Effects of hydrilla on water quality in Lake Seminole

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Hq Effects of Hydrilla on Water Quality in Lake Seminole
by David Partridge
Georgia Department ofNatural Resources Wildlife Resources Division Social Circle, Georgia
March 1996 This study was funded in part by the Federal Aid in Sport Fish Restoration program under Grant F-28, Georgia

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Figure 4. Dissolved oxygen (mean 1 SE ) on the surface in hydrilla beds (. ) and open water habitats (. ) during 1994 and 1995 in Lake Seminole, Georgia. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes minimum desirable dissolved oxygen for warmwater fishes (Swingle 1969).
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Figure 5. Dissolved oxygen (mean 1 SE) at 0.5-m depth in hydrilla beds (. ) and open water habitats (. ) during 1994 and 1995 in Lake Seminole, Georgia. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes minimum desirable dissolved oxygen for warmwater fishes (Swingle 1969).
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Figure 6. Dissolved oxygen (mean 1 SE) at 1-m depth in hydrilla beds (. ) and open water habitats (. ) during 1994 and 1995 in Lake Seminole, Georgia. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes minimum desirable dissolved oxygen for warmwater fishes (Swingle 1969).
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Figure 7. Dissolved oxygen (mean 1 SE) at 1.5-m depth in hydrilla beds (. ) and open water habitats (.) during 1994 and 1995 in Lake Seminole, Georgia. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes minimum desirable dissolved oxygen for warmwater fishes (Swingle 1969).
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appeared to have lower DO concentrations than the Spring Creek area during 1995 (Figs. 5, 6, and 7).
Mean depth (m) in hydrilla beds (1.6 0.07, N = 51) was less than Ce. :0; 0.01) mean depth
in open water habitats (4.3 0.26, N = 51; :0; 0.01). Dissolved oxygen concentrations on the bottom ofhydrilla beds (2.5 0.36, N = 51) were lower than (p:o; 0.05) on the bottom of open
water habitats (4.9 0.37, N = 51; :0; 0.05). Overall, mean water temperature was similar
between hydrilla beds (24.5 0.21, N = 216) and open water (24.5 0.16, N = 469). However, mean water temperatures were higher (:0; 0.01) in 1995 than in 1994 for both habitats (Table 1). Mean DO was negatively correlated with mean water temperature (r = -0.68, :0; 0.01).
DISCUSSION There was no indication that hydrilla affected water temperature or surface pH; however, the results of this study do suggest that hydrilla reduced DO to concentrations that could potentially have negative impacts on sport fish populations in Lake Seminole. Much has been written about the dissolved oxygen requirements offish (Boyd 1990). The minimum DO concentration at which largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochrius) can survive for 24 hours is around 1.0 mg/I (Moss and Scott 1961; Swingle 1969). Dissolved oxygen concentrations measured in hydrilla beds during this study were usually above lethal levels; however, DO concentrations below 5 mg/I, which were common in hydrilla beds, can have wide ranging impacts on fish.
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Table 1. Water temperature COC, 1 SE) in hydrilla and open water habitats during 1994 and
1995 on Lake Seminole.

Habitat

1994 Mean water temp

1995 Mean water temp

Hydrilla Open

23.3 0.30, N = 90 22.7 0.21, N = 191

25.4 0.25, N = 126
25.8 0.21, N = 278

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Swingle (1969) suggested that DO concentrations below 5 mg/I are undesirable for warmwater fisheries. Growth and food conversion efficiency of largemouth bass, yellow perch (Perca flavescens) and channel catfish (Ictalurus punctatus) have been shown to be reduced when DO concentrations are lower than 4 mg/I (Stewart et al. 1967; Carlson et aI.1980). Dissolved oxygen concentrations at depths ofO.5-m and deeper in hydrilla beds were commonly lower than 4.0 mg/I during summer. In addition, wide diurnal fluctuations in DO concentrations, which can occur in dense vegetation beds (Frodge et al. 1990), can reduce fish growth (Stewart et al. 1967). Previous research has shown that fish can detect and will avoid areas oflow DO concentration (Davis 1975). Largemouth bass have shown strong avoidance of concentrations near I. 5 mg/I (Cross et al. 1987; Whitmore et al. 1960), and some avoidance at levels up to 4.5 mg/I (Whitmore et aI.1960). Bluegill have shown avoidance of DO concentrations less than 3 mg/I (Whitmore et al. 1960). Dissolved oxygen concentrations at the Flint River stations during 1995 were commonly at or below levels which have been shown to result in fish avoidance behavior. Avoidance of these areas during periods oflow DO could result in the loss of much available habitat, and may also concentrate adult largemouth bass in areas with suitable DO concentrations, possibly making them more vulnerable to angling, and increasing their susceptibility to disease.
Low DO concentrations can also affect fish spawning and early life stages. Survival of larval and embryonic stages oflargemouth bass are not affected at DO concentrations of 3 mg/I and higher, while survival at concentrations of2 mg/I is near zero (McDaniel 1993). Larval and embryonic stages in bluegill have been shown to survive 4-hour exposure to 1.8 mg/I and 0.5 mg/I, respectively (McDaniel 1993). There is some evidence that DO concentrations in hydrilla
14

beds in Lake Seminole at least approach concentrations that have been shown to affect survival of larval and embryonic stages of fish.
Hydrilla has been shown to reduce DO concentrations to lower levels than native, or other exotic species of vegetation (Honnell et al. 1993). The suggested mechanism by which hydrilla reduces DO is the ability of this vegetation to form dense canopies which prevent the exchange of oxygen at the air-water interface (Honnell et al. 1993). In addition, decomposition of aquatic macrophytes during periods of senescence can severely reduce DO concentrations (Boyd 1990).
DO concentrations were measured during the early morning hours, at a time which would likely reflect the greatest impact ofhydrilla. Therefore, the results of this study provide little insight concerning the impact ofhydrilla on DO concentrations during other times of the day. Photosynthetic rates exceed respiration during daylight hours, and as a result DO concentrations can be high in the upper layers of submersed plant beds. However, due to extensive surface canopies, underlying shaded portions can have low DO concentrations even during these periods of intense photosynthetic activity (Buscemi 1958; Frodge et al. 1990). There appeared to be some evidence that the impact ofhydrilla on DO concentrations exhibited both spatial (Spring Creek versus Flint River), and temporal (1994 versus 1995) variation. The significant negative correlation between DO concentration and water temperature suggest that the variation in DO between years may be related to the significantly higher water temperatures in 1995. High water flow as a result of a 500-year flood event which occurred on the Flint River during July 1994 may have contributed to this relationship. Regardless, this variation makes it difficult to generalize about the impact ofhydrilla on DO concentrations from year to year, or on a lake-wide basis.
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There is evidence that hydrilla in Lake Seminole can lower DO concentrations to levels that can adversely impact growth, reduce available habitat, and influence survival of larval and embryonic fish stages. However, our sampling was limited to two relatively small areas of the lake during early morning hours. It is therefore difficult to draw conclusions as to the extent that hydrilla influences DO concentrations and, subsequently fish populations, on a lakewide scale.
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LITERATURE CITED
Andrews, 1. W., T. Murai, and G. Gibbons. 1973. The influence of dissolved oxygen on the growth of channel catfish. Transactions of the American Fisheries Society 102:835-838.
Boyd, C. E. 1990. Water quality in ponds for aquaculture. Birmingham Publishing Company, Birmingham, Alabama.
Brungs, W. A. 1971. Chronic effects oflow dissolved oxygen concentrations on the fathead minnow (Pimephales promelas). Journal of the Fisheries Research Board of Canada 28:1119-1123.
Buscemi, P. A. 1958. Littoral oxygen depletion produced by a cover of Elodea canadensis. Oikos 9:239-245.
Carlson, A. R., 1. Blocker, and L. 1. Herman. 1980. Growth and survival of channel catfish and yellow perch exposed to lowered constant and diurnally fluctuating dissolved oxygen concentrations. Progressive Fish-Culturist 42:73-78.
Carpenter, S., and D. M. Lodge. 1986. Effects of submersed macrophytes on ecosystem processes. Aquatic Botany 26:341-370.
Cross, R. E., R. L. Eisenhauer, and S. E. Brown. 1987. Biological effects of water level fluctuations in the upper reaches of the St. Johns River. Florida Game and Freshwater Fish Commission, Final Report D-J project F-33, Tallahassee.
Davis, 1. C. 1975. Minimal dissolved oxygen requirements of aquatic life with emphasis on Canadian species: A review. Journal of the Fisheries Research Board of Canada 32:22952332.
Engel, S. 1985. Aquatic community interactions of submerged macrophytes. Wisconsin Department of Natural Resources, Technical Bulletin Number 156, Madison.
Frodge, 1. D., G. L. Thomas, and 1. B. Pauley. 1990. Effects of canopy formation by floating and submersed aquatic macrophytes on the water quality of two shallow Pacific Northwest lakes. Aqu~tic Botany 38:231-248.
Gholson, Jr., A. K. 1984. History of aquatic weeds in Lake Seminole. Aquatics 6:21-22.
Honnell, D. R., 1. D. Madsen, and R. M. Smart. 1992. Effects of selected plants on water quality in pond ecosystems. Pages 30-34 in Proceedings, 26th Annual Meeting Aquatic Plant Control Research Program, Miscellaneous paper A-92-2. U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Mississippi.
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The Georgia Department of Natural Resources receives Federal Aid in Sport Fish and Wildlife Restoration. Under Title VI of the 1964 civil Rights Act, section 504 of the Rehabilitation Act of 1973, the Age Discrimination Act of 1975, and Title IX of the Education Amendments of 1972, the U. S. Department of the Interior prohibits discrimination on the basis of race, color, national origin, age, sex or disability. If you believe that you have been discriminated against in any program, activity or facility as described above, or if you desire further information please write to:
The Office for Human Resources U.S. Fish and Wildlife Service U.S. Department of the Interior Washington, D.C. 20240

