota mesotrophic lakes (e.g., Valley and Drake 2007; O’Hara et al. 2010).


25

Table 5. Common aquatic plant species sampled during past MDNR Fisheries lake surveys. Species were deemed common if they were either noted as “common” or “abundant” or if they

were encountered greater than 10 percent of the sample stations or transects surveyed.

Date Common Name Species Name Growth Form

9/5/1973 Hardstem bulrush Schoenoplectus acutus Emergent

Wildrice Zizania palustris Emergent

Yellow waterlily Nuphar variegatum Floating Leaf

Bushy pondweed Najas flexilis Submersed

Clasping-leaf pondweed Potamogeton richardsonii Submersed

Muskgrass Chara sp. Submersed

Canada waterweed Elodea canadensis Submersed

Sago pondweed Stuckenia pectinatus Submersed

Coontail Ceratophyllum demersum Submersed

7/10/1990 Wildrice Zizania palustris Emergent Hardstem bulrush Schoenoplectus acutus Emergent

Flatstem pondweed Potamogeton zosteriformis Submersed

Bushy pondweed Najas flexilis Submersed

Sago pondweed Stuckenia pectinata Submersed

Clasping-leaf pondweed Potamogeton richardsonii Submersed

Muskgrass Chara sp. Submersed

8/12/1998 Arrowhead Sagittaria sp. Emergent

Hardstem bulrush Scirpus acutus Emergent

Sedge Carex sp. Emergent

Narrow-leaved cattail Typha angustifolia Emergent

Broad-leaved cattail Typha latifolia Emergent

Yellow waterlily Nuphar variegatum Floating leaf Clasping-leaf pondweed Potamogeton richardsonii Submersed

Illinois pondweed Potamogeton illinoensis Submersed Whitestem pondweed Potamogeton praelongus Submersed Bushy pondweed Najas flexilis Submersed Coontail Ceratophyllum dermersum Submersed

Leafy pondweed Potamogeton foliosus Submersed Muskgrass Chara sp. Submersed

Needlerush Eleocharis acicularis Submersed Filamentous algae Mare’s tail Hippuris vulgaris Submersed Stonewort Nitella Submersed Straight-leaf pondweed Potamogeton strictifolius Submersed Flatstem pondweed Potamogeton zosteriformis Submersed

Sago pondweed Stuckenia pectinata Submersed Common bladderwort Utricularia vulgaris Submersed


26

Table 6. Plant species observed on June 6, 2007, along the Elk Lake north shore by the MN County Biological Survey

Common Name Species Name Growth Form Coontail Ceratophyllum demersum Submersed Northern watermilfoil Myriophyllum sibiricum Submersed Fries’ pondweed Potamogeton friesii Submersed Claspingleaf pondweed Potamogeton richardsoni Submersed Flatstem pondweed Potamogeton zosteriformis Submersed Common sago pondweed Stuckenia pectinata Submersed Blunt-tipped sago pondweed Stuckenia filiformis Submersed Greater bladderwort Utricularia vulgaris Submersed Ivy-leaved (star) duckweed Lemna trisulca Free floating Yellow water lily Nuphar variegata Floating leaf White water lily Nymphaea odorata ssp. tuberosa Floating leaf Floating leaf pondweed Potamogeton natans Floating leaf Hard-stem bulrush Schoenoplectus acutus Emergent Soft stem bulrush Schoenoplectus tabernaemontani Emergent Sweet flag Acorus americanus Emergent Wild calla Calla palustris Emergent Lake sedge Carex lacustris Emergent Beaked sedge Carex utriculata Emergent Bald spikerush Eleocharis erythropoda Emergent Water horsetail Equisetum fluviatile Emergent Common reed grass, Cane Phragmites australis Emergent Broad-leaved arrowhead Sagittaria latifolia Emergent Narrow-leaved cattail Typha angustifolia Emergent Wild rice Zizania palustris Emergent


27

Table 7. Percent frequency of occurrence of aquatic plant species at depths ≤ 15 feet sampled during point-intercept surveys on July 28, 2008, July 6, 2009, and July 7, 2010, at Elk Lake, Clearwater County, MN

Common Name Species Name Growth Form

Frequency (%) 2008 2009 2010

All rooted species 98.6 93.9 98.6 Flatstem pondweed Potamogeton

zosteriformis Submersed 57.5 28.8 39.4

Bushy pondweed Najas flexilis Submersed 53.4 43.9 54.9 Coontail Ceratophyllum

demersum Submersed 31.5 33.3 42.2

Wild rice Zizania palustris Emergent 31.5 21.2 23.9 Star duckweed Lemna trisulca Submersed 30.1 30.3 40.8 Sago pondweed Stuckenia pectinata Submersed 28.8 6.1 23.9 Muskgrass Chara sp. Submersed 21.9 18.2 28.2 Hardstem bulrush Scirpus acutus Emergent 17.8 25.8 15.5 Canada waterweed Elodea canadensis Submersed 15.1 7.6 8.4 Floating-leaf pondweed Potamogeton natans Submersed 15.1 10.6 4.2 Clasping-leaf pondweed Potamogeton

richardsonii Submersed 12.3 6.1 12.7

Filamentous algae 11.0 21.2 18.3 Fries pondweed Potamogeton friesii Submersed 11.0 28.8 26.8 White waterlily Nymphaea odorata Floating-

leaf 9.6 10.6 1.4

Bladderwort Utricularia vulgaris Submersed 4.1 4.5 1.4 Large-leaf pondweed Potamogeton

