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Brines, Emily K.

Rubenstein School of Environment and Natural Resources

2004

MS

Introduction of the exotic zebra mussel, Dreissena polymorpha, has resulted in measurable decreases in phytoplankton biomass in many freshwater systems. These changes might appear to be beneficial because eutrophication is a major concern in freshwater systems; however, such a view does not take into account which algal species are being reduced or what changes in the foodweb might occur along with the intense filter feeding of zebra mussels. It has been suggested that zebra mussels may foster the development of cyanobacteria even as they drastically reduce other phytoplankton. There is also an indication that some herbivorous zooplankton may be capable of preventing the development of high densities of cyanobacteria. Because herbivorous zooplankton are common in systems that zebra mussels may invade and both of these groups consume phytoplankton, these small grazers and larger filter feeders may compete.
One of the primary objectives of this study was to follow up on research that has suggested a link between zebra mussel presence and the dominance of potentially toxic cyanobacteria and to explore whether the presence of large herbivorous grazers may dampen the effects of zebra mussels on cyanobacteria. A related objective of this study was to determine whether zebra mussel filtration affects toxin production by cyanobacteria. The impact of zebra mussels on nutrient concentrations and ratios was also assessed in this study. Lastly how zebra mussels may impact other members of the lower planktonic foodweb, specifically protozoans, rotifers and nauplii, was investigated. This study employed laboratory mesocosms to investigate independent and combined impacts of zebra mussels and zooplankton on a relatively natural plankton community of Lake Champlain. Experiments were conducted in 200L mesocosms filled with natural lake water filtered through a 150um sieve to remove the large zooplankton. Treatments included a zooplankton addition, a zebra mussel addition, and a combined zebra mussel + zooplankton treatment. Over a 96-hr experimental period, samples for nitrogen and phosphorus, chlorophyll a, microcystin, and plankton species composition were collected and later analyzed.
Zebra mussels rapidly decreased total phytoplankton abundance, including the cyanobacteria. In this experiment, however, these algae were not very abundant nor were there large colonial forms present. Zebra mussels also caused significant declines in protozoans, rotifers and nauplii. Soluble reactive phosphorus and total nitrogen increased over time in zebra mussel treatments and the changes in these nutrients resulted in a decrease in N:P ratios in those treatments. No impacts of macro zooplankton were observed, however, macrozooplankton abundances in these treatments were far lower than planned. Zebra mussels clearly are capable of strongly impacting the lower planktonic foodweb; it is also possible that these effects will be felt by higher trophic levels of the planktonic foodweb.