factors influencing the recovery of submersed macrophytes in four coastal marshes of lake ontario

WETLANDS, Vol. 18, No. 2. June 1998, pp. 256-265 G; 1998, The Society of Wetland Scientists

F A C T O R S I N F L U E N C I N G THE R E C O V E R Y OF S U B M E R S E D M A C R O P H Y T E S IN F O U R COASTAL M A R S H E S OF LAKE ONTARIO

Eric R S. Sager, Thomas H. Whillans, and Michael G. Fox Environmental and Resource Studies Program

Trent University Peterborough, Ontario, Canada

K9J 7B8

Abstract: In this study, we investigated the impact of carp and turbidity on the growth of macrophytes from propagules in four Lake Ontario marshes with low submersed macrophyte abundance. A healthy propagule bank was transplanted into 4 m z carp exclosures (5-cm-mesh cages), turbidity exclosures (enclosed in plastic), and open control sites, with four replicates per treatment used in each marsh. Carp exclosures were intended to protect the transplanted propagule banks from carp and other large aquatic organisms; turbidity exclosures were intended to also reduce wind exposure and inflowing suspended sediments, thus increasing the amount of light reaching bottom sediments. The mean density of shoots produced in the turbidity exclosures (256 4- 46 shoots.m -2) was significantly higher than that produced in carp enclosures (20 ± 7 shoots.m -z) and open controls (10 + 5 shoots.m--'); above-ground biomass (AGB) was also significantly greater in turbidity exclosures. The difference in protection afforded the developing submersed macrophyte shoots can be attributed to the lower concentration of total suspended solids and greater level of light penetration in the turbidity exctosures. There was a strong linear relationship between photosynthetically active radiation (PAR) reaching the substrate surface and shoot growth in terms of both shoot density and AGB. The growth response was more sensitive to PAR in the field than it was in a growth chamber, suggesting that light levels are more critical to shoot development when multiple stressors are involved. The two marshes exposed to high wave energy had very high levels of suspended solids, and the introduced propagule bank was eroded away in controls and carp exclosures. In such marshes, both turbidity and exposure would have to be addressed for macrophyte recovery.

Key Words: macrophyte, turbidity, recovery, Great Lakes coastal marshes, carp

I N T R O D U C T I O N

The International Joint Commiss ion of the United States and Canada identified 43 Areas of Concern (AOC) throughout the Great Lakes based upon chronic water quality problems. The disappem'ance and sub- sequent lack of recovery of submersed rooted macrophytes is a common problem for many AOC. Macro- phytes play a crucial role in the littoral zone of aquatic systems (Wetzel 1983) such as Great Lakes coastal marshes. They dissipate the energy of wind and wave action, while the roots reduce the amount of bot tom sediment resuspension (Spence t982). They also serve as effective traps for inorganic and organic particulates (Foote and Kadlec 1988, Barko et aL 1991) and pro- vide suitable shelter and forage for waterfowl, inver- tebrates, and fish (Lodge 1991).

Many studies have focused on light as being the major limiting factor for recovery of submersed vegetation. The two factors often responsible for increasing light attenuation in nearshore areas are increases

in phytoplamkton populat ions and increases in the sus- pension of abiotic particulates (Sand-Jensen and Bo- rum 1991). Both factors might ptay a significant role in altering light penetration in disturbed systems (Chambers and Kal f f 1985), and i f the contribution of each could be predicted, flaen better informed management decisions could be made about restoring appropriate light environments.

A number of studies have been conducted to assess the direct influence of biota, particularly the common carp (Cyprinus carpio L.), on submersed and emergent plants. King and Hunt (1967), Macrae (1979), and Cri~ velli (1983) placed various densities of adult carp in enclosures and monitored their effect on existing vegetation. Other studies used exclosures to exclude adult carp and monitored changes to existing or transplanted vegetat ion within these exclosures (Dieter et al. 1991, O ' D a c r e 1992, Kahl 1993). All o f these studies have noted significant effects on vegetation density andJor biomass where plants were unprotected. Carp foraging

256

Sager et al., MACROPHYTE RECOVERY 1N COASTAL MARSHES 257

behaviour directly affects aquatic vegetation through the uprooting of mature plants (Macrae 1979). An indirect result of carp foraging is the resuspension of bottom sediments and the reduction of light intensity in the water column.

Few studies have examined the effects of biota on submersed vegetation in degraded coastal marshes and nearshore areas, but two such studies suggest that the light environment, rafher than plant dislocation or damage, is the major factor preventing reestablishment of this vegetation. Carter and Rybicki (t985) transplanted sprigs and tubers of Vall isneria amer icana Michx. from an 01igohaline estuary to six freshwater tidal sites in the Potomac River. The transplants were placed in several types of exclosures to assess the im- pacts of animal mid waterfowl grazing. They found that even in the presence of grazing activitb,, transplants were generally successful as long as an adequate light environment was present. Dushenko et al. ( t990) carried out a series of transplant experiments in the Bay of Quinte, Lake Ontario using propagutes of submersed plant species that were cultured under greenhouse conditions. They also included an herbivory component that addressed the impact of carp. Their study did not find any significant herbivory effects; instead fluctuating water levels along with inadequate water clarity were the major fhctors inhibiting recovery.

The goal of this study was to determine whether exclusion of carp and/or turbidity would aid the recovery of submersed vegetation in coastal marshes of Lake Ontario. The following hypotheses were tested in four coastal marshes with low abundances of submersed macrophytes.

1) Large carp are inhibiting the regenerative germination and growth of submersed macrophyte propagules in coastal marshes. Reductions in plant biomass due to carp may result from non-consumptive destruction (King and Hunt 1967, Macrae 1979, Cfivelli 1983, Lodge 1991). This could entail direct physical disturbance when foraging for benthic prey among es- tablished or regenerating macrophyte beds and during spawning activities. Indirect stress would also increase sediment resuspension and a reduction in light. This study was designed to measure the direct effects of carp.

2) High turbidity is inhibiting the regenerative germination and growth of submersed macrophyte propagules in degraded coastal marshes. High concentrations of suspended solids may reduce incident light to levels that are inadequate for germination and growth of submersed propagt~les. Another possible result of high levels of turbidity is increased rates of sedimentation, effectively burying the propagule bank or ger- minating plants (O'Dacre 1992, Westcott et al. 1997).

3) Germination of a propagule bank could occur under low light levels that would not sustain growth. Westcott et aL (1997) believed that one possible cause for the low densities of viable propagules in degraded marshes was that seedlings were not able to sustain growth after germination.

METHODS

Field Experiment

Exclosure experiments were carried out in t993 at four marshes tocated along the northwest shore of Lake Ontario: Port Britain, Oshawa Second Marsh, Grindstone Creek, and Hickory Island within Cootes Paradise (Figure 1). All of these marshes have high levels of turbidity and very little submersed vegetation. Westcott et at. (t997) found almost no submersed macrophyte propagules in the sediments of three of these sites (Grindstone Creek was not included). Com- mon carp were observed to be numerous in summer at all of these marshes.

Twelve sites were selected rmqdomty within each marsh for placement of exclosure treatments: four were set up as controls, four as carp exclosures, and four as turbidity exclosures. Each unit was an open box made of 5 × 5 cm wooden frmnes, 2 m in length and width, and 1.1 tn in height (4 m ~ area enclosed). Units were placed in water depths of 0.7 to 1.0 m, sunk 30-40 cm into the bottom sediments, and all four corners were wired to metal stakes. Carp exctosuxes were made of 5 × 5 cm wooden frames with 5-cm- mesh galvanized chicken wire. Turbidity exclosures had an additional layer of 6 ml clear plastic placed on top of the chicken wire. Controls were open except for the four corner stakes marking their location.

Four plastic germination trays (38.1 × 50.8 × 12.7 cm) containing 3 to 5 cm (depth) of propagute bank taken from Presqu'ile Bay to the east on Lake Ontario were stocked in each of the 12 exctosures. Details on collection and preparation of the Presqu'ile propagule b a ~ are provided by Sager (1995). A second set of control germination trays was stocked in poots in a greenhouse (see below). After 8 weeks, trays in both the field and greenhouse were harvested, and data on plant species and abundance were collected. The number of shoots from each of the four trays in each treatment were counted, and shoots were transported back to the lab, where they were washed with tap water, identified to species, and then dried at 60 ° C for 36 h. Total above-ground biomass (AGB) was determined for each treatment at each site by adding the shoot biomass from each of the four trays in an exclosure and standaxdiziug per m 2.

Weekly trips were made to each of the marshes to

258 WETLANDS, Volume 18, No. 2, t998

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: i : 1 . 1 Lake Ontario i !;

1. Hickory Island and Grindstone Creek 2. Second Marsh 3, Port Britain 4, Presqu'ile Marsh

Figure 1. Location of field sites relative to Lake Ontario.

monitor the condition o f the treatments and to coIlect biophysical data on factors affecting light attenuation. Carp and turbidity exclosures were monitored for the various water-column parameters, as it was assumed that carp exclosures were indicative of the open water conditions. During each visit to a site, all carp and turbidity exclosures were sampled from a boat for total i r radiance (pho tosyn the t i ca l ly act ive radiat ion, or PAR), temperature, and depth. A 1-2 L water sample was taken from the center of each exclosure at roughly half the distance to the bottom for determination of total suspended solids (TSS) and chlorophyll a concentration.

PAR was measured with a LiCor radiometer (LI- 1858) and underwater sensor (LI-192SB). Light intensity readings (txE-m-2-sec ~) were taken at depth inter- vals of 10 cm through the water column. Smaller in- tervals were used in shallower areas. Total extinction

(EeA~) was calculated as the slope of the naturM log of light intensity (PAR) as a function of depth.

Percent of surface light incident at the sediment surface (% Io) was calculated from light extinction coefficients by using an equation adopted from Painter (1990):

~z ~ e h l ( l o ) .~ (z*Epar)

where Io is irradiance just below the surface of the water and I x is the calculated irradiance at depth z. "Fne average Io for the summer was taken to be 1157 p,E.m-2-sec -~, which was the average PAR just below the water surface at the four marshes over the duration of the field experiment. % Io was calculated as 100*(I z /Io). It should be noted that the average t o is represen- tative of light levels between i0:00 a.m.-2:00 p.m. which was the approximate time that each marsh was sampled over ~ e course of the field season.

Sager et al., M A C R O P H Y T E R E C O V E R Y IN C O A S T A L M A R S H E S 259

Methods for determination of chlorophyll a and TSS were adapted f rom Environment Canada (1979), with final concentrations of chlorophyll a determined using the Trichromatic method (Wetzel and Likens 1979).

A rough measure of exposure was needed to quan- tify the energy environment of each location. Simple mean fetch was calculated for all treatments at each site by taking the mean of the distance to the closest land mass measured at each of the eight compass points (Loftus et al. 1992). The fetch of all 12 treatments at each site was then averaged to give one gen- eral index of exposure for each site.

Greenhouse Controls

Four germination trays were also set up in wading pools (2.5 m 2) and placed in the Trent Universi ty greenhouse on M a y 31. These were the control treatments to indicate seed viability for the transplanted propagule bank. Haag (1983) found that shoot densities are a fairly good indicator of the viable propagule bank that is present in the sediment. Pools were filled with tap water and al lowed to sit for 3 days to de- chlorinate the water before the trays were introduced. Compar ison with field-derived shoot b iomass was in- appropriate because the low m a x i m u m water depth in the wading pools (20 cm) would have a direct impact on shoot morphology and shoot length (Spence 1982). Therefore, shoot density was the only measure for comparing germination between the field and greenhouse treatments.

Growth Chamber Exper iment

A controlled experiment was set up over an 8-week period in a growth chamber to determine a critical light level at which germination and growth of the Pres- qu ' i le propagule bank could occur. Values of 1%, 5%, 10%, and 30% I o were used to give a realistic repre- sentation of light conditions that were encountered in the field. I o was taken to be the average summer value for the four marshes, as noted above, and the corre- sponding PAR values were approximately 12, 58, 116, and 350 txE-m-2.sec -l, respectively.

Each treatment was replicated four times and con- sisted of a 22.8 L aquarium filled to a depth of 15 cm with dechlorinated tap water and a removable sediment rack containing the Presqu ' i le propagule bank, 3 cm thick. Water was added throughout the duration of the experiment to maintain a constant level. A 14 h light/ 10 h dark photoperiod was maintained throughout the 60-day experiment. The 30% Io was achieved using a high intensity discharge mercury halide bulb (400W), whereas the 10% Io treatment was achieved using a combination of 120 watt bulbs with soft white fluo-

rescent tubes. The light source for the remaining two treatments was GE Gro and Sho wide spectrum fluo- rescent tubes, except that the 1% treatment had a layer of neutral density fiberglass screening to reduce irradiance (Goldsborough and Kemp 1988). All light levels were initially measured with a LiCor radiometer (LI 1858) and underwater sensor (LI 192SB). Tem- perature of the growth chamber was maintained at 22 to 23 ° C, which is within the range of op t imum germinat ion temperatures for many of the reproductive propagules of submersed plants (Barko et al. 1982).

At harvest, all shoots were removed, counted, and identified to species. All shoots were then cleaned and dried at 60 ° C for 36 h, and total AGB was measured for each replicate. Total shoot densities, which included the macroalga Chara, and angiosperm shoot densities were calculated. This was done to ensure that the macroalgae were not the only plants germinat ing at the lower light levels, as they can better survive such conditions relative to the angiosperms (Sand-Jensen and Madsen 1991), which were of interest in this study.

Statistical Analyses

The field exper iment was analyzed as a mixed model two-way analysis of variance (ANOVA) with four replicates per treatment and marsh (Zar 1974). An average of the four germinat ion trays was tabulated to yield a single value of density and AGB for each of the 12 treatment exclosures. When computing proba- bilities, the marsh was considered a fixed variable and the treatment was considered a random variable. When significant differences arose, Tukey 's HSD tests were conducted to determine which sites or treatments were different. Means compar isons between sites were calculated using the mean square error term f rom the A N O V A error term (Zar 1974). Compar isons of treatments within marshes were made using a one-way ANOVA.

Comparisons among treatments in the growth chamber experiment were made with Tukey 's HSD test. Least squares regression lines were used to describe the relationships between plant growth and light parameters. A probabil i ty value of 0.05 was defined as significant in all statistical tests.

RESULTS

Field Exper iment

Shoot Density and Above Ground Biomass. Significant differences in shoot density and AGB among field treatments and marshes were found, with both parameters having significantly greater values in turbidity exclosures than in carp exclosures or field controls (Ta-

260 W E T L A N D S , V o l u m e 18, No. 2, 1998

Table 1. Results of two-way ANOVA (Model I) for density and above ground biomass. (GK = Grindstone Creek; HI = Hickory Island; PB = Port Britain; SM -- Second Marsh; T = Turbidity exclosure; C = Carp exclosure; O = Open controls).

Tukey's Multiple Variable a F b Probability Comparison

Density Site 10.74 p < 0.005 PB, GK > HI, SM Treatment 45.9 p < 0.001 T > C, O Site × Trt. ° 0.61 p = 0.723

AGB Site 5.21 p < 0.05 PB > GK, HI, SM Treatment 8.28 p < 0.001 T > C, O Site × Trt5 0.96 p = 0,469

All data Log~0 transformed to achieve a normal distribution. b All F stats use the following DF: Site = 3, 6; Treatment = 2, 36; Site × Trt. - 6, 36. Trt. = Treatment.

bles I and 2). Por t Br i t a in and G r i n d s t o n e C r e e k m ar shes p r o d u c e d a s ign i f i can t ly h ighe r dens i ty o f shoots than the o ther two marshes , but w h e n A G B was assessed , Por t Br i ta in was s ign i f i can t ly d i f fe ren t than H i c k o r y I s l and and S e c o n d M a r s h (Table 1, F i g u r e 2). G r e e n h o u s e con t ro l s p r o d u c e d a much h ighe r dens i ty o f shoots than any o f the t r ea tments in the f ield (Table 2). A t all si tes, the d o m i n a n t spec ies found in tu rb id i ty exc losu re s was Najas flexilis Wil ld . and Chara sp., wh i l e the d o m i n a n t spec ies f o u n d in ca rp exc losu re s was Potamogeton pectinatus L. Chlorophyll a, Suspended Solids, Eeae, and % I o. Turb id i ty exc losu re s had s ign i f i can t ly l o w e r levels o f TSS , c h l o r o p h y l l a concen t ra t ion , and % I o than ca rp exc losu re s (Tables 3 - 4 ) . C h l o r o p h y l l a concen - t ra t ion was s ign i f i can t ly grea te r and EpAR was s ignif i - can t ly l o w e r at H i c k o r y I s l and enc losu res than those o f the o ther three marshes . T S S and % Io in Por t Br i t - ain enc losu res we re s ign i f i can t ly g rea te r than those o f the o ther marshes .

Temperature, Depth, and Fetch. W a t e r dep ths in t r ea tmen t si tes used at G r i n d s t o n e C r e e k and H i c k o r y I s l and were 8 - 1 1 c m deeper , on average , than at s i tes in Por t Br i ta in and S e c o n d M a r s h (Table 5). D e sp i t e the sha l l ower water , s i tes in Por t Br i ta in and S e c o n d M a r s h were s ign i f ican t ly c o o l e r than those in G r i n d - s tone C r e e k and H i c k o r y Is land. H i c k o r y I s l and had the la rges t c a l cu l a t ed fe tch at r o u g h l y 367 m. This was f o l l o w e d by S e c o n d M a r s h (95 m), Por t Br i ta in (78 m), and Gr inds tone C r e e k (77 m).

Cr i t i ca l L igh t E x p e r i m e n t

Shoo t dens i t i es in the 1% and 5% l o t r ea tments were s ign i f ican t ly l o w e r than those in the 10% and 30% Io

Table 2. Mean shoot densities and AGB (+ SE) of each treatment when all sites are pooled.

Shoot Density AGB

Treatment (Sht-m -2) (g-m -~) N

Carp exclosures 20.0 -+ 6.8 4.21 _+ 1.6 16 Open 9.6 -4- 4.9 1.3 -- 0.85 16 Turbidity exclosures 255.6 + 46.2 36.8 -+ 14.0 16 Greenhouse controls 1545 - 292 NA 4

t r ea tmen t s ; this was the case when bo th total dens i ty and a n g i o s p e r m dens i ty a lone were c o n s i d e r e d (F igure 3a). In the case o f A G B , the o n l y s ign i f ican t differences were b e t w e e n the 1% I o and the 10% and 30% I o t r ea tmen t s (F igure 3b). The co r re l a t ion b e t w e e n shoo t dens i ty and A G B was s t ronge r under the con- t r o l l e d cond i t i ons of the cr i t ica l l ight e x p e r i m e n t ( r 2 = 0 .90) than in the f ie ld (r 2 = 0.52).

R e l a t i o n s h i p s B e t w e e n L igh t In tens i ty and Shoo t G r o w t h

Table 6 s u m m a r i z e s the leas t squares r e g r e s s i o n re- l a t i onsh ips b e t w e e n P A R and shoot d e n s i t y / A G B . B o t h r e l a t i onsh ip s were h igh ly s ign i f ican t in bo th the f ie ld and cr i t ica l l ight expe r imen t s . Howeve r , the s lopes o f the bes t fit l ines d i f fe red s ign i f i can t ly be - t w e e n the two e x p e r i m e n t s for bo th b i o m a s s ( p = 0 .001) and dens i ty ( p=0 .007 ) . S h o o t dens i ty and A G B in the f ie ld were a ppa re n t l y m o r e r e s p o n s i v e to in- c reases in P A R than was the case in the g r o w t h chamber. Howeve r , p l an t g rowth occu r r ed in the g r o w t h c h a m b e r at l igh t leve ls a p p r o a c h i n g 10 ixE.m Z.sec 1, w he re a s s e v e r e l y i m p a i r e d g e r m i n a t i o n and g r o w t h occu r r ed in the f ie ld at l igh t l eve ls o f 25 ixE.m 2.sec 1.

R e l a t i o n s h i p s b e t w e e n shoot de ns i t y and sed imen t - su r face P A R were s igni f icant in all o f the marshes , and r e l a t i onsh ip s b e t w e e n A G B and s e d i m e n t P A R were s ign i f ican t on ly at Por t Br i ta in and G r i n d s t o n e C r e e k (Table 7). P A R e x p l a i n e d a h igher pe rcen t o f the to ta l v a r i a n c e in shoot dens i t ies at Por t Br i ta in and Gr ind- s tone Creek , w h i c h a lso had a r e d u c e d fetch. The re- g r e s s ion for shoot dens i ty and P A R e x p l a i n e d 73% o f the total va r i ance at H i c k o r y Is land, w h i c h had the h ighe s t fetch. A b s o l u t e dens i t i es we re s ign i f i can t ly l o w e r at this site, as wel l as at S e c o n d Marsh , bu t w h a t l i t t le g r o w t h d id occur st i l l a p p e a r e d to d e p e n d upon l ight , as m o s t g e r m i n a t i o n occu r red wi th in the tu rb id- i ty e xc lo su re s where e x p o s u r e to w i n d and w a v e en- e r g y was r e d u c e d and the a m o u n t o f l ight r e a c h i n g the s e d i m e n t su r face was increased .


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Figure 2. A) Mean above-ground biomass (AGB) of all three treatments at each site. B) Mean shoot densities of three treatments for each site. Error bars represent standard errors. (PB =- Port Britain; SM = Second Marsh; GK = Grindstone Creek; HI = Hickory Island; T = Turbidity exclosure; C = Carp exclosure; O = Open controls).

D ISCUSSION

Field Exper iment

Aside from the absence of a viable native propagule bank, recovery of submersed vegetat ion at all four of the marshes seemed to be influenced by suspended ma- terial. TSS increased light attenuation and also increased the rates of sedimentation, which effect ively buried the introduced propagule bank. Shoot densit ies and AGB were greater in the turbidi ty exclosures where the effects of a number of inhibi t ing agents were dampened. Shoot densities were much higher at Port

Britain and Grindstone Creek, relat ive to the other two sites. Since there was neither a measure of carp density nor complete isolat ion of carp effects (i.e., indirect effects), it is imposs ib le to determine whether or not these differences were caused by carp. Given the differences between sites when carp were excluded, some other factors also appear to be exert ing pressures.

The direct impact of carp (Cyprinus carpio) was not obvious. Propagule transplants within the carp exclosures did not germinate in significantly greater num- bers than the open controls, but there were quali tative differences in what remained of the propagule bank,

262 W E T L A N D S , V o l u m e 18, No. 2, 1998

Table 3. Results of two-way ANOVA (Model I) of limnological parameters. (GK = Grindstone Creek; HI = Hickory Island; PB = Port Britain; SM = Second Marsh; T = Tur- bidity exclosure; C = Carp exclosure).

Tukey's Means Variable . F ° Probability Comparison

TSS Site Treatment Site × Trt5

Chl. a. Site Treatment Site X Trt. c

%Io Site Treatment Site × Trt5

EpAR Site Treatment Site x Trt. c

17.38 p < 0.025 HI, GK, SM > PB 116.5 p < 0.001 C > T

3.45 p < 0.02

47.44 p < 0.005 HI, SM > GK, PB 50.85 p < 0.001 C > T

1.098 p = 0.352

18.8 p < 0.025 PB > SM, GK, HI 71.72 p < 0.001 T > C

5.29 p < 0.002

53.7 p < 0.005 HI > SM, G K > PB 91.74 p < 0.001 C > T

0.55 p = 0.65

, All data Log,o transformed to achieve a normal distribution; %I,, transformed with arcsine-square root. b All F stats use the following DF: Treat = 1, 167; Site = 3, 3; Site × Trt. = 3, 167. ° Trt. = Treatment.

Trays p laced in carp exc losu re s in p ro tec ted loca t ions had the t ransplanted p ropagu le bank still intact, but they were c o v e r e d wi th fine par t ic les that had sett led out o f the wa te r co lumn. Trays that were exposed to ambien t cond i t ions in open t rea tments but p l aced in she l te red s i tuat ions were usual ly devo id o f the trans- p lan ted bank, sugges t ing that there was direct disturbance. This cou ld have been carp that were fo rag ing in the rich organic transplant , but muskra ts , birds, and other fish cannot be ru led out. Trays f r o m carp exc lo-

Table 5. Mean temperature and depth for each site (± SE). Mean depths were calculated from all treatments (turbidity exclosures, carp exclosures, open controls) while mean temperatures were calculated from data collected from turbidity and carp exclosures.

Site Temp (°C) ~ Depth (cm)'-

Grindstone Creek 25.9 ± 0.3" 83 ± 2.0 ~ Hickory Island 25.6 ± 0.2" 83 ± 2.0" Port Britain 23.8 ± 0.6 b 75 ± 2.0 b Second Marsh 24.3 - 0.4 b 72 - 2.@

~.2 Same letters indicate means that are not significantly different (p < 0.05) as determined by Tukey's means comparison test.

sures and open t rea tments that were set in m o r e exposed e n v i r o n m e n t s s h o w e d c o m p l e t e loss o f the prop- agu le bank, and this is m o s t l ikely due to di rect w a v e effects .

The turbidi ty exc losures r educed the a m o u n t o f total suspended solids, inc reased the a m o u n t o f l ight incident upon the s ed imen t surface, and r educed the physical impac t s o f exposu re and carp. T h e y w o u l d l ike ly p rov ide a m o r e favorab le e n v i r o n m e n t for the g rowth o f submersed plants as c o m p a r e d to the carp exc lo - sures.

P r ev ious s tudies h a v e s t ressed the impor t ance o f l ight as one o f the factors l imi t ing the co lon iza t ion pat terns o f s u b m e r s e d vege t a t i on (Chamber s and Ka l f f 1985, D u s h e n k o 1990, M c A l l i s t e r 1991, van D i j k et al. 1992, M c L a u g l i n 1993). S u s p e n d e d ino rgan ic ma- terial makes up the ma jo r i ty o f the total suspended mat te r at all four sites and is r espons ib le for a lmos t all par t icula te l ight a t tenuat ion (Sager 1996). S u s p e n d e d sol ids w e r e r e spons ib le for inh ib i t ing the l ight envi - r onm en t and b u r y i n g the t ransplanted p ropagu le trays. The sha l low m e a n depths and gent le s lopes o f these marshes m a k e them suscept ib le to r e suspens ion o f bot- t o m sed iments f r o m w i n d and w a v e activity. H a k a n s o n and Jansson (1983) p red ic ted the area o f a lake in-

Table 4. Mean values of TSS, chlorophyll a, mean light extinction coefficients (EpAR), and percent of surface light on bottom sediments (%Io) for treatments at each site (± SE). Mean represents the mean value calculated for all sites combined. (T = turbidity exclosure; C = carp exclosure; GK = Grindstone Creek; HI = Hickory Island; PB = Port Britain; SM = Second Marsh).

GK HI PB SM Mean (n = 20) (n = 20) (n = 24) (n = 24) (n = 88)

TSS-C (rag.1 -~) 34.8 ± 2.6 53.3 ± 3.6 27.1 ± 1.7 35.1 + 4.4 37.0 + 1.9 TSS-T (mg.1 ~) 16.4 ± 2.1 26.0 - 1.5 9.5 ± 0.8 22.4 - 2.3 18.2 - 1.1 Chl.a-C (mg.1 t) 29.6 ± 2.6 76.6 - 5.3 22.1 ± 2.2 56.6 ± 4.8 45.6 - 3.0 Chl.a-T (mg-1-1) 19.7 - 3.7 37.4 - 4.5 12.4 ± 1.4 42.7 ± 4.7 28.2 + 2.3 EeAR-T 3.7 -+ 0.3 4.6 ± 0.2 2.2 ± 0. I 3.8 ± 0.2 3.5 ± 0.1 EpAR-C 5.6 ± 0.4 6.9 ± 0.3 4.0 ± 0.3 5.4 - 0.4 5.4 -+ 0.2 %Io-T 6.8 ± 0.2 3.4 ± 0.7 23.3 ± 2.0 8.3 ± 1.1 11.1 ± 1.1 %Io-C 2.5 ± 0.7 0.5 ± 0.1 7.6 - 1.4 4.1 ± 0.9 3.9 ± 0.6


,00o[ 7 A E .~ 800 0 0

- C 6oo

60 t- 400 "0

o 200 t -

1% 5% 10% 30%

% i ° (based upon s u m m e r average)

20 B a

a

E

cO 60

I0 E ab 0

-

_Q

0 < b

0 I ---T'-" 7 1% 5% 10% 30%

% Io (based upon summer average)

Figure 3. A) Shoot density results from the critical light experiment. Error bars represent standard errors (n = 4). ("To- tals" represent angiosperm densities plus densities of Chara; "Angiosperms" represents shoot densities of only submersed angiosperms.) B) Results of AGB from critical light experiment. Error bars represent standard errors (n = 4). For both figures, the same letters indicate means that are not significantly different (p < 0.05) as determined by Tukey's means comparison test.

vo lved with the eros ion and transport of sed iments using m e a n depth, fetch, and total area of open water. The i r mode l predicts that, a s suming a m e a n depth of 0 .6 -0 .7 m, 6 2 - 7 2 % of the area of Cootes Paradise , 2 0 - 2 2 % of Port Bri tain, and 4 1 - 4 7 % of Second Marsh exper ience chronic erosion of bo t tom sediments . This ca lcu la t ion was no t carried out for Gr inds tone Creek as it was assumed that the major i ty of the suspended mater ia l s or ig inated f rom the creek watershed. Trays p laced in carp exclosures at Gr inds tone Creek did no t demons t ra te any signs of sed imenta t ion compared to trays f rom some of the exclosures at other sites, even though TSS concent ra t ions were comparab le to the other sites. This suggests that a sui table e n v i r o n m e n t

Table 6. Results of least squares regression of above ground biomass (AGB) and shoot density with sediment photosynthetically active radiation (PAR) for both the field study (all sites pooled) and the critical light experiment. All logarithmic transformations are base 10.

Independent Dependent Variable Variable Equation r 2 p

Log(PAR) Log(Density) Critical Light

Experiment y = 0.33x + 2.04 0.50 0.002 Field Study y = 1.35x - 0.4 0.69 0.001

Log(AGB) Critical Light

Experiment y = 0.89x - 0.95 0.73 0.001 Field Study y = 1.93x - 2.66 0.65 0.001

* n = 16 for the cri t ical l ight ex p e r imen t ; n = 3 2 for the field study.

did not exist for mater ia l to settle out of the water and is l ikely related to the fact that this site was in f luenced by the flow of Gr inds tone Creek.

S i n c e all of the sites were i ncuba t ed with the same propagule bank, all p ropagules were exposed to the same sed iment condi t ions prior to t ransplant ing . Tem- peratures were s l ight ly different , bu t all fell wi th in the o p t i m u m range for growth of submersed plants. That leaves the a m o u n t of l ight and site exposure as be ing the on ly measu red di f ferences be t w e e n sites and treat- men t s that migh t be cont ro l l ing the di f ferent rates of germina t ion . The fact that no t even Chara was able to grow, g iven its a b u n d a n c e in the p ropagu le b a n k and abi l i ty to grow under lower l ight levels than mos t rooted plants , also suggests that l ight is no t the on ly l im- i t ing var iable in the field. It is h igh ly un l ike ly that

Table 7. Results of least squares regression of above ground biomass (AGB) and shoot density with sediment photosynthetically active radiation (PAR) for each site. Both treatments are pooled for each site. All logarithmic transformations are base 10 (n = 8).

Independent Dependent Variable Variable Equation r 2 p

Log(PAR) Grindstone Creek Hickory Island Port Britain Second Marsh

Grindstone Creek Hickory Island Port Britain Second Marsh

Log(Density) y = -2 .91 + 3.03x 0.84 0.001 y = -0 .76 + 1.74x 0.73 0.007 y = -1 .01 + 1.49x 0.90 0.00t y = 0.25 + 0.97x 0.59 0.045

Log(AGB) y = - 4 .70 + 3,23x 0.64 0.02 y = -3 .37 + 2,54x 0.53 0.15 y -- -1 .35 + 1.29x 0.60 0.025 y = -1 .02 + 0.99x 0.33 0.24

264 W E T L A N D S , Volume 18, No. 2, 1998

nutrient limitation could be a factor as all trays were incubated with the same propagule bank. Thus, exposure and sedimentation could be additional stressors.

The amount of suspended matter appeared to be directly related to the exposure of each site, with the two more exposed sites, Second Marsh and Hickory Island, having higher concentrations of TSS. With little or no natural vegetation present to reduce resuspension, wind could readily stir up bottom sediments. The cal- culation of the area involved in lake sediment erosion also shows that at Second Marsh and Cootes Paradise, nearly 50% or more of total marsh area is contributing to sediment erosion processes. For recovery of submersed vegetation to occur at these sites, the issue of exposure must be addressed. This value is only 25% for the smaller Port Britain marsh where resuspension due to carp and tributary loading likely contributes more to the sediment load.

Critical Light Experiment

van Dijk and van Vierssen (1991) report a light compensation point of approximately 40 txE.m 2.sec 1 for P. pectinatus, and Sand-Jensen and Madsen (1991) report a range of 2-5 txE-m 2-sec 1 for most macroalgae. In the critical light experiment of this study, angiosperm shoot densities reached levels of almost 200 shoots.m ~- in both the 1% and 5% treatments, but total AGB for these two treatments was very low (<5 g.m 2). Therefore, any additional stresses imposed upon these plants would be detrimental. These shoot densities could be explained by initial growth of these plants being supported by starch reserves in the propagule or seed (Wetzel 1983). The field experiment produced % Io values ranging between 0.2 and 28.0%, with plants being present in treatments that received as little as 1.8% Io. Turbidity exclosures receiving a mean of less than 5 % Io generally had very low total AGB values relative to total densities. In the few ex- ceptions (replicates at Grindstone Creek, Hickory Is- land, and Second Marsh) where AGB was fairly sub- stantial, this was due to the fact that these treatments were colonized by P. pectinatus, which is often able to survive in turbid waters by creating a dense floating canopy at the surface of the water (van Dijk et al, 1992).

In summary, there was very little growth in the carp exclosures, which suggests that large carp alone are not directly preventing recovery of these plants. Rath- er, the results from the turbidity exclosures showed that if turbidity and exposure were reduced (and fish are excluded) then a favorable environment for plant reestablishment could be created.

The turbidity exclosures worked with varying de- grees of success, and this was dependent upon the total

exposure of a site. Turbidity exclosures at Second Marsh and Hickory Island had the poorest success in terms of germination and survival of plants from the germination trays. Historically, both of these sites had significant stands of emergent vegetation in areas that are presently open water. Such stands would have bro- ken up the wave energy associated with strong winds and protected submersed plants from becoming dis- lodged. They would have also increased water clarity by allowing suspended materials to settle out (Foote and Kadlec 1988).

Present conditions at all four sites are such that suspended materials are blocking necessary light and, at all sites except Grindstone Creek, are also settling out of the water column and burying plants and propagule banks. The suspended sediments could be the result o f carp or other animal activity and/or wind and wave activity, The high energy sites had very high levels of suspended solids, and the introduced propagule bank was eroded away. Thus, both turbidity and exposure would have to be addressed for macrophyte recovery. On the other hand, the more protected sites (Port Brit- ain and Grindstone Creek) provided a more hospitable environment for growth. High levels of turbidity still created poor light environments, but turbidity exclosures were able to remedy this problem.

A C K N O W L E D G M E N T S

Funding is acknowledged from the Great Lakes University Research Fund through Trent University and from the Tri-Council Eco-Research Grant through McMaster University, Hamilton, Ontario. The authors also recognize L. Simser of the Royal Botanical Gar- dens of Hamilton, Ontario, S. Millard of the Depart- ment of Fisheries and Oceans at Burlington, Ontario, P. E. Sager and H. J. Harris of the University of Wis- consin-Green Bay, and E. Nol and S. Symington of Trent University for technical and personal assistance in the field.

L ITERATURE CITED

Barko, J. W., D. G. Hardin, and M. S. Mathews. 1982. Growth and morphology of submersed freshwater macrophytes in relation to light and temperature. Canadian Journal of Botany 60:877 887.

Barko, J. W., D. Gunnison, and S. R. Carpenter. 1991. Sediment interaction with submersed macrophyte growth and community dynamics. Aquatic Botany 41:41 65.

Carter, V. and N. B. Rybicki. 1985. The effects of grazers and light penetration on the survival and transplants of Vallisneria americana Michx. in the tidal Potomac River, Maryland. Aquatic Bot- any 23:197-213.

Chambers, P A. and J. Kalff. 1985. Depth distribution and biomass of submersed aquatic macrophyte communities in relation to Sec- chi depth. Canadian Journal of Fisheries and Aquatic Sciences 42: 701-709.

Crivelli, A. J. 1983. The destruction of aquatic vegetation by carp:

S a g e r et al., M A C R O P H Y T E R E C O V E R Y I N C O A S T A L M A R S H E S 2 6 5

A comparison between southern France and the United States. Hydrobiologia 106:37 41.

Dieter, C, D., C. R. Berry Jr., and B, Kolterman. 1991. Fish enclosures for research in marshes.Wetlands l 1 : 173-177.

Dushenko, W. T 1990. Physical and chemical factors affecting submerged aquatic macrophytes in the Bay of Quinte, Lake Ontario. Ph.D. Thesis. Queens University, Kingston, ON, Canada.

Dushenko, W. T., A. Crowder, and B. Cameron. 1990. Revegetation in the Bay of Quinte, Lake Ontario: Preliminary lab and field experiments, p. 245-254. In 1. Kusler and R. Smardon (eds.) Pro- ceedings of an International Symposium: Wetlands of the Great Lakes, Protection and Restoration Policies; Status of the Science. Niagara Falls, NY, USA.

Environment Canada. 1979. Analytical methods manual. Inland Wa- ters Directorate, Water Quality Branch, Ottawa, ON, Canada.

Foote, A. L. and J. A. Kadlec. 1988. Effects of wave energy on plant establishment in shallow lacustrine wetlands. Journal of Freshwater Ecology 4:523 532.

Goldsborough, W. J. and W. M. Kemp. 1988. Light responses of a submersed macrophyte: Implications for survival in turbid tidal waters. Ecology 69:1756-1786.

Haag, R. W. 1983. Emergence of seedlings of aquatic macrophytes from lake sediments. Canadian Journal of Botany 61 : 148-156.

Hakanson, L. and M. Jansson. 1983. Principles of Lake Sedimen- tology. Springer-Verlag. New York, NY, USA.

Kahl, R. 1993. Aquatic macrophyte ecology in the Upper Winne- bago pool lakes, Wisconsin. Department of Natural Resources, Madison, WI, USA. Tech. Bull. No. 182.

King, D. R. and G. S. Hunt. 1967. Effect of carp on vegetation in a Lake Erie marsh. Journal of Wildlife Management 31:181-188.

Lodge, D. M. 1991. Herbivory on freshwater macrophytes, Aquatic Botany 41 : 195-224.

Loftus, K_ K_, R. B. Sayer, R. E. Elliot, A. A. Crowder, and J. M. Casselman. i992. A geographic information system based model of aquatic vegetation and piscivore habitat in the Bay of Quinte. Bay of Quinte Remedial Action Plan, Kingston, ON, Canada. Tech. Report No. 14.

Macrae, D. A. 1979. The impact of carp on the summer production of aquatic vegetation as indicated by an enclosure experiment and food habitats study. M.S. Thesis. Trent University, Peterborongh, ON, Canada.

McAllister, L. S. 1991. Factors influencing the distribution of submersed macrophytes in Green Bay, Lake Michigan: A focus on light attenuation and Vallisneria americana. M.S. Thesis. Unive~ sity of Wisconsin Green Bay. Green Bay, WI, USA.

McLauglin, A. 1993. Submergent wetland distribution in the Bay of Quinte: A link to abiotic factors and emergent wetland distribution. M.S. Thesis. Queen's University, Kingston, ON, Canada.

O'Dacre, T. 1992. An exclosure experiment to determine the effect of carp, Cyprinus carpio, on submersed macrophytes in Lake On- tario shoreline marshes. Honors Thesis. Trent University, Peter- borough, ON, Canada.

Painter, D. S. 1990. Establishing habitat goals and response in an area of concern using a geographical information system, p. 36- 42. In J. Kusler and R. Smardon (eds.) Proceedings of an Inter- national Symposium: Wetlands of the Great Lakes, Protection and Restoration Policies; Status of the Science. Niagara Falls, NY, USA.

Sager, E. R S. 1995. Factors influencing the recovery of submersed vegetation in four Lake Ontario coastal marshes. M.S. Thesis. Trent University, Peterborough, ON, Canada.

Sager, E. R S. 1996. Factors influencing the light environment in Cootes Paradise, Hamilton Harbour, and other coastal marshes of Lake Ontario. Water Quality Research Journal of Canada 31:553 575.

Sand-Jensen, K. and J. Borum. 199l. Interactions among phyto- plankton, periphyton, and macrophytes in temperate freshwater estuaries. Aquatic Botany 41 : 137-175.

Sand-Jensen, K. and T. V. Madsen. 1991. Minimum light require- ments of submerged freshwater macrophytes in laboratory growth experiments. Journal of Ecology 79:749-764.

Spence, D. H. N. 1982. The zonation of plants in freshwater lakes, p. 37-125. In A. MacFayden and E. D. Fords (eds.) Advances in Ecological Research, Vol. 12. Academic Press, New York, NY, USA.

van Dijk, G. M.. A. W. Breukelaar, and R. Gijlstra, 1992. Impact of light climate history on seasonal dynamics of a field population of Potamogeton pectinatus L. during a three year period (1986- 1988). Aquatic Botany 43:17-41.

van Dijk, G. M. and W. van Vierssen. 1991. Survival of a Pota- mogeton pectinatus L, population under various light conditions in a shallow eutrophic lake (Lake Veluwe) in The Netherlands, Aquatic Botany 39:121 129.

Westcott, K., T H. Whillans, and M. G. Fox. 1997, Viability and abundance of seeds of submerged macrophytes in the sediment of disturbed and reference shoreline marshes in Lake Ontario. Ca- nadian Journal of Botany 75:451-456.

Wetzel, R. G. and G. E. Likens. 1979_ Limnological Analyses. W. B. Saunders Co., Philadelphia, PA, USA.

Wetzcl, R. G. 1983. Limnology; Second Edition. Saunders College Publishing, New York, NY, USA.

Zar, J. H. 1974. Biostatistical Analysis; Second Edition. Prentice- Hall, Englewood Cliffs, NJ, USA,

Manuscript received 30 January 1997; revisions received 29 Sep- tember 1997 and 17 February 1998; accepted 3 March 1998.

factors influencing the recovery of submersed macrophytes in four coastal marshes of lake ontario

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