lake management (muck removal) and hurricane impacts to the trophic state of lake tohopekaliga,...

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Lake and Reservoir Management 24:57-68, 2008© Copyright by the North American Lake Management Society 2008

Lake management (muck removal) and hurricane impacts to the trophic state of Lake Tohopekaliga,

Florida

Mark V. Hoyer, Roger W. Bachmann and Daniel E. Canfield, Jr.Department of Fisheries and Aquatic Sciences,

University of Florida/Institute of Food and Agricultural Sciences, 7922 NW 71st Street, Gainesville, FL 32653, USA

Abstract

Hoyer, M.V., R.W. Bachmann and D.E. Canfield, Jr. 2008. Lake management (muck removal) and hurricane impacts to the trophic state of Lake Tohopekaliga, Florida. Lake Reserv. Manage. 24:57–68.

Lake Tohopekaliga is a large (surface area 9,800 ha) and shallow (mean depth 2.1 m) natural lake in central Florida. Cultural eutrophication and lake water level stabilization led to accelerated growth of invasive native and non-native aquatic macrophytes, resulting in the buildup of thick deposits of organic matter along the shoreline. Those shoreline areas were often devoid of oxygen, and the muck buildup filled feeding grounds for wading birds and spawning fish. Muck build up has also reduced aesthetics and boat access. To remove organic accumulation, the lake water level was dropped and heavy equipment was used to scrape the plants and dead organic materials from the underlying sand substrates from more than 1,420 ha of the littoral zone. Most of this material was heaped into large piles in shallow parts of the lake to form 29 artificial islands with basal areas from 0.4 to 3.3 ha each. Our study was designed to determine: (1) amount of nutrients stored in islands relative to annual inflows, (2) nutrient release to the lake from the islands, and (3) changes in lake trophic state due to the muck scraping and construction of the islands.

The lake enhancement project was completed in late summer 2004, and the average thickness of organic materials in the scrapped areas was reduced from 46 cm to 1.6 cm, improving access and aesthetics tremendously. The islands stored several times the annual inflow of total phosphorus (TP, 3.1 times) and Total Nitrogen (TN, 6.5 times) and thus could potentially affect the lake’s trophic state by leaching nutrients. Our study of water quality in the vicin-ity of the islands indicates that the islands had no statistically significant impact on the water chemistry of the lake through leaching of nutrients. In the 2 years following the muck removal, substantial increases in average TP (39%), chlorophyll (56%), and color (53%) and a decline in dissolved oxygen (−10%) were found in open water stations. An unintended complication to our experimental design was the occurrence of 3 major hurricanes with high winds and heavy rainfalls that passed over the Lake Tohopekaliga area immediately following the muck removal project. To account for the effects of hurricane activity we examined monthly TP, TN, chlorophyll, Secchi depth data, and quarterly color values measured for 55 relatively small (median surface area 33 ha), nearby lakes. Our sample of 55 nearby lakes showed significant increases in TP (8.2%), TN (4.1%), chlorophyll (20.1%), and water color (23.8%), and decreases in Secchi depth (−8.2%) coinciding with the passage of the hurricanes. Additionally, data from a larger control lake (Kissimmee, surface area 19,800 ha) located 10 km south of Lake Tohopekaliga showed a much larger increase in total phosphorus (66%). Therefore, some or possibly all of the differences we measured before and after scraping could have been the result of low quality water (high nutrients and organic color) flushed into the lake following the heavy rains (93 cm in August and September of 2004) accompanying the storms. The effects of muck removal cannot be completely separated from those of hurricanes because they both occurred at the same time. However, aquatic plant (Florida Department of Environmental Protection) and water chemistry (Florida Fish and Wildlife Conservation Commission) data collected after this project was completed show that submersed aquatic macrophytes in Lake Tohopekaliga have returned and total phosphorus and chlorophyll concentrations are down to levels measured prior to muck scraping and hurricane impacts. Thus, the changes in water chemistry caused by muck removal and/or the hurricanes were relatively short lived (approximately 2 years).

Key words: eutrophication, lake management, littoral zone, sediment removal, trophic state

Hoyer, Bachmann and Canfield

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Lake Tohopekaliga is a large (surface area 9,800 ha), shallow (mean depth 2.1 m), natural lake located in the central part of the Florida peninsula south of Orlando, Florida (Osceola County). A major lake management problem started in the 1950s when the first municipal wastewater discharges reached the lake (Williams 2001). As a result, significant deterioration in water quality and aquatic habitat were evi-dent by 1969. Annual phosphorus loading peaked in 1980 at 112,000 kg/yr. In 1982, it was estimated that 42–48% (60,000 kg) of the total phosphorus load and 41–49% of the total nitrogen load entering Lake Tohopekaliga came from wastewater treatment plants (Jones et al. 1983). Since that time, lake management activities have caused a steady decline in wastewater effluent reaching the lake, thus decreasing the wastewater treatment plant phosphorus discharge to Lake Tohopekaliga from 87,000 kg in 1981 to 1,500 kg in 1988 (Dierberg et al. 1988, Williams 2001). Prior to the clean up, however, culturally increased phosphorus concentrations in the lake caused increases in algal production, organic sedi-mentation, and accelerated lake succession.

