THE IMPORTANCE OF SIZE-FREQUENCY RELATIONSHIPS FOR PREDICTING ECOLOGICAL IMPACT OF ZEBRA MUSSEL POPULATIONS

Although most physiological processes of bivalves are highly size-depe ndent in a non-linear manner, often only total densities of population s of freshwater bivalves such as the zebra mussel are reported rather than size-frequency information. This can cause serious errors when tr ying to predict or assess the environmental impacts of these filter fe eders on planktonic communities or the role of their pseudofeces in tr ansferring materials from the plankton to the benthos. We used a bioen ergetics model to examine the effect that differing size-frequency dis tribution has on influencing total phytoplankton consumption and pseud ofeces production. We constructed different size-frequency distributio ns of 1000 zebra mussels with the same mean length or same mean body m ass for comparison. In addition, we used several size-frequency distri butions from the published literature. The size-frequency distribution of a population had a tremendous impact on both total consumption and pseudofeces production with rates varying by more than an order of ma gnitude (43.5 g consumption by 1000 smaller mussels to 654 g for a pop ulation dominated by large mussels). These data emphasize the importan ce of knowing not only population density, but population size structu re in order to accurately understand and predict the impacts of zebra mussels, or any filter feeder on pelagic and benthic communities. This work also demonstrates the usