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Effects of Selected Exotic and Native Aquatic Plant Communities on Water Temperature and Dissolved Oxygen
Introduction
Many studies have examined the distribution of aquatic plants in relation to water chemistry parameters or, in essence, the influence of water environment on the growth and metabolism of aquatic plants. Among these are the studies of Moyle (1945), Seddon (1972), Pip (1979), Hellquist (1980), and Kadono (1982). However, fewer studies have examined the influence that aquatic plants exert on their water environment. Plants interact with water through dynamic processes such as chemical uptake and excretion, physical processes such as shading and reduced circulation, and ecosystem processes such as organic matter production and decomposition (Carpenter and Lodge 1986). Perhaps one of the reasons for the relative lack of studies is the difficulty in differentiating between the natural geochemical variability among lakes and the biological/chemical effects of different aquatic plant species on their environment.
The ponds located at the Lewisville Aquatic Ecosystem Research Facility (LAERF) in Lewisville, Texas (Photo 1), provided a unique opportunity to examine the impacts of different aquatic plant species on water chemistry. An earlier study (Honnell, Madsen, and Smart 1992) examined the effect of aquatic plants on four specific water quality parameters (temperature, dissolved oxygen (DO), pH, and conductivity) using ponds filled with lake water of initially similar chemistry. That earlier study has since been expanded to include additional water chemistry parameters over an extended period. The information on DO and temperature presented in this article was collected as part of the extended investigation.
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| Photo 1 |
Methods
Study Ponds
Vegetation types selected for this study included monocultures of three exotic species, a monoculture of a native species, and a mixed native plant community. The exotics included the free-floating aquatic plant waterhyacinth (
Eichhornia crassipes), and two submersed species, hydrilla ( Hydrilla verticillata), and Eurasian watermilfoil (Myriophyllum spicatum). The native monoculture was the floating-leaved species American pondweed (Potamogeton nodosus), and the mixed native plant community included American pondweed, southern naiad (Najas guadalupensis), coontail (
Ceratophyllum demersum), and muskgrass (Chara vulgaris).
The surface area of each study pond was approximately 0.75 acre (3,000 square meters), with an average depth of approximately 1 meter. The ponds are supplied with water from Lewisville Lake, an adjacent Corps of Engineers reservoir. The ponds can be filled and drained independently, allowing for control of the hydrologic regime. Each study pond contained a pier to provide access to the deepest portion of the pond, which facilitated sampling and observation. The piers were located away from the inlet/outlet structure to minimize the influence of incoming water. Each pond was also equipped with a permanent staff gauge for recording water levels.
Sampling
Ponds were monitored once every 2 weeks (bi-weekly), at approximately the same time of day each trip, beginning in March 1991 and ending in December 1992. Temperature and DO of the surface waters were measured in situ during each sampling trip using a calibrated Hydrolab Surveyor II (Photo 2).
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| Photo 2 |
In addition to the routine biweekly monitoring, temperature and DO were monitored on a semicontinuous basis during the summer months of 1991 and 1992. This monitoring was accomplished with calibrated Hydrolab datasondes. The datasondes were anchored at the end of the piers, approximately 5 meters from the shoreline, and were positioned so that the sensors were approximately 20 centimeters below the surface of the water. Data were logged hourly for 1-week intervals throughout the growing season. Only datasonde data for the period May through October 1991 and 1992 were used.
Data analysis was performed using parametric analysis of variance and nonparametric Kruskal-Wallis statistical tests. The statistics were computed for the minimum, maximum, average, and range for each 24-hour day of recorded information for each pond type, and on biweekly data from routine chemical monitoring.
Results and Discussion
Averaged over the growing season, surface (20-centimeter depth) water temperature did not differ among the ponds. The growing season mean daily average temperature of the ponds was approximately 25o C (Figure 1
). During the winter months, essentially no differences in temperatures occurred among the ponds. However, during the summer months, as plant canopies developed, the ponds began to show differences in temperatures. During the period from mid-June to the end of August 1992, the native mixed pond, which did not develop a surface canopy, had the highest temperatures (Figure 2). The pondweed and waterhyacinth ponds, both of which developed extensive canopies of leaves at the water surface, exhibited temperatures that were cooler than the other ponds. This was evident during the summer months of both 1991 and 1992.
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| Figure 1 |
Figure 2 |
Because of their shallow depth and frequent mixing by wind, the LAERF ponds' temperatures tended to track average air temperatures fairly closely (Smart, Snow, and Dick 1992). In larger water bodies in which water temperatures lag behind average daily temperatures, vegetation may more profoundly alter water temperature by reducing water mixing and allowing warming of surface water (Juget and Rostan 1973, Seki, Takahashi, and Ichimura 1979). Both floating-leaved (Juget and Rostan 1979) and submersed species (Dale and Gillespie 1977) can create significant temperature gradients in which surface water is warmed several degrees above open-water temperatures. Bottom water temperatures change little over the day (Dale and Gillespie 1977).
Mean daily DO concentrations in the hydrilla and waterhyacinth ponds were significantly lower than in the other ponds (
Figure 3). Dissolved oxygen concentrations were highest in the native community ponds. Mean daily average DO levels in the waterhyacinth pond were below 5 milligrams per liter, which is undesirable for warmwater fisheries (Boyd 1979). Mean daily minimum DO concentrations in all but the mixed native community ponds were also below the 5-milligram per liter DO stress level. Dissolved oxygen levels in the ponds appear to be related to the surface area covered by vegetation. Extensive surface coverage restricts the exchange of oxygen across the air-water interface, resulting in levels below saturation (Figure 3).
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| Figure 3 |
A substantial population of any canopy-forming species, exotic or native, could lead to critically low levels of dissolved oxygen, especially in the early morning period. More severe problems associated with extensive aquatic plant populations can also occur during senescence and decomposition, when DO concentrations may decline to near zero.
Dense mats of floating and submersed plants have been observed to deplete oxygen, particularly at night. During the daytime, large gradients in oxygen concentration may also be observed in submersed plant populations, with high concentrations in the upper layers and low concentrations in underlying, shaded portions (Buscemi 1958, Frodge, Thomas, and Pauley 1990). The floating plant waterhyacinth restricts gas exchange the most (Reddy 1985), followed by hydrilla, pondweed, and watermilfoil, all of which formed surface mats. The mixed native community did not develop an extensive surface canopy and thus did not restrict the entry of atmospheric oxygen into the water.
Conclusion
Based on a preliminary analysis of the results of a larger investigation, dissolved oxygen is the parameter most significantly affected by the different types of plant communities. The primary influence of plant type on dissolved oxygen is associated with canopy formation. Oxygen depletion beneath plant canopies is likely due to the physical barrier that floating plants or leaves provide between the atmosphere and the water (Figure 4). This barrier reduces wind-driven water movement and impedes reaeration. In free-floating species, such as waterhyacinth, or floating-leaved species, such as American pondweed, photosynthesis contributes little oxygen to the water because gas exchange occurs between aerial leaves and the atmosphere, bypassing the water column. Decomposition of decaying plant material also exerts an oxygen demand, further reducing ambient oxygen concentrations.
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| Figure 4 |
All aquatic plants exert certain influences on the aquatic environment by reducing water flows, changing flow patterns, increasing sedimentation, and altering chemical composition of the water. Massive populations of any aquatic plant species, exotic or native, may cause diminished water quality and lowered oxygen availability. However, many exotic problem species often cause seriously degraded water quality and depleted DO levels. These exotics are often associated with water quality problems because their monoculture populations usually cover large expanses and also because they develop extensive canopies at the water surface. Both of these traits contribute to degraded water quality conditions.
In this study, the native species American pondweed also caused lowered DO levels. This species forms a surface canopy of floating leaves, and therefore can cause water quality problems similar to those associated with exotic species. While in the study pondweed covered much of the surface of the small pond, this species rarely forms extensive monospecific populations in larger bodies of water. Thus, it is unlikely that American pondweed causes significant water quality problems in most aquatic systems.
Aquatic plants are an important component of the aquatic ecosystem because they are the key interfaces between sediment, the water column, and the atmosphere in stream and lake ecosystems, controlling both productivity and biogeochemical cycles as well as structuring aquatic habitats (Carpenter and Lodge 1986). Although they are restricted in their distribution by their aquatic environment, aquatic plants in turn modify this environment by direct biological, chemical, and physical processes.
References
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