Decreases in water level fluctuations (water level stabiliza-tion) led to other management problems at Lake Tohope-kaliga. For the period of record from 1942 to 1964, Lake Tohopekaliga fluctuated between 59.40 ft MSL and 48.93 ft MSL, a range of 10.47 ft vertical (United States Geological Survey, unpubl. data; Fig. 1). As a part of flood control pro-grams, a lock and spillway structure (S-61) was completed on the outlet in January 1964 and a reduced fluctuation range was implemented. From 1964 to 1970 the elevation of Lake Tohopekaliga was controlled between 56.09 ft MSL to 51.35 ft MSL, a difference of 4.74 ft (Wegener and Williams 1974). The regulation schedule was then revised, reducing fluctua-tions to 3.0 ft vertical (55.0 ft MSL to 52 ft MSL) with a 1 in 3 year drop to 51.5 ft MSL.

Reduced water level fluctuations permitted development of formerly flooded lands around the lake, but had unintended consequences in the littoral areas. Limited water level fluctua-tions in Florida allow expansive monocultures of emergent aquatic vegetation to develop in the littoral zone (Hoyer and Canfield 1997). These conditions are also favorable for some submersed and floating-leaved aquatic vegetation. Increases in aquatic vegetation result in accumulation of organic matter, especially from exotic aquatic plants like hydrilla (Hydrilla verticillata) and water hyacinth (Echhornia crassipes). Ex-pansive monocultures of native emergent vegetation, such as pickerelweed (Pontederia cordata) and cattails (Typha spp.) also produce tremendous amounts of leaf litter (Hoyer and Canfield 1997). Organic matter trapped in stem and root structures of emergent and floating-leaved plants such as spatterdock (Nuphar luteum) can create tussocks (floating plant islands with an organic base) when anaerobic gasses accumulate on the bottom, causing mats to break loose and float to the surface. The historical record of aquatic plants

in Lake Tohopekaliga supports this mechanism of tussock formation (Hurkey 1957; Florida Department of Environ-mental Protection, Bureau of Invasive Plant Management 1982–1995).

High water events in Lake Tohopekaliga possibly resulted in uprooting plants and sediment deposition on the normally dry floodplain where they remained as water levels dropped. Conversely, organic sediments were exposed to drying and oxidation during drought conditions. Both mechanisms, which functioned to reduce the accumulation of organic mat-ter and create a diverse, dynamic, aquatic plant community in the littoral zone, were lost when the range in water level fluctuation was reduced from approximately 10 ft to 3 ft.

Aerial photographs of Lake Tohopekaliga from 1944 and 1996 (Fig. 2) show the dramatic increase in tussocks and accumulated organic matter along the shoreline. This rapid accumulation of organic matter greatly accelerated lake suc-cession, threatening to decrease the life span of Lake Toho-pekaliga. Degraded fish and wildlife habitats have occurred because the tussock areas are devoid of oxygen. Aesthetics and boat access for citizens who wish to use these areas of Lake Tohopekaliga also decreased.

Recent development around the lake has precluded a return to the former range in water levels with its natural rejuve-nation of the littoral zones; therefore, an alternative plan was selected to remove the accumulated aquatic plants and organic sediments. The Florida Fish and Wildlife Conserva-tion Commission (FFWCC) initiated a Lake Tohopekaliga enhancement project in summer 2004 designed to meet the

Figure 1.-Annual average water level for Lake Tohopekaliga from 1944 to 2004. The time water level control structure S61 came on line and the time three hurricanes passed over the lake are marked.

Lake management (muck removal) and hurricane impacts to the trophic state of Lake Tohopekaliga, Florida

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following objectives: (1) offset lake succession (primarily shoreline accretion) that resulted from cultural eutrophica-tion, water level stabilization, expansion of invasive aquatic macrophytes, and especially the accumulation of organic material from aquatic plant monocultures in the littoral zone; (2) improve lake access and aesthetics; and (3) restore fish and wildlife habitat toward historic plant community char-acteristics and improve sportfishing opportunities.

First, the lake water level was lowered to allow littoral sedi-ments to dry out. Next, heavy equipment was used to scrape the plants and dead organic materials from 1,420 ha of the underlying sand substrates in the littoral zone. Some of these materials were trucked out of the lake basin, but due to the high costs required to transport this material long distances and a lack of nearby disposal sites, most was heaped into large piles in shallow parts of the lake to form 29 artificial islands with basal areas from 0.4 to 3.3 ha each.

The first 2 objectives that FFWCC set for the Lake Tohope-kaliga enhancement project were accomplished. The average depth of approximately 46 cm of organic matter in the littoral areas before the lake enhancement project was reduced to 1.6 cm in the scraped areas (Hoyer et al. 2006). Thus, the enhancement project did offset increased lake succession caused by cultural eutrophication, water level stabilization, and expansion of invasive aquatic macrophytes, which also increased lake access and aesthetics of Lake Tohopekaliga for the citizens of Florida who wish to enjoy and preserve Florida’s lake systems. The final objective to restore fish and wildlife habitat toward historic plant community character-istics is being examined with other projects and will take a longer time to evaluate.

Many lakes in Florida and other parts of the world suffer from increased lake succession caused by cultural eutrophication, water level stabilization, and expansion of invasive aquatic macrophytes (Sheffer 1998, Rouwer and Roelofs 2001, Cooke et al. 2005). When management actions call for the removal of sediments in these lakes it is generally done by dredging with some form of upland disposal. Thus, to deter-mine if future management of accumulated organic matter with in-lake disposal was appropriate for Florida lakes, rep-resentatives of the major state and federal resource agencies working in Florida recommended a number of monitoring studies on various aspects of Lake Tohopekaliga’s ecosys-tems, such as fish populations, aquatic bird populations, plant and animal populations on the islands, and longevity of the islands.

Our study was initiated to address a major concern about the impacts to water chemistry, primarily the possibility for eutrophication caused by nutrients leaching from the islands to the open waters of Lake Tohopekaliga. Studies on dredging activities in other lakes generally show short term increases in nutrients then decreases to a lower stable state (Horne and Goldman 1994, Cook et al. 2005). Our study was designed to determine: (1) the amount of nutrients stored in islands relative to annual inflows, 2) nutrient release to the lake from the islands, and (3) changes in lake trophic state due to muck scraping and construction of the islands.

MethodsFeatures of islandsA Trimble ProXR global positioning data logging system (GPS) was used to measure the areas and volumes of each of the 29 islands (Fig. 3) before the water levels in the lake were raised. The perimeter and contours were mapped separately for each island using approximately a 15-m logging interval around the island perimeter and a 15-m spaced grid across the island for the contours. Approximately every 15 m, a coordi-nate point was recorded for 20 sec and averaged to create a

Figure 2.-Historical aerial photographs of Lake Tohopekaliga taken from collections of photographs taken by U.S. Department of Agriculture. The photographs from 1944, 1956, 1979, and 1996 show the rapid expansion of organic matter in the littoral zone of the lake.


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data point containing latitude, longitude, and altitude (MSL). Eighteen months later a subsample of 15 islands (every other island) was measured again to determine actual material losses. The perimeters of the islands were mapped at the scraped surface/piled muck interface. After water surrounded the islands, a depth pole was used to find the estimated edge of the islands before GPS readings were taken. The average altitude of all perimeter points was subtracted from the av-erage altitude of all contour points to calculate the average height of each island. The average height of an island was multiplied by its basal area to determine volume.

On the day that initial areas and volumes of individual islands were measured, 3 separate cores were taken from the top of each island. The cores, taken using a post-hole digger, were spaced uniformly around the islands depend-ing on the shape of the island. The depth (approximately 61 cm) and diameter (approximately 15 cm) of each core were measured to determine the volume of material removed. The material was weighed, shredded with garden clippers and a subsample taken for later analysis. Bulk density of the island material was calculated by dividing the wet weight of the material by the volume of the core (kg wet wt/L). At the laboratory, a smaller subsample of core material was dried at 100 °C and weighed to determine percent moisture content. Approximately 5 g of the dried material was ground to a powder, weighed, burned at 550 °C, and weighed again to estimate organic content. An additional small subsample (approximately 5 g) of the dried material was ground to a powder for analysis of total phosphorus, total nitrogen and carbon concentrations following the methods of Schelske et al. (1986). Nutrient concentrations in sediments are expressed as amount per unit dry mass.

Sediment cores (145) were taken to determine the thickness of organic sediment in all scraped areas around islands, the effectiveness of the scraping program, and for future deter-mination of rate of organic sediment accumulation in Lake Tohopekaliga. Sediment cores were taken once at each of 5 stations around each of the 29 wildlife islands to determine depth (cm) of the organic material on the lake bottom im-mediately after water filled the lake. The latitudes and longi-tudes of all sediment core stations were recorded using GPS equipment for future reference. The cores were taken with a clear plastic tube 3.8 cm. in diameter that was pushed into the sediment, sealed and removed. The thickness of organic sediment above the sand base in the core was measured us-ing a meter stick. Two cores were taken behind each island between the island and the shoreline water interface, and 3 cores were taken along a transect set perpendicular to the shoreline approximately 300 m away from each island.

