The biological and physical interactions of Mysis relicta in Lake Ontario

Understanding and quantifying the impact of biological and physical factors on M. relicta in the Lake Ontario food web requires information on Mysis interactions with it's physical and biological environment. The primary objective of this study was to determine the trophic interactions between M. relicta, its prey and predators. This was performed by first determining the biological and physical factors governing M. relicta distribution in Lake Ontario. Consequentially, I compared the direct effects mysids have on zooplankton with the direct and indirect effects planktivorous fish have on zooplankton in the lake.;The triangular trophic interactions between mysids, planktivorous fish, and zooplankton were examined by determining the portion of zooplankton biomass consumed by fish and mysids (direct effects) and the impact that the presence of predators has on the potential consumption of zooplankton by mysids (indirect effects). Models developed for acoustically determining mysid abundance and for predicting mysid diel vertical migration were used in conjunction with bioenergetics and functional response models to estimate the consumption of zooplankton by mysids. Fish consumption of zooplankton was estimated by using acoustically determined fish abundance estimates and bioenergetics models. In the acoustic studies, mysid acoustic target strength (TS) was found to be higher than organisms of similar shape and size in marine systems. Modeled TS values matched those measured in the field.;In the DVM studies, the model accurately predicted the mysid vertical distribution at two stations, during three seasons. Although, mysids displayed a preference for a relatively narrow range of light and temperature conditions, their vertical distribution was found to be determined predominately by the position of the thermocline in the water column.;In contrast to previous studies, mysids were found to consume zooplankton at rates comparable to and at times exceeding those estimated for planktivorous fish. This suggest that mysids and zooplankton cannot be combined as a single trophic level available to planktivores in the Lake Ontario food web. Moreover, it was clear from the results that the indirect effects exhibited by the fish on zooplankton cannot be neglected. Any future changes to the Lake Ontario community structure, especially to the planktivores in the lake, may have a significant impact on the flow of energy through the food web.