Honnell, D. R., 1. D. Madsen, and R. M. Smart. 1993. Effects of selected exotic and native aquatic plant communities on water temperature and dissolved oxygen. Pages 1-6 in Technical Notes for the Aquatic Plant Research Program, Miscellaneous paper A-93-2. U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Mississippi.
McDaniel, M. D. 1993. Point-source discharge. Pages 1-56 in C. F. Bryan and D. A Rutherford, editors. Impacts ofwarrnwater streams: guidelines for evaluation. Southern Division, American Fisheries Society, Little Rock, Arkansas.
Moss, D. D., and D. C. Scott. 1961. Dissolved oxygen requirements of three species offish. Transactions of the American Fisheries Society 90:377-393.
Plumb, 1. A, 1. M. Grizzle, and 1. Defigueiredo. 1976. Necrosis and bacterial infection in channel catfish (IctaJurus punctatus) following hypoxia. Journal of Wildlife Diseases 12:247-253.
Stewart, N. E., D. L. Shumway, and P. Doudoroff. 1967. Influence of oxygen concentration on growth ofjuvenile largemouth bass. Journal of the Fisheries Research Board of Canada 24:475-494.
Swingle, H. S. 1969. Methods for analysis of waters, organic matter, and pond bottom soils used in fisheries research. Auburn University, Alabama.
Whitmore, C. M., C. E. Warren, and P. Douderoff. 1960. Avoidance reactions ofsalmonid and centrarchid fishes to low oxygen concentrations. Transactions of the American Fisheries Society 89: 17-26.
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FINAL REPORT

State: Georgia

Project Number: F-28

Project Type: Research

Grant Title: Southwest Region Fisheries Investigations

Study XXXVI Title: Effects of Hydrilla on Water Quality in Lake Seminole.

Period Covered: 1 June 1994 to 1 July 1996

Study Objectives: To determine ifhydrilla growth in Lake Seminole has negatively impacted water quality.

ABSTRACT Water quality parameters were monitored monthly during 1994 and 1995 in hydriIla (HydriIla verticillata) beds and in open water areas of Lake Seminole to determine if extensive hydrilla beds were negatively impacting water quality. During both years, pH was similar between habitat types and never fell outside of the range that is considered optimal for sport fish production. Water temperatures were also similar between habitat types. Dissolved oxygen (DO) was consistently lower in hydrilla beds than in open water areas, and was commonly below the suggested minimal level for warmwater fishes of 5 mg/1. Mean DO on the bottom ofhydrilla beds (2.5 mg/I) was significantly less than on the bottom of open water areas (4.9 mg/I). This study suggests that hydrilla may be reducing DO to levels that negatively impact sport fish populations in Lake Seminole.

INTRODUCTION Lake Seminole is a 15,100 hectare U.S. Army Corps of Engineers impoundment located at the confluence of the Flint and Chattahoochee Rivers in southwest Georgia. The clear, shallow (mean depth of3 meters), and nutrient-laden waters ofLake Seminole coupled with mild temperatures and a long growing season, provide an ideal habitat for the growth of aquatic vegetation (Gholson 1984). The reservoir's main aquatic macrophyte is the exotic hydrilla (Hydrilla verticillata) which was first discovered in the reservoir in 1967. Hydrilla spread rapidly, increasing from 0.40 hectares in 1967 to a peak of 9,660 hectares in 1992. Since peaking in 1992, hydrilla has decreased, with an estimated 7,330 hectares ofhydrilla in 1995 (D. Morgan, U. S. Army Corps of Engineers, personal communication). The proliferation ofhydrilla has raised concern that this aquatic plant may be adversely impacting water quality in Lake Seminole. Aquatic plants can alter limnological processes in aquatic ecosystems (Engel 1985; Carpenter and Lodge 1986). Extensive aquatic plant populations can reduce dissolved oxygen (DO) levels below those which are desirable for warmwater fisheries (Honnell et al. 1992; Honnell et al. 1993). While extremely low DO can directly result in fish death (Moss and Scott 1961), prolonged exposure to levels above those which directly result in death can have wide ranging effects on fish. Prolonged exposure to low DO can reduce growth (Stewart et al. 1967; Andrews et al. 1973), decrease survival of fry (Brungs 1971), and cause bacterial infections in fish (Plumb et al. 1976). Although not as common a problem as dissolved oxygen, extreme fluctuations in pH associated with aquatic plant beds have the potential to create problems for fish. During periods of intense photosynthetic activity, aquatic plants can cause large diel fluctuations in pH (Boyd 1990; Honnell et al. 1992). Low pH has been suggested to result in slow fish growth
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(Swingle 1969), whereas high pH is suspected to reduce survival offish fry (Boyd 1990). The purpose of this study was to detennine if hydrilla has negatively impacted water
quality in Lake Seminole, and to draw inferences, based on the results of this study and previous research, regarding the potential impacts on fish populations.
METHODS Water quality data were collected monthly (June through October) during 1994 and 1995 at six fixed stations on Lake Seminole. Three stations were sampled on both the Flint River and Spring Creek arms (Fig. 1). At each station, dissolved oxygen and water temperature (YSI model SIB) profiles were recorded at the surface and at 0.5 m intervals, and pH (Fisher scientific pH pen or Hach model FFIA test kit) was recorded on the surface. At each station, water quality parameters were recorded within hydrilla beds, and in an open water area immediately adjacent to hydrilla beds. All samples were collected as close to sunrise as possible (range 0630 hours to 1130 hours). For each reservoir arm, stations were combined to compute mean DO and pH values for each habitat type, sample date, and depth combination. Mean water temperature was computed for each habitat and year combination. Analysis of variance (ANOVA) was used to test for differences in mean water quality parameters between hydrilla beds and open water areas. For DO and pH, discrete comparisons were made for the Spring Creek and Flint River arms for each sample date and depth.
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Chattahoochee River

Spring Creek

Figure 1. Map of Lake Seminole, showing water quality sampling stations. 4

RESULTS Surface pH values were similar in hydrilla and open water habitats during both years (Fig. 2). In addition, mean pH values in both habitats never fell outside of those values which are considered optimal for fish production (Swingle 1969). However, DO did exhibit differences between habitats. In both the Flint River and Spring Creek areas, mean DO was lower at all depths in hydrilla than in open water habitats (Fig. 3). For both years combined, mean DO in hydrilla beds fell below 5 mg/I at 1-m, and 1.5-m in the Flint River and Spring Creek stations, respectively. In open water, mean DO was greater than 5 mg/I for all depths less than 5.5-m. Examination of DO concentrations by depth, within the same station and date, revealed some differences in DO between habitats. Surface DO concentrations were seldom below 5 mg/I in either habitat; however, surface DO was significantly lower in hydriIla beds on six sample dates (Fig. 4). For all remaining sample dates and depths, mean DO concentrations were lower in hydrilla than in the open water habitat, and in many cases mean DO concentrations were significantly lower in hydrilla beds (Figs. 5, 6, and 7). For depths up to 1.5-m, mean DO never fell below 5 mg/I in open water. By comparison, mean DO in hydrilla beds was less than 5 mg/I for 34% of the dates and depths sampled, and was always at or below 5 mg/I from late-July through September at depths greater than O.5-m. Although comparisons between areas (i.e., Flint River and Spring Creek) and years were not subjected to statistical analysis, visual comparisons suggest that the Flint River area had lower DO concentrations during 1995 when compared to 1994. In addition, the Flint River area
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Figure 2. pH (mean 1 SE) on the surface in hydrilla beds (. ) and open water areas (. ) of Lake Seminole, Georgia during 1994 and 1995. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes optimal range for fish production (Swingle 1969).

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Figure 3. Dissolved oxygen (mean 1 SE ) versus depth in hydrilla beds (. ) and open water habitats (. ) during 1994 and 1995 in Lake Seminole, Georgia. Abbreviations are: SC=Spring Creek, FR= Flint River. Dashed line denotes minimum desirable dissolved oxygen for warmwater fishes (Swingle 1969).

7