ECOSYSTEM LEVEL IMPACTS OF AN EXOTIC SPECIES: HERBIVORY AND NUTRIENT C

This Progress Report submitted by Headworth, Jenifer Lyn and Michael J. Vanni [e-mailto: headwojl@miaimu.muohio.edu ] on 10/24/97.

Invading species can greatly alter an ecosystem, their impacts on food web dynamics have large implications in issues of global change (Vitousek, 1996). Historically, aquatic ecosystems have experienced a large number of non-indigenous species introductions, both deliberate and accidental (mills, 1995). It is often hard to assess the ecosystem level impacts of these introductions as they often occur in conjunction with other invasions or other large scale modifications in the system. A controlled laboratory study in which an invasive species is introduced to a microcosm isolated from the whole ecosystem would us to understand on the potential impacts of the exotic species. Zebra mussels, Dreissena polymorpha, are an excellent candidate for this study as it is projected that they will soon be a component in most freshwater ecosystems in the United States and Canada (Schneider, 1994).
Zebra mussels were introduced to the North American Great Lakes in 1988 via the ballast water of ships. Zebra mussels rapidly throughout the United States, they have been sighted as far west as California (Strayer et al, 1994). Although much has been studied on the physiology and distribution of the mussels, little is known about the impacts they have on food web dynamics. Zebra mussels van have direct and indirect impacts on aquatic ecosystems. They can affect phytoplankton by consumption and nutrient recycling, potentially affecting entire food webs. I investigated how invasion by zebra mussels may affect phytoplankton by making lab measurements of algal consumption and nutrient recycling in water samples from three mussel-free Ohio Lakes. Algal biomass was reduced 81% in Acton Lake water, 84% in Sharon Lake water and only 18% in Miami Whitewater Lake water. Phosphorus excretion rate was 83 mg P/g tissue DW /hr, 36 mg P/g tissue DW /hr and 56 mg P/g tissue DW /hr in the three lakes respectively and varied seasonally in Acton Lake. Nitrogen excretion rate by mussels averaged 392 mg N/g tissue DW /hr. Nitrogen:phosphorus ratios averaged 6.5 by mass which may promote blooms of blue-green taxa. Phytoplankton were also isolated in nutrient permeable chambers and exposed to nutrient recycling by zebra mussels but not to consumption. Algal biomass increased by 2-5 X, depending on the temperature, when exposed to nutrients recycled by the mussels. These results indicate that zebra mussels can have substantial effects on phytoplankton through consumption and recycling of nitrogen and phosphorus, and could greatly alter a lake ecosystem when they invade.

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Fascinating

This Progress Report submitted by Dr. C [e-mailto: HaysC@muohio.edu ] on 10/28/97.

Your work sounds fascinating and relevant!

Hays

Ecosystem Level Impacts of Zebra Mussels - Research Design

This Progress Report submitted by Jenifer L. Headworth [e-mailto: headwojl@miamu.muohio.edu ] on 10/27/97.

The Indirect and Direct Ecosystem Level Impacts of _Dreissena polymorpha_ (zebra mussel) - Research Design

This study consists on three major stages of research: The firsts stage addresses the direct impacts ZM's can have on phytoplankton populations via herbivory. To do this, I allowed mussels to feed on phytoplankon for up to twelve hours. Throughout the feeding period, I removed aliquots of the water the mussels were in for analysis of chlorophyll a, particulate C,N,and P, and preserved phytoplankton for identification and enumeration. From these measurements, I was able to calculate ingestion and clearance rates and also feeding selectivity indices. In the second stage of the experiment, I measured the excretion rates of soluble nitrogen and phosphorus by the zebra mussels. To do this, I placed the mussels in lake water that was filtered free of all seston. I removed aliquots of the water at time intervals of 0,1,2,4,6 and 8 hours for analysis of ammonia and soluble reactive phosphorus. In the third stage of the experiment, I separated the direct impacts of herbivory from the indirect impacts of growth stimulation by excreted nutrients. During this stage, I isolated phytoplankton in chambers permeable to water and nutrients but prevented herbivory by the mussels. The phytoplankton in the chambers were exposed to nutrients excreted by the mussels. I measured chlorophyll a and preserved some of the plankton for identification and enumeration.

These expriments were done with natural phytoplankton assemblages from three Ohio reservoirs, The experiments were also performed many times over the course of the season with Acton Lake phytoplankton.

Outline of Senior Project

This Progress Report submitted by Dr. C [e-mailto: HaysC@muohio.edu ] on 10/25/97.

Hello Jenifer,

Thanks for the post! Could you also place your outline of your study?

Hays

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