Water chemistryIn the early 1980s, the South Florida Water Management District (SFWMD) set up 4 long-term open water chemistry monitoring stations (BO2, BO4, BO6, and BO9) in Lake Tohopekaliga (Fig. 3). We continued sampling at these same stations to examine potential whole-lake changes that may have occurred after the Lake Tohopekaliga enhancement project. An unintended complication to our experimental design was the occurrence of 3 hurricanes with high winds and heavy rainfalls that passed over the Lake Tohopekaliga area immediately following the refilling of the lake.

To determine if the islands were changing the water chemistry in their vicinity, we selected 3 islands (I, G and N: Fig 3) for short (3 months of monthly sampling) and long-term (2 years of quarterly sampling) examination of water chemistry im-pacts. These islands were selected because they were located closest to the 4 SFWMD water chemistry monitoring stations. At each of the 3 islands, 3 water chemistry sampling stations were selected along a transect 25 m, 75 m, and 150 m from the water-island interface toward the main lake. These sta-

Figure 3.-Map of Lake Tohopekaliga with the locations of all created islands (solid circles). Solid circles with a ring around them are islands where water chemistry sampling transects are located. Island N is used as an example of how the water chemistry sampling transects were set. The location of 4 long-term water chemistry sampling stations is also marked with white circles.


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tions were identified with the individual island’s letter and a station number 1, 2, and 3 corresponding to 25 m, 75 m, and 150 m, respectively (e.g., L1, L2, and L3). Approximately 400 m to one side of each transect, 3 additional, equally spaced water chemistry sampling stations were selected along a parallel transect approximately the same distance out into the main lake. These stations were considered a control and were identified with the individual island letter, the letter C for control, and station number 1, 2, and 3 corresponding to 25 m, 75 m, and 150 m of the island transect, respectively (e.g., LC1, LC2, and LC3). The latitude and longitude for these stations were recorded with GPS equipment to facili-tate repeat sampling. Water sampling began in August 2004 when the water level in Lake Tohopekaliga filled to low pool, and water surrounded the islands. Surface water samples (approximately 0.5 m below surface) were collected at each station using 1-L acid-washed, triple-rinsed Nalgene bottles. Water samples were placed on ice and transported within 24 hours to the Department of Fisheries and Aquatic Sciences water chemistry laboratory at the University of Florida for the analyses of total phosphorus (µg/L), total nitrogen (µg/L), chlorophyll (µg/L), and color (Pt-Co units). At each sta-tion, Secchi depth (m) was recorded, and a Yellow Springs Instrument Model 35 meter was used to measure dissolved oxygen (mg/L). All stations were sampled monthly for the first 3 months and then quarterly for a total of 10 sampling dates between August 2004 and June 2006.

Total phosphorus concentrations were determined using the procedures of Murphy and Riley (1962), with a persulfate digestion (Menzel and Corwin 1965). Total nitrogen con-centrations were determined by oxidizing water samples with persulfate and determining nitrate-nitrogen with second derivative spectroscopy (D’Elia et al. 1977, Simal et al. 1985, Wollin 1987). Chlorophyll concentrations were determined spectrophotometrically (Method 10200 H; APHA 1998) following pigment extraction with ethanol (Sartory and Grobbelaar 1984). Color was determined by spectroscopy (Bowling et al. 1986).

Statistical proceduresAnalysis of variance was used to determine differences in water chemistry among islands and differences between water chemistry measured along the control transects and the island transects for the first 3 months and all 10 months of repeated sampling at fixed locations. To identify localized impacts to water chemistry from leaching, another analysis of variance was used to determine significant differences among the sam-pling stations off the islands. Finally, an analysis of variance was used to determine differences in water chemistry among data collected before and after the lake enhancement project at the 4 long-term water chemistry sampling stations. Before analyses, all data were log10-tranformed to accommodate heteroscedasticity (Sokal and Rohlf 1981). All data collected

at the 4 SFWMD long-term monitoring stations were plotted with the last 10 years of SFWMD data collected prior to the enhancement project to determine if the water chemistry immediately after the enhancement project fell outside of the 95% confidence limits measured during the 10 years immediately prior to the enhancement project.

To account for the effects of the hurricane activity we ex-amined the monthly total phosphorus concentrations and quarterly color values for 55 adjacent lakes in Osceloa and Orange counties (Fig. 3) sampled by Florida LAKEWATCH in 2004. For each lake we calculated an annual average value for total phosphorus, total nitrogen, chlorophyll, Secchi depth and water color in 2004 (Florida LAKEWATCH 2006). We then calculated the average values for these variables for the months before and after the hurricanes and expressed them as percentages of the 2004 annual means. An analysis of variance was used to compare the mean percentages for these variables in the 55 lakes for the time before and after the hurricanes passed over Lake Tohopekaliga. Additionally, similar data were available for Lake Kissimmee from 1996 through June 2006 (Florida LAKEWATCH 2006), the same time period analyzed for the SFWMD long-term open water chemistry monitoring stations. The Lake Kissimmee data were examined as an additional control to Lake Tohopeka-liga because of its similar size (approximately 19,800 ha), location (approximately 10 km south of Lake Tohopekaliga) and because it had no muck scraping activity prior to the hurricane time period. All statistical computations were performed using various procedures in the JMP statistical package (SAS 2000).

ResultsAmount of nutrients stored in the new islands relative to annual inputsUsing GPS equipment and measuring poles, personnel from FFWCC estimated that 6.5 × 106 m3 of material were removed from 14.2 × 106 m2 of Lake Tohopekaliga littoral areas (Mann et al. 2004). These numbers yield an estimate of 46 cm for an average thickness of organic matter in littoral areas prior to the enhancement project. In 2005, using data from 145 cores, the average thickness of organic matter in the scraped areas was 1.6 cm, ranging from 0.0 cm to 16.0 cm. These data show that the scraping project was effective and provide a good baseline measurement for determining a sediment accumulation rate in the future.

In 2004, before water surrounded the islands, the basal areas of the 29 islands averaged 0.92 ha, ranging from 0.42 ha to 3.33 ha, with a total footprint in Lake Tohopekaliga of ap-proximately 27 ha (Table 1). Bulk density and percent wet weight of sediment cores taken from the islands averaged 1.3 kg/L wet wt and 34%, respectively. The cores also showed


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that the organic content averaged 11% and total phosphorus and total nitrogen concentrations averaged 0.12 mg/g dry wt and 3.0 mg/g dry wt, respectively.

We found a significant amount of the nutrients phosphorus (75,000 kg) and nitrogen (2,081,000 kg) stored in the 29 islands created in this project. To put these amounts into perspective, we looked at the nutrient budgets developed by James et al. (1994) who estimated nutrient loading to Lake Tohopekaliga for 1982 through 1992 by combining stream flow data with corresponding measurements of nutrients in the inflowing waters. Because all wastewater inputs had been diverted from the lake by January 1988, loadings for the period 1988–1992 were used as the best estimates of the

loadings during the current study. The average annual phos-phorus loads of 24,000 kg and nitrogen of 319,000 kg are less than the amounts of nutrients contained in all the islands. The islands hold approximately 3.1 times the estimated annual load for phosphorus and 6.5 times the estimated annual load for nitrogen. If all of these nutrients became available to the lake’s water column in a moderate-to-short time frame, they could cause significant eutrophication, so there was a basis for the initial concern from some professionals about the transfer of nutrients from the islands to the water.

Table 1.-Statistics for areas, volumes, and some physical and chemical characteristics of materials in the 29 islands created in Lake Tohopekaliga in 2004.

Parameter Mean Minimum Maximum Standard Error

Island Characteristics:

Island Area (ha) 0.92 0.42 3.33 0.09 Island Volume (m3) 31575 1649 116747 4241

Island Material Characteristics:

Bulk Density (kg/L wet wt) 1.3 0.9 1.8 0.03 Percent Wet Weight (%) 34 17 57 1.6 Total Phosphorus (mg/g dry wt) 0.12 0.05 0.23 0.01 Total Nitrogen (mg/g dry wt) 3 1.6 7.6 0.25

Table 2.-Statistics for water chemistry data collected from transects off 3 different islands (treatment transects) and control transects located approximately 400 m away, both located in Lake Tohopekaliga. The statistics are from samples taken on 10 different sampling days between August 2004 and July 2006.

Control Treatment Chemistry Island Mean Std Err Mean Std Err

Total Phosphorus (µg/L) G 69 4 69 4 I 43 2 46 3 N 83 5 88 7

Total Nitrogen (µg/L) G 1121 45 1121 49 I 823 11 868 18 N 1336 40 1372 54

Chlorophyll (µg/L) G 35 3 36 3 I 14 1 15 1 N 49 4 50 4

Secchi (m) G 0.8 0.04 0.9 0.05 I 1.0 0.03 0.9 0.03 N 0.7 0.03 0.7 0.03

Color (Pt-Co) G 92 7 88 6 I 98 6 103 6 N 86 6 94 8

Dissolved Oxygen (mg/L) G 7.1 0.3 7.0 0.3 I 6.6 0.3 6.5 0.3 N 6.3 0.4 6.1 0.4


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Effects of the hurricanes eroding the new islandsAt about the time the lake was refilled with water, 3 strong hurricanes passed through central Florida and affected Lake Tohopekaliga. On 13 August 2004, Hurricane Charley passed over the lake with sustained winds of 98 km/h and gusts of 120 km/h; on 5 September, Hurricane Frances passed over with sustained winds of 89 km/h and gusts of 139 km/h; and on 26 September, Hurricane Jeanne also impacted Lake Tohopekaliga with sustained winds of 98 km/h and gusts of 124 km/h. This unusual combination of major storms brought a tremendous rainfall of approximately 92 cm in August and September compared to the average of 38 cm for the same 2 months in 2000 through 2003 (South Florida Water Manage-ment District’s rain gauge S61-R located at the south side of Lake Tohopekaliga). The strong winds also ripped up large amounts of aquatic vegetation. Volume measurements of 15 islands in 2006 showed that the islands lost an average of 21% of their mass, and personal observations on the first sampling date after the hurricanes passed over (October 2004) suggest that the majority of this loss occurred during the 3 hurricanes.

Effects of islands on local water chemistryThe means for water quality variables for the transects near the islands and the controls for all 10 dates were recorded by island and treatment (Table 2). These data showed that the islands had very small and statistically insignificant ef-fects on the quality of water surrounding them. Two sets of analysis of variance were run, one on short-term data (first 3 months of data) and the second using all 10 sampling dates. The analyses were run using each individual water chemis-try variable (total phosphorus, total nitrogen, chlorophyll, Secchi depth, color and dissolved oxygen) as a dependent variable and island (general location in the lake), treatment (island transect versus control transect), Island*treatment interaction, month (seasonal variable) and month*treatment interaction as independent variables. For both data sets, all analyses showed that each whole model was significant. All analyses also showed that only island and month showed sig-nificant effects. Treatment, treatment*island interaction, and month*treatment interaction showed no significant effects.

Using only stations off the islands, additional analyses of vari-ance were run to determine if the stations closest to the island were different from those farther away. Again the analyses were run using each individual water chemistry variable (total phosphorus, total nitrogen, chlorophyll, Secchi depth, color and dissolved oxygen) as a dependent variable and station (stations 1, 2, and 3 correspond to 25 m, 75 m, and 150 m from the water-island interface) as the independent variable. All analyses showed no significant effect for station as the independent variable.

Effects of muck scraping and new islands on water chemistry at SFWMD monitoring stationsAnother set of analysis of variance was run using each water chemistry variable (total phosphorus, total nitrogen, chlorophyll, Secchi depth, color, and dissolved oxygen) as dependent variables measured at the 4 long-term sampling stations (Table 3). Group data (before muck scraping = data collected 1996–2004 and after muck scraping = data col-lected 2004–2006), station (B02, B04, B06, and B09), and station*group interaction were considered as independent variables. All analyses showed that the whole model was significant. All analyses also showed that station location had a significant effect suggesting that the water chemistry of Lake Tohopekaliga varies spatially, as did the analyses of transects from different islands. Only analyses on color, total phosphorus, chlorophyll and dissolved oxygen showed a significant group effect, while all other variables showed no significant group effect. The least squares means differences suggest that color, total phosphorus, and chlorophyll values were higher after muck scraping, and oxygen concentrations were lower after muck scraping. However, examining the plots of raw data versus date (Fig. 4), each water chemistry variable collected after muck scraping falls well within the distribution of data collected at Lake Tohopekaliga between 1996 and 2004, which was before muck scraping.

The 55 lakes near Lake Tohopekaliga also showed signifi-cant changes in water quality relative to the 3 hurricanes. The analyses of those 55 lakes (Fig. 5) showed that total phosphorus (8.2%), total nitrogen (4.1%), chlorophyll (20.1%), and water color (23.8%) after the hurricanes were significantly higher as a percentage of the average 2004 data than data collected before the hurricanes, while Secchi depth was significantly less (−8.2%). The median size of these 55 lakes, however, was only 33 ha compared to 9800 ha for Lake Tohopekaliga. Data from Lake Kissimmee, a larger control lake, was similar to the SFWMD long-term data collected in Lake Tohopekaliga. Lake Kissimmee’s total phosphorus, total nitrogen, and color all significantly increased after the hurricanes, while Secchi depth decreased (Table 3). Chlorophyll concentrations actually decreased in Lake Kissimmee following the hurricanes, while it increased in Lake Tohopekaliga. Thus, there is strong evidence that the hurricane rain, wind and wave action effects dramatically confound potential impacts caused by muck scraping and island construction in Lake Tohopekaliga.


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DiscussionDid the islands cause changes in surrounding water chemistry?Removing sediment, muck and/or tussock material from lakes brings the potential of negative impacts due to mobilization of nutrients and/or toxic substances (Peterson 1979). In fact, placing dredged leachable materials close to a lake is not a recommended lake management practice (Horne and Goldman 1994, Cooke et al. 2005). Our results showed that the creation of islands placed 3.1 and 6.5 times the annual inflow of phosphorus and nitrogen in Lake Tohopekaliga, respectively. Our study of water quality in the vicinity of the islands indicates that the islands were not having a significant impact on the water chemistry of the lake through leaching of nutrients. A statistical comparison of the values for total phosphorus, total nitrogen, chlorophyll, Secchi depth, color

and dissolved oxygen between samples collected in transects adjacent to 3 islands and corresponding control transects ap-proximately 400 m away from the islands showed no statisti-cal differences. Additionally, the individual stations placed different distances from the islands showed no significant differences in water chemistry.

Currents around the islands are possibly mixing the nutrients, inhibiting our ability to measure differences. However, after the initial flooding, submersed aquatic macrophytes grew rapidly, and personal observations of these macrophytes throughout the project suggest no such currents other than occasional wind and boat traffic. Another possibility is that rapid filling of the lake followed by tremendous hurricane wind and wave action removed the majority of the leachable materials before our sampling began. However, contrary to conventional wisdom, our data suggest that the islands are

Table 3.-Statistics for water chemistry data collected from 4 SFWMD long-term water chemistry sampling stations located in Lake Tohopekaliga and Florida LAKEWATCH data for Lake Kissimmee. The statistics are from samples taken before (1996–2004) and after (August 2004 and July 2006) hurricanes passed directly over the lakes.

1996-2003 2004-2006 Chemistry Station Mean Std Err Mean Std Err

Total Phosphorus (µg/L) BO2 68 3 91 16 BO4 35 2 42 3 BO6 48 2 71 7 BO9 51 2 80 7 Kissimmee 48 2 80 5

Total Nitrogen (µg/L) BO2 1128 35 1036 54 BO4 904 25 831 20 BO6 1114 45 1088 72 BO9 1152 36 1244 71 Kissimmee 1269 40 1452 38

Chlorophyll (µg/L) BO2 17 2 21 4 BO4 10 1 12 2 BO6 23 2 32 4 BO9 22 2 48 7 Kissimmee 33 3 29 6

Secchi (m) BO2 0.9 0.0 0.8 0.1 BO4 1.1 0.0 1.1 0.1 BO6 0.7 0.0 0.9 0.0 BO9 0.9 0.0 0.7 0.0 Kissimmee 0.9 0.0 0.7 0.1

Color (Pt-Co) BO2 111 7 143 16 BO4 55 3 97 9 BO6 59 5 100 14 BO9 56 4 87 11 Kissimmee 84 8 150 18

Dissolved Oxygen (mg/L) BO2 7.2 0.2 6.0 0.7 BO4 7.7 0.2 7.0 0.4 BO6 8.7 0.2 7.5 0.5 BO9 8.3 0.2 7.7 0.5


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Figure 4.-Plots of date versus 6 water chemistry variables measured at all 4 SFWMD long-term water chemistry stations located in Lake Tohopekaliga. The data span 8 years before the lake enhancement project and 10 dates in 2 years after the lake enhancement project. The dashed line represents the 95% of the distribution from the 8 years of data collected before the enhancement project.


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not impacting the waters immediately surrounding the islands in Lake Tohopekaliga.

Did the muck scraping and island creation followed by reflooding affect water chemistry at the SFWMD Long Term Monitoring stations?Large-scale sediment removal occurred in Lake Tohopeka-liga. Normally, dredging sediments increases nutrients in the lake, at least for a few years (Cooke et al. 2005). However, muck and tussock removal in Lake Tohopekaliga occurred when the lake was lowered and the site was not flooded, so smaller effects on water chemistry might be expected on reflooding. In general we found smaller water effects on eutrophication than with underwater dredging. For example, in Lilly Lake, Wisconsin, nitrate + ammonia rose from <1 to almost 6 mg/L during dredging and took 2 years to recover, while total phosphorus was little affected (Dunst, reported in Cooke et al. 2005). Chlorophyll in Lilly Lake increased 6-fold during dredging in response to the increase in inorganic nitrogen, and dissolved oxygen fell by 50%. Ortho-phosphate concentrations doubled during dredging at Lake Herman (Brashier, cited in Dunst et al. 1974). We did not measure nitrate, ammonia, or orthophosphate, but comparing Lake Tohopekaliga data with Lilly lake removal found somewhat smaller increases in total phosphorus (39%) and chlorophyll (55%), and decreases in dissolved oxygen (−11%).

Because of the confounding effects of the 3 hurricanes, some caution is required in interpreting the data from the SFWMD long-term monitoring stations. While statistical analysis in-dicated that levels of total phosphorus, chlorophyll and color were higher and dissolved oxygen were lower following the enhancement project, all or part of these differences could have been the result of low quality water (high nutrient, and organic content) flushed into the lake following the heavy rains (93 cm) accompanying the storms. Since the humic materials that produce water color in Florida lakes originate in the watershed, the fact that water color in some of the Lake Tohopekaliga samples exceeded 200 Pt-Co (Fig. 4) immedi-ately after the hurricanes indicates an external source for this material. Our sample of 55 nearby lakes showed significant increases total phosphorus, total nitrogen, chlorophyll and water color, while Secchi depth showed significant decreases coinciding with the passage of the hurricanes. The percent-ages of change were somewhat less than those experienced in Lake Tohopekaliga; however, the median surface area of the 55 nearby lakes was only 33 ha, compared to the surface area of Lake Tohopekaliga of 9,800 ha. Lakes with larger surface areas will tend to be impacted more by wind and wave action due to their large fetch and potential for resuspension and wave energy (Bachmann et al. 2000).

Data for our larger control lake showed that Lake Kissimmee’s total phosphorus and total nitrogen concentration increased

66% and 14 %, respectively from before (1996 to 2003) to after (2004 to 2006) the hurricanes, while Lake Tohopekaliga showed 39% and 31% increases respectively. Similarly, lake average total phosphorus concentration in Lake Okeechobee, Florida (approximate surface area 1,800 km2), increased from 90 µg/L 2 weeks prior to Hurricane Irene (fall 1999) to 220 µg/L (a 144% increase) 2 weeks after the hurricane (Havens et al. 2001). These large lake comparisons are much closer to data collected in Lake Tohopekaliga than those with the smaller lakes, suggesting that the hurricanes do impact larger lakes more than smaller lakes.

Another potential reason for the increased nutrients observed in the long-term sampling data is related to the abundance of aquatic macrophytes before and after the enhancement project. Each year between 1995 and 2004, hydrilla coverage exceeded 60% (Bell 2006), which is above the abundance where aquatic macrophytes have the ability to impact whole lake trophic state parameters (Canfield et al. 1983). Lakes with large amounts of aquatic macrophytes tend to have clear water with low nutrients and chlorophyll. Immediately fol-lowing the enhancement project there were few submersed aquatic macrophytes in the lake because they were scraped

Figure 5.-A comparison of total phosphorus (TP), total nitrogen (TN), Secchi depth, color, and chlorophyll data collected in 55 lakes located in the vicinity of Lake Tohopekaliga. The data were collected in 2004 and were split according to whether samples were taken before or after the August and September hurricanes passed over the lakes. The bars represent the means for monthly samples expressed as percent deviation from the 2004 annual averages. The error bars represent the 95% confidence limits on the means.


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away and/or scoured by hurricane winds. Additionally, lake color went up to approximately 200 (Pt-Co) after the hur-ricanes, which has been shown to decrease available light, and probably postponed the re-establishment of submersed aquatic macrophytes (Bachmann et al. 2002).

ConclusionNo matter how carefully a project is planned, something may still go wrong, a saying adapted from a line in “To a Mouse,” by Robert Burns: “The best laid schemes o’ mice an’ men / Gang aft a-gley.” There is no way to completely separate the affects of the muck scraping from those of the 3 hurricanes because they occurred at the same time. We know from the literature that both factors can elevate nutrient concentrations and impact other aspects of water chemistry. However, it is evident from plots of the distribution of the last 10 years of data that for each variable the majority of data collected after muck scraping and hurricanes passage fall within the distribution of recent historical data (Fig. 4). Additional data collected October 2006 after this project was completed (Florida Department of Environmental Protection, unpubl. data) show that submersed aquatic plant abundance in Lake Tohopekaliga had increased to almost 45% cover-age. With this increase in aquatic macrophyte abundance, total phosphorus and chlorophyll concentrations measured November 2006 at 2 open water stations close to BO2 and BO9 (FFWCC unpubl. data, average total phosphorus was 64 µg/L and average chlorophyll was 20 µg/L) are also down to levels measured prior to muck scraping and hur-ricane passage. Thus, impacts to water chemistry of Lake Tohopekaliga caused by muck scraping and/or hurricane passage were relatively short lived (approximately 2 years) and similar in duration to dredging projects discussed by Cooke et al. (2005).

When properly conducted, sediment removal is an effective lake management tool. Sediment removal is conducted for several reasons including nutrient removal, removal of toxic substances, and to offset lake succession due to sediment accumulation, as was the case for the Lake Tohopekaliga enhancement project. Removal of sediment from lake sys-tems always creates some environmental concerns (Peterson 1979). One of the most common is the potential for eutro-phication due to the liberation of nutrients. Data presented in this paper shows that the successful Lake Tohopekaliga enhancement project did not cause a long-term eutrophica-tion problem.

AcknowledgmentsWe thank Alex Horne for major contributions to this manu-script and two anonymous reviewers for additional comments that also improved this manuscript. This project was funded

by the Florida Fish and Wildlife Conservation Commission (FFWCC). We thank all of the Florida LAKEWATCH and FFWCC personnel who help on this large lake enhancement project.

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