amplifolius Submersed 4.1 0 0

Swamp horsetail Equisetum fluviatile Emergent 4.1 6.1 2.8 Northern watermilfoil Myriophyllum sibiricum Submersed 2.7 0 0 Illinois pondweed Potamogeton

illinoensis Submersed 2.7 4.5 1.4

Robbins pondweed Potamogeton robbinsii Submersed 2.7 0 0 Bulrush Scirpus sp. Emergent 2.7 3.0 0 White-stem pondweed Potamogeton

praelongus Submersed 1.4 1.5 2.8

Broad-leaf cattail Typha latifolia Emergent 1.4 4.5 0 Narrow-leaf cattail Typha angustifolia Emergent 1.4 0 0 Narrow-leaf pondweed Potamogeton sp. Submersed 1.4 3.0 0 Arrowhead Sagittaria sp. Emergent 1.4 0 1.4 Cattail Typha sp. Emergent 0 4.5 0 Wild celery Vallisneria americana Submersed 0 4.5 0 Yellow waterlily Nuphar variegata Floating-

leaf 0 1.5 1.4

Water star-grass Heteranthera dubia Submersed 0 0 2.8 White waterlily Nymphaea sp. Floating-

leaf 0 0 8.4

Leafy pondweed Potamogeton foliosus Submersed 0 0 2.8 Variable-leaf pondweed Potamogeton

gramineus Submersed 0 0 1.4

Small pondweed Potamogeton pusillus Submersed 0 0 1.4

Max depth of veg growth (ft)a 11 10 16.6 aDepth of 95 percent of all plant occurrences


28

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

-70

-30

-50

-10

-40

-20

-10

-30

-40

-10

-20

-40

-60

2008 Veg Biovolume % Water Column

86

0

µ

0 0.1 0.2 0.05 Miles

Figure 21. Percent of water column occupied by vegetation (biovolume) in Elk Lake in summer 2008. Data were collected using hydroacoustics and mapped using kriging interpolation (methods described by Valley et al. 2005).

↑N


29

Figure 22. Percent of water column occupied by vegetation (biovolume) as a function of depth. A non-parameteric regression smoother was used to model this relationship (see methods described by Valley et al. 2005).

0 5 10 15 20 25 30

Depth (ft)

Perc

ent B

iovo

lum

e

020

40 60

80 100


30

Water quality Standard summer-mean water quality data for 2008-2010 are presented in Table 8, and raw data results are provided in Appendix B. In addition, major cations, anions, and total organic carbon were collected in May, July, and October and those values and typical ranges as derived from the National Lakes Assessment (NLA) database for Minnesota are summarized in Table 9. The NLA was a statistically-based survey of the nations lakes administered by the EPA in 2007. The typical range provided in Table 9 is based on 64 Minnesota lakes that were included in that NLA study and is intended to provide a regional perspective.

Table 8. Elk Lake 2008-2010 summer mean water quality. Typical range based on Northern Lakes and Forest ecoregion reference lakes (Heiskary and Wilson 2005) noted for comparison.

Parameter Elk Lake 2008

Elk Lake 2009

Elk Lake 2010

NLF

Total phosphorus (µg/L) 21 18 15 14 - 27 Chlorophyll mean (µg/L) 5 6 5 4 - 10 Chlorophyll max (µg/L) 8 8 9 <15 Secchi disk (feet) (meters)

8.2 2.5

8.9 2.7

9.5 2.9

8 – 15 2.4 – 4.6

Total Kjeldahl Nitrogen (mg/L) 0.6 0.6 0.5 <0.4 – 0.75

Alkalinity (mg/L) 157 160 155 40 - 140 Color (Pt-Co Units) 5 7 10 10 - 35 pH (SU) 8.4 8.6 8.8 7.2 – 8.3 Chloride (mg/L) - - 0.7 0.6 – 1.2 Total suspended solids (mg/L) 1.9 3.3 2.6 <1 - 2 Total suspended inorganic solids (mg/L) 0.3 1.2 0.7 <1 - 2 Conductivity (umhos/cm) 267 283 260 50 - 250 Total nitrogen: Total phosphorus ratio 29:1 33:1 33:1 25:1 - 35:1

Table 9. Annual mean values for cations, anions, and organic carbon. Interquartile range (referred to as typical range) based on 64 lakes included in the 2007 National Lake Assessment study included for perspective.

Parameter1 Elk Elk Elk NLA IQ Range

Ion balance

µeq/L µeq/L

2008 2009 2010 2007 2009 2010 Ca (mg/L) - 32.1 32.7 19.1 - 33.7 cations 1,602 1,632 Mg (mg/L) - 16.2 16.6 6.7 - 26.9 1,333 1,366 K (mg/L) 1.7 1.6 1.7 0.9 - 4.8 41 43 Na (mg/L) 6.6 6.7 6.9 2.2 - 9.0 291 300 Fe (µg/L) - 92.2 37.7 sum 3,267 3,341 Si (mg/L) - 9.7 7.4 3.1-13.5 Alk (mg/L 157 160 155 anions 3,200 3,100 SO4 (mg/L) Below Detection Level 2.2 - 14.1 - -Cl (mg/L) - - 0.7 1.5 - 18.4 - 20 DOC (mg/L) - 6.5 6.4 sum 3,200 3,120 TOC (mg/L) 7 6.9 6.4 7.3 - 14.2 1Cations and anions expressed as element (e.g. Ca as Ca); alkalinity expressed as CaCO3


31

Dissolved oxygen profiles

Dissolved oxygen (DO) profiles were taken monthly at site 101 (Figure 23). DO levels remained above 5 milligrams per liter (mg/L) in the epilimnion (upper, warmer layer) during the entire monitoring period. A metalimnetic maxima, which is common in oligo-mesotrophic lakes, was noted in May and June. DO levels were above 5 mg/L in the hypolimnion (lower, cooler layer) to a depth of 9 meters (29.5 feet) in May, September, and October. During the summer months of June, July, and August, the DO drops below 5 mg/L at 6 meters (19.7 feet) indicating oxygen demand from the decomposition of organic materials in the hypolimnetic water and bottom sediments and a lack of oxygen production. Additionally, low DO in the hypolimnion allows for an increase of phosphorus release from the sediments. This is evidenced by a sharp increase in depth phosphorus levels from July through October (Figure 25).

Temperature profiles

Temperature profiles were also taken monthly at site 101 (Figure 23). The lake formed a shallow thermocline in May and June at a depth of 2 to 3 meters (6.6-9.8 feet). The thermocline descended and remained at a depth of approximately 5 meters (16.4 feet) in July, August, and October and was at a depth of 10 meters (32.8 feet) in September. Surface temperatures at site 101 peaked at 24 degrees Celsius (°C) in July and water temperatures at the bottom remained within a half a degree of 5 °C throughout the summer.

A comparison was made between July 1985 and July 2010 profiles (Figure 24). Based on this comparison, thermocline depth developed at approximately the same depth in both years (Figure 24); however, the 2010 profile shows that the surface temperature was nearly two degrees warmer and the depth temperature was one degree warmer. The DO profiles in contrast were almost identical between the two time periods.


32

Figure 23. Elk Lake 2010 dissolved oxygen and temperature profiles

Dissolved Oxygen (mg/L)

0 5 10 15 20

0

5

10

15

20

25

30

35

Temperature (Degrees C)

0 5 10 15 20 25 30

0

5

10

15

20

25

30

35

Dep

th (m

) D

epth

(m)

May June July August September October

May June July August September October


33

Figure 24. Elk Lake July 1985 and 2010 dissolved oxygen & temperature profile comparison

Temperature (Degrees C) & DO (mg/L)0 5 10 15 20 25 30

0

5

10

15

20

25

30

Total phosphorus

Total phosphorus (TP) concentrations for Elk Lake (site 101) averaged 15 µg/L in 2010 (Table 8). This average was within the typical range of concentrations for NLF reference lakes. TP concentrations peaked at 20 µg/L in May and declined in June and July upon stratification (Figure 25). TP increased slightly in August and was fairly consistent continuing through October. TP concentrations collected near the lake sediment were similar to those at the surface in May (following spring overturn) but increased when the lake was stratified, peaking in September (Figure 26). This coincided with anoxic conditions near the bottom (Figure 23) from June through October. The three-year average for Elk Lake surface TP was 18 µg/L.

Both external (watershed) and internal (sediments, plants, and fish) sources can contribute to TP levels in lakes. TP in Elk Lake remained relatively stable throughout the summer. There was some moderate precipitation in mid-June through mid-July and more significant rain events in mid-August through September (Figure 7). Runoff from precipitation can be a significant source of nutrient input to a lake; however, since a majority of the land use within the drainage network surrounding the lake is forested with several other lakes and wetlands it is likely that that watershed inputs were rather minimal during the summer (Figure 5).

Hypolimnetic TP for Elk Lake ranged from a low of 21 µg/L to a high of 271 µg/L (Figure 27). Across the three years of available data, hypolimnetic TP was elevated when compared to epilimnetic TP on most dates, particularly during late summer and fall. The magnitude and timing of “elevated” hypolimnetic P varied among the three years. This bears further examination and closer inspection of the depth of sample collection relative to the DO and temperature profiles. For example, July through October of 2010 elevated hypolimnetic TP corresponded to a time period when DO was < 2.0 mg/L (Figure 23). Low DO above the sediments can allow for release of phosphorus from the sediments. This “internal loading” of TP may contribute to the slight increase of TP across the season following spring and fall overturn (Figures 26 and 27). The continued and more detailed data collections and synthesis by USGS will allow for an improved understanding of sediment P release and P cycling as a function of DO, temperature and lake mixing.

Dep

th (m

)

July 2010 Temp July 2010 DO

July 1985 Temp July 1985 DO


34

Chlorophyll-a

Chlorophyll-a (chl-a) concentrations provide an estimate of the amount of algal production in a lake. During summer 2010, chl-a concentrations for Elk Lake (site 101) ranged from 3 µg/L to 9 µg/L (Figure 25), with an average of 5 µg/L (Table 8). This is also the average for all three years of sampling (2008 - 2010). Chl-a concentrations from each sampling event within those three years were below the typical range of 4-10 µg/L for the NLF ecoregion. Concentrations greater than 20 µg/L will typically be perceived as a nuisance (Heiskary and Walker, 1988). As such, no nuisance algal blooms were observed in 2008, 2009, or 2010.

Secchi disk transparency

Secchi disk transparency on Elk Lake averaged 2.9 meters (9.5 feet) at site 101 during the summer of 2010 (Table 8). The average Secchi depth is within the typical range of values for the NLF ecoregion. The change in the transparency of Elk Lake during each sampling event closely mirrored the changes in nutrient availability (TP) and algal production (chl-a). The Secchi disk transparency reached a low of 2.3 meters (7.5 feet) in October and a high of 3.6 meters (11.8 feet) in July and August (Figure 25).

Figure 25. Elk Lake 2010 total phosphorus (surface) and chlorophyll-a concentrations, and Secchi depth

40 0

Conc

entr

atio

n (u

g/L)

30 2

20 4

10 6

0 8

2010 Monitoring Months

Surface TP Chl-a Secchi

May June July Aug Sept Oct D

epth

(m)


35

Figure 26. Elk Lake 2010 surface and depth total phosphorous comparison

0 50

100 150 200 250 300

May June July Aug Sept

Oct

Surface TP Depth TP

Figure 27. Elk Lake surface and depth total phosphorous three year comparison

0

50

100

150

200

250

300

Tota

l Pho

spho

rous

(ug/

L)

Surface TP Depth TP


36

Dissolved minerals and organic carbon

Dissolved minerals and organic carbon were measured in 2009 and 2010 as part of the long-term monitoring of Elk Lake and other Sentinel lakes. This includes some of the standard lake assessment measures of total suspended solids (TSS), alkalinity, conductivity and color (Table 8) as well as major cations, anions, silica, iron and organic carbon (Table 9). While several of these parameters have “typical” ecoregion-based concentrations (e.g., Table 8), some do not. For parameters without ecoregion–based comparisons, data from the 2007 NLA study were used to provide perspective on reported concentrations (Table 9). Since the NLA lakes were selected randomly, they provide a reasonable basis for describing typical ranges and distributions at the state-wide level.

TSS is within the range of expected values for NLF reference lakes (Table 8) and most of the TSS can be attributed to organic suspended solids (TSS minus TSIS), i.e., suspended algae. The low color value (Table 8) indicates the water is clear with a minimal amount of dissolved organic carbon (DOC). As such, total organic carbon (TOC) is rather low and the majority of the TOC (or all of the TOC for 2010) is in the DOC form, which is consistent with the state-wide data (Table 8). Lakes that receive a majority of their water inputs from forest and wetland runoff often have correspondingly higher color and TOC values as a result of incompletely dissolved organic matter (plants, leaves, and other organic material).

Alkalinity and conductivity are above the typical range for NLF lakes and are indicative of hard water (Table 8). Most cation and anion concentrations were quite stable across sample events and years (Table 9), which is consistent with the literature. Magnesium (Mg), sodium (Na), potassium (K), and chloride (Cl) are noted to be relatively conservative and undergo only minor spatial and temporal change (Wetzel 2001). Mg is required by algae to produce chlorophyll-a and Ca is used by rooted plants. Silica (Si), which is required by diatoms to form their “glass” shells, declined slightly from spring to spring.

Ca and Mg are the dominant cations and concentrations of both were within the typical range of the statewide data for both 2009 and 2010 (Table 10). The other two major cations, Na and K, were also within the typical range. Bicarbonate is the dominant anion, followed by Cl. Cl is within the typical range for NLF reference lakes (Table 8); however, it is low relative to state-wide NLA data (Table 10). Sulfate results were below the detection limits of the analyzing lab equipment and were reported as a non-detect. The average cation and anion balances (cation-anions expressed as a percentage of cations) for 2009 and 2000 were within 5 percent and 10 percent, which is well within values exhibited by the NLA lakes.


37

Table 10. Elk Lake cation, anion, and total organic carbon measurements

Date Ca1 Mg Na K Alk Cl TOC Si

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

5/22/2008 - - 6.9 2 170 - 6.6 -

7/16/2008 - - 6.7 1.5 150 - 7.4 -

10/8/2008 - - 6.3 1.7 150 - 7 -

5/20/2009 34.9 16.0 6.6 1.6 170 - 6.7 11

7/15/2009 29.1 16.0 6.6 1.5 150 - 7.5 7.1

10/20/2009 32.3 16.5 6.9 1.6 160 - 6.4 11

5/27/2010 37.4 17.1 7.2 2 170 0.8 6.5 8.1

6/29/2010 - - - - 150 0.7 - -

7/22/2010 27.4 15.8 6.4 1.4 140 - 6.7 5.6

8/26/2010 - - - - 150 0.7 - -

9/28/2010 - - - - 160 0.7 - -

10/12/2010 33.4 16.9 7.2 1.6 160 0.8 6.1 8.5 Average 32.4 16.4 6.8 1.7 157 0.7 6.8 8.6 NLA IQ range (mg/L)

19.133.7

6.726.9

2.2-9.0 0.9-4.8 1.5-18.4 7.3-14.2 3.1-13.5

µeq/L 1617 1349 296 43 3140 20

NLA typical range provided as a basis for comparison. 1Microequivalents (µeq/L) based on average value. Ion concentrations expressed as element (e.g., Ca as Ca).

Zooplankton Zooplankton samples were analyzed by Jodie Hirsch and Gary Montz at the MDNR Division of Ecological and Water Resources. A summary report was prepared that included information for all the Sentinel lakes sampled in 2008 (Hirsch 2009). Results from 2009 and 2010 were charted by MPCA staff and will be included in the discussion below.

Mean annual density and mean annual biomass for Elk Lake ranked low among NLF lakes (excluding NLF border lakes) within the Sentinel lakes program (Figures 28 and 29). Hirsch (2009) determined that, in general, as the amount of TP and chl-a increases (i.e., trophic state) so too does the relative abundance (biomass) of zooplankton. This appears to be the case for Elk Lake and the other NLF lakes (Figures 25 and 29). Similar to other NLF lakes, biomass peaked in the early summer and declined as the season progressed (Figure 29).

The zooplankton community over the three-year period of record was dominated by copepods. Cladocerans did appear to make up a larger proportion of the community later in the summer of 2008 and did exceed the copepod population. The cladoceran community declined during the summer months of both 2009 and 2010 (Figure 32). This also coincides with declining levels of TP in Elk Lake for 2009 and 2010.


38

Figure 28. Mean annual zooplankton densities for Northern Lakes and Forests Sentinel lakes

120

100

80

60

40

20

0

Elk

Hill

Portage

Red Sand

South Twin

Ten Mile

Biom

ass

(ug/

L)D

ensi

ties

(#/L

)

2008 2009 2010

Year

Figure 29. Mean annual zooplankton biomass for Northern Lakes and Forests Sentinel lakes

300

250

200

150

100

50

0

Elk

Hill

Portage

Red Sand

South Twin

Ten Mile

2008 2009 2010

Year


39

Den

siti

es (#

/L)

3

2.5

2

1.5

1

0.5

0

Figure 30. Mean monthly 2010 zooplankton densities for Elk Lake

May June July Aug Sept Oct

Month

Cladoceran Density Daphnia Density

Figure 31. Mean monthly 2010 zooplankton biomass for Elk Lake

0 2 4 6 8

10 12 14 16 18 20

Biom

ass

(ug/

L)

May June July Aug Sept Oct

Month

Cladoceran Biomass Daphnia Biomass


40

Figure 32. Percent composition by organism count

0%

10%

20%

30%

40%

50%

60%

70%

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100%

May

-08

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Copepods Cladocerans


41

National Lake Assessment Summary for Elk Lake Minnesota participated in EPA’s NLA that was conducted during 2007. This survey was part of EPA’s ongoing statistically-based surveys for assessing the health of the Nation’s lakes, rivers, wetlands, coastal waters, and estuaries. Over 900 lakes in the lower continental 48 states were included in this survey and of these, 50 Minnesota lakes were selected. These lakes were sampled according to EPA protocols by MPCA, MDNR, the U.S. Forest Service, and other collaborators during the summer of 2007. Details on the overall survey may be found on the NLA webpage at: http://water.epa.gov/type/lakes/lakessurvey_index.cfm.

Additionally, reference lakes representative of the various ecoregions represented in the survey were selected and sampled by EPA. Elk Lake was one of those lakes. It was selected based on its characteristics and the fact that it had been previously used as an ecoregion reference lake by MPCA. Each lake in the survey was sampled on one occasion (with the exception of two lakes that had repeat visits for statistical purposes). Elk was sampled on August 9, 2007, by EPA staff. Water chemistry data for Elk Lake was included in an assessment of Minnesota’s NLA data and those results may be found in Heiskary and Lindon (2010) http://www.pca.state.mn.us/index.php/view-document.html?gid=14986.

One aspect of the NLA survey that has relevance for the Elk Lake Sentinel report was the collection of sediment cores. Short sediment cores were collected with a 45 centimeter (cm) modified K-B corer from each of the NLA lakes. The upper and lower 1 cm of sediment was collected from the core for diatom analysis. The upper slice was intended to represent present-day and the lower slice was intended to represent historic conditions. Dating of the cores was not completed so the exact date for each of the slices is unknown. Diatoms in each slice were identified and enumerated and standard models were used to infer various water chemistry parameters. These models are based on well-documented studies that allow for the inference of lake water chemistry based on diatom community composition. Details on collection methods may be found at http://water.epa.gov/type/lakes/upload/2007_11_26_lakes_lakessurvey_pdf_lakes_field_op_manual.p df.

The core from Elk Lake was considered to be of good quality with the results deemed reliable by EPA analysts. Results that compare observed water chemistry and diatom-inferred for the top and bottom slices are presented in Table 11. Based on these results, good agreement among observed and inferred present-day TP and pH were noted, with fair agreement for TN and conductivity. A comparison of the top and bottom core slices suggests that Elk Lake may have been more eutrophic at some point in its history than is currently the case today; however, without actual dating of the core, it is difficult to draw any specific conclusions on these results.

Table 11. Comparison of National Lake Assessment sediment core observed water chemistry and diatom inferred chemistry. Statistical significance of difference is noted.

Observed Inferred top

Inferred bottom

Diff. (topbottom)

Sig. diff.

TP µg/L 9 8 56 -48 Yes TN µg/L 529 469 1055 -586 Yes Conductivity (µmhos/cm)

264 362 292 70 Yes

pH 8.5 8.6 8.4 0.2 No


42

http://water.epa.gov/type/lakes/lakessurvey_index.cfm�

http://www.pca.state.mn.us/index.php/view-document.html?gid=14986�

http://water.epa.gov/type/lakes/upload/2007_11_26_lakes_lakessurvey_pdf_lakes_field_op_manual.pdf�

http://water.epa.gov/type/lakes/upload/2007_11_26_lakes_lakessurvey_pdf_lakes_field_op_manual.pdf�

Sediment cores were previously taken in 1978 and 1982 from Elk Lake as a part of an effort to describe Holocene climate change (Dean et al. 1984). In contrast to the NLA cores, these were long cores collected by a piston corer and these cores were dated and used to describe long-term climatic changes for this portion of Minnesota. A somewhat more recent analysis of the cores was completed by Bradbury et al. (2002). They describe climatic and environmental change in Elk Lake (over a 1,500-year period) with an emphasis on primary productivity. This study reveals much in terms of long-term change in Elk Lake and the surrounding watershed. In addition to distinct climate signatures in the sediment, Bradbury et al. (2002), note there was a distinct increase in nutrients about 100 years prior. This would correspond to European settlement in the watershed and a period marked by extensive logging in northern Minnesota. Although, recent forest classification and aging documents that only 1/3 of Elk Lake’s watershed was logged, anecdotal evidence suggests log drives within Elk Lake’s watershed and through the lake enroute to Lake Itasca may have had a measurable effect on the lake’s limnology (Bradbury et al. 2002). These authors do note there was some uncertainty as to the accuracy of the 1980 observed TP, but thought it likely that TP was at least 18 µg/L, which would be within the range of summer-means from the Sentinel work (15-21 µg/L). Based on this account, it seems likely that the NLA findings indicate a valid and significant change in TP (Table 11). It will be important for these previous studies to be consulted as future coring work on Elk Lake is conducted.

Trophic State Index One way to evaluate the trophic status of a lake and to interpret the relationship between TP, chl-a, and Secchi disk transparency is Carlson’s Trophic State Index (TSI) (Carlson 1977). TSI values are calculated as follows:

Total Phosphorus TSI (TSIP) = 14.42 ln (TP) + 4.15

Chlorophyll-a TSI (TSIC) = 9.81 ln (chl-a) + 30.6

Secchi disk TSI (TSIS) = 60 – 14.41 ln (SD)

TP and chl-a are in µg/L and Secchi disk is in meters. TSI values range from 0 (ultra-oligotrophic) to 100 (hypereutrophic). In this index, each increase of ten units represents a doubling of algal biomass. Comparisons of the individual TSI measures provides a bases for assessing the relationship among TP, chl-a, and Secchi (Figure 36). In general, the TSI values are in fairly close correspondence with each other. Based on an average TSI score of 45 in 2010, Elk Lake would be characterized as mesotrophic. When compared with the TSI score (and corresponding values) from 2008 and 2009, Elk Lake has maintained the characteristics of a mesotrophic lake.

Trophic Status Trends One aspect of lake monitoring is to assess trends in the condition of the lakes. This analysis is based on data gathered through the MPCA’s Citizens’ Lake Monitoring Program or data collected by local groups and then stored in STORET. A review of data in STORET indicates there is a poor amount of data for Elk Lake to describe annual variability and to statistically assess trends. In general, for trend assessment, we seek a minimum of eight years of consistent data. While there is a large amount of water transparency data for Elk Lake, chemistry data is lacking. Based on yearly TSI averages calculated for 2008 through 2010, Elk Lake can be classified as mesotrophic (Figure 36).


43

Individual summer-mean TP and chl-a data provide further insight into trends and variability (Figure 33). The three-year average TP for Elk Lake is 18 ± 3 µg/L. Standard error, expressed as a percent of the long-term mean, represents the coefficient of variation (CV) of the mean. For Elk Lake, the CV equals 16 percent, which is fairly typical for oligo-mesotrophic Minnesota lakes. Chl-a values are also low, with a three-year mean of 2.8 ± 0.6 µg/L and a CV of 21 percent of the mean. Secchi disk transparency has been fairly high, with a long-term mean of 3.2 ± 0.6 m (Figure 34). The CV is 19 percent of the mean, which suggests minimal variability, which is typical for lakes that exhibit a long-term trend. Secchi disk values for 2008 through 2010 have been less than the long-term mean, indicating a recent reduction in water clarity; however, these values have improved each year. As with TP and chl-a, the Secchi disk values indicate mesotrophic conditions.

Historical precipitation records, collected in Itasca State Park, may provide some insight into potential nutrient sources influencing observed trends. Based on precipitation records from 1960 to 2010, mean annual precipitation is 26.9 inches and showing a slight decrease over the period of record (Figure 9). Mean for period of record is indicated by a solid blue line and simple linear regression by a red dashed line. The year 2008 was significantly drier, with an average measurement of 15.5 inches. In 2009, precipitation amounts increased, with an average measurement of 22.8 inches, which was still below the long-term average. In 2010, precipitation measurements showed a significant increase, at 29.1 inches, placing that year above the long-term mean. This above average precipitation coincided with a deeper Secchi disk value than the previous year (Figure 35). When historical precipitation is compared to historical Secchi disk values, in general, years with higher levels of rainfall show an increase in water clarity. Additionally, linear regression trendlines for Secchi depth (blue dashed line) and precipitation (red dashed line) indicate a similar decline (Figure 35). Future monitoring that includes lake level measurements should provide further insight into this relationship.

Figure 33. Elk Lake three-year summer-mean total phosphorus and chlorophyll-a

Conc

entr

atio

n (u

g/L)

40 35 30 25 20 15 10

5 0

2008 2009 2010 Year

Surface TP Chl-a


44

Figure 34. Elk Lake long-term summer-mean Secchi disk depth.Long-term mean noted by dashed blue line.

0 0.5

1 1.5

2 2.5

3 3.5

4 4.5

5

Dep

th (m

)

Year

Figure 35. Elk Lake historical Secchi transparency and precipitation comparison

Year

0 0

Secchi Depth Precipitation

Prec

ipit

atio

n (in

ches

)

Dep

th (m

) 2 20

4 40

6 60


45

Figure 36. Carlson’s Trophic State Index for Elk Lake R.E. Carlson

TSI < 30 Classical Oligotrophy: Clear water, oxygen throughout the year in the hypolimnion, salmonid fisheries in deep lakes.

TSI 30–40 Deeper lakes still exhibit classical oligotrophy, but some shallower lakeswill become anoxic

in the hypolimnion during the summer. TSI 40–50 Water moderately clear, but increasing probability of anoxia in hypolimnion during

summer. TSI 50–60 Lower boundary of classical eutrophy: Decreased transparency, anoxic hypolimnia during

the summer, macrophyte problems evident, warm-water fisheries only. TSI 60–70 Dominance of blue-green algae, algal scum probable, extensive macrophyte problems. TSI 70–80 Heavy algal blooms possible throughout the summer, dense macrophyte beds, but extent

limited by light penetration. Often would be classified as hypereutrophic. TSI > 80 Algal scum, summer fish kills, few macrophytes, dominance of rough fish.

NLF Ecoregion Range: Elk 2008: Elk 2009: Elk 2010:

OLIGOTROPHIC MESOTROPHIC EUTROPHIC HYPEREUTROPHIC 20 25 30 35 40 45 50 55 60 65 70 75 80 TROPHIC STATE INDEX 15 10 8 7 6 5 4 3 2 1.5 1 0.5 0.3 TRANSPARENCY (METERS) 0.5 1 2 3 4 5 7 10 15 20 30 40 60 80 100 150 CHLOROPHYLL-A (µg/L) 3 5 7 10 15 20 25 30 40 50 60 80 100 150 TOTAL PHOSPHORUS (µg/L) After Moore, l. and K. Thornton, [Ed.]1988. Lake and Reservoir Restoration Guidance Manual. USEPA>EPA 440/5-88-002.

Modeling Numerous complex mathematical models are available for estimating nutrient and water budgets for lakes. These models can be used to relate the flow of water and nutrients from a lake's watershed to observed conditions in the lake. Alternatively, they may be used for estimating changes in the quality of the lake as a result of altering nutrient inputs to the lake (e.g., changing land uses in the watershed) or altering the flow or amount of water that enters the lake. To analyze the 2010 water quality of Elk Lake, the Minnesota Lake Eutrophication Analysis Procedures (MINLEAP) model (Wilson and Walker, 1989) was used. A comparison of MNLEAP predicted vs. observed values is presented in Table 12.

MINLEAP was developed by MPCA staff based on an analysis of data collected from the ecoregion reference lakes. It is intended to be used as a screening tool for estimating lake conditions with minimal input data and is described in greater detail in Wilson and Walker (1989). The model predicts in-lake TP from these inputs and subsequently predicts chl-a based on a regression equation of TP and Secchi based on a regression equation based on chl-a. For analysis of Elk Lake, MINLEAP was applied as a basis for comparing the observed (2010) TP, chl-a, and Secchi values with those predicted by the model based on the lake size and depth and the area of the watershed.

Elk Lake is located in the NLF ecoregion and the model was run using NLF ecoregion-based inputs. The observed TP, chl-a, and Secchi values for Elk Lake are very similar to the predicted values. This simply means that the observed TP is consistent with what is expected for a lake of its size, depth, and watershed area in the NLF ecoregion. The model predicted TP loading at 112 kilograms per year (kg/yr). This result is likely a good estimate given that the observed TP matches the predicted values. The areal water load to the lake is estimated at 1.8 meters per year (m/yr) and estimated water residence time is on the order of three to four years; however, it is important to note that this estimate only considers watershed runoff and precipitation on the lake and does not account for groundwater inputs that are likely significant in lakes like Elk Lake. An additional subroutine in the MINLEAP model estimates the “background” TP for the lake based on its alkalinity and mean depth and a regression equation developed by Vighi and Chiaudani (1985). For Elk Lake this value is estimated at 22 µg/L, which is within than the NLF nutrient criteria (Table 12).

The MINLEAP model does not indicate the actual source of nutrient loading to the lake; however, based on its watershed to lake area ratio, land use composition, and the morphology of Elk Lake, it is probable that the model provides a reasonable estimate of the nutrient loading rate to Elk Lake (Table 12). The model estimates are derived from typical runoff nutrient concentrations from a forest-dominated watershed, combined with that contributed directly on the surface of the lake via wet and dry deposition.

Table 12. Minnesota Lake Eutrophication Analysis Procedures model results for Elk Lake

Parameter 2010 Elk Lake Observed

MINLEAP Predicted NLF Ecoregion

TP (µg/L) 15 18 Chl-a (µg /L) 5 5 Secchi (m) 2.9 3.1 P loading rate (kg/yr) - 112 P retention (%) - 67 P inflow conc. (µg/L) - 57 Water Load (m/yr) - 1.8 Outflow volume (hm3/yr) - 1.9 Residence time (yrs) - 3.6 Vighi & Chiaudani 21.6


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303(d) Assessment and Goal Setting The federal Clean Water Act requires states to adopt water quality standards to protect waters from pollution. These standards define how much of a pollutant can be in the water and still allow it to meet designated uses, such as drinking water, fishing, and swimming. The standards are set on a wide range of pollutants, including bacteria, nutrients, turbidity and mercury. A water body is “impaired” if it fails to meet one or more water quality standards.

Under Section 303(d) of the Clean Water Act, the state is required to asses all waters of the state to determine if they meet water quality standards. Waters that do not meet standards (i.e., impaired waters) are added to the 303(d) list and updated every even-numbered year. In order for a lake to be considered impaired for aquatic recreation use, the average TP concentration must exceed the water quality standard for its ecoregion. In addition, either the chl-a concentration for the lake must exceed the standard or the Secchi data for the lake must be below the standard. A minimum of eight samples collected over two or more years are needed to conduct the assessment. There are numerous other water quality standards for which we assess Minnesota’s water resources. An example is mercury found in fish tissue. If a water body is listed, an investigative Total Maximum Daily Load (TMDL) study must be conducted to determine the sources and extent of pollution, and to establish pollutant reduction goals needed to restore the resource to meet the determined water quality standards for its ecoregion. In Minnesota, the MPCA is responsible for performing assessment activities, listing impaired waters, and ensuring that TMDL studies are carried out.

Elk Lake was assessed based on NLF ecoregional standards (Table 13). Both the 2010 and long-term mean for Elk Lake have remained below 30 µg/L. Likewise, chl-a and Secchi are in full compliance with the NLF ecoregion standard. Based on these results, Elk Lake was assessed as fully supportive of aquatic recreational use based on 305(b) and 303(d) assessment standards that the MPCA conducts in support of the Clean Water Act. These assessments are submitted to the EPA on a biennial basis. Because of its good water quality, Elk Lake is a good candidate for protection and every effort should be made to ensure long-term maintenance of its high water quality.

Table 13. Eutrophication standards by ecoregion and lake type (Heiskary and Wilson, 2005).Elk Lake 2010 and three-year means provided for comparison.

Ecoregion TP Chl-a Secchi µg/L µg/L meters

NLF – Lake trout (Class 2A) < 12 < 3 > 4.8 NLF – Stream trout (Class 2A) < 20 < 6 > 2.5 NLF – Aquatic Rec. Use (Class 2B) < 30 < 9 > 2.0 NCHF – Stream trout (Class 2a) < 20 < 6 > 2.5 NCHF – Aquatic Rec. Use (Class 2b) < 40 < 14 > 1.4 NCHF – Aquatic Rec. Use (Class 2b) Shallow lakes < 60 < 20 > 1.0 WCBP & NGP – Aquatic Rec. Use (Class 2B) < 65 < 22 > 0.9 WCBP & NGP – Aquatic Rec. Use (Class 2b) Shallow lakes < 90 < 30 > 0.7 Elk Lake 2010 15 5 2.9 Elk Lake three-year mean 18 5 2.8


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Appendix A

Ice-on and ice-off records for Elk Lake

Lake Name Lake ID Ice Off Date Ice On Date

Elk 15-0010 17-Apr-88 02-Dec-87

Elk 15-0010 29-Apr-89 28-Nov-88

Elk 15-0010 24-Apr-90 24-Nov-89

Elk 15-0010 18-Apr-91 29-Nov-90

Elk 15-0010 18-Apr-93 25-Nov-91

Elk 15-0010 18-Apr-94 26-Nov-92

Elk 15-0010 14-Apr-00 27-Nov-93

Elk 15-0010 17-Apr-03 10-Dec-99

Elk 15-0010 19-Apr-04 26-Nov-03

Elk 15-0010 22-Apr-09


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Appendix B

Lake surface water quality data for Elk Lake for 2008-2010 All water quality data can be accessed at: http://www.pca.state.mn.us/data/eda/STresults.cfm?stID=29-0250&stOR=MNPCA1

Lake Name Lake ID Sample Date Site ID Secchi TP Chl-a Alkalinity Chloride TKN Color, Apparent TSS

Meters µg/L µg/L mg/L mg/L mg/L PCU mg/L

Elk 15-0010 5/15/2008 101 2.1

Elk 15-0010 5/15/2008 201 2.1

Elk 15-0010 5/22/2008 101 2 31 8 170 0.8 5 2

Elk 15-0010 6/1/2008 101 2.3

Elk 15-0010 6/1/2008 201 2.3

Elk 15-0010 6/24/2008 101 2.1 19 3

Elk 15-0010 6/26/2008 101 2

Elk 15-0010 6/26/2008 201 1.9

Elk 15-0010 7/9/2008 101 2.7

Elk 15-0010 7/9/2008 201 2.7

Elk 15-0010 7/16/2008 101 3.7 20 2 150 0.5 5 2.4

Elk 15-0010 8/1/2008 101 2

Elk 15-0010 8/1/2008 201 1.9

Elk 15-0010 8/7/2008 101 1.5

Elk 15-0010 8/7/2008 201 1.5

Elk 15-0010 8/26/2008 101 2.8 15 4 0.6

Elk 15-0010 9/12/2008 101 3.4

Elk 15-0010 9/12/2008 201 3.4

Elk 15-0010 9/17/2008 101 3.5 17 6 0.6

Elk 15-0010 10/8/2008 101 3 21 7 150 0.5 5 1.2

Elk 15-0010 5/20/2009 101 2.3 25 8 170 0.6 5 3.6

Elk 15-0010 6/10/2009 101 2.1 20 7 0.6


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http://www.pca.state.mn.us/data/eda/STresults.cfm?stID=29-0250&stOR=MNPCA1�

Lake Name Lake ID Sample Date Site ID Secchi TP Chl-a Alkalinity Chloride TKN Color, Apparent TSS

Elk 15-0010 7/15/2009 101 2.2 16 5 150 0.5 5 4

Elk 15-0010 8/12/2009 101 4.2 13 3 0.5

Elk 15-0010 9/24/2009 101 3 14 5 0.6

Elk 15-0010 10/20/2009 101 2.4 22 6 160 0.6 10 2.4

Elk 15-0010 5/27/2010 101 2.7 21 3 170 0.8 0.5 20 3.2

Elk 15-0010 6/29/2010 101 2.4 13 3 150 0.7 0.6 5 4

Elk 15-0010 7/22/2010 101 3.6 11 2 140 Non Detect 0.7 5 2.4

Elk 15-0010 7/26/2010 101 3.7

Elk 15-0010 8/6/2010 101 3.4

Elk 15-0010 8/26/2010 101 3.6 15 4 150 0.7 0.5 10 1.6

Elk 15-0010 9/13/2010 101 3

Elk 15-0010 9/28/2010 101 2.8 14 9 160 0.7 0.4 10 2.4

Elk 15-0010 10/12/2010 101 2.3 16 8 160 0.8 0.5 10 2


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2010 lake assessment of elk lake (15-0010), clearwater ... · elk lake (15-0010) clearwater county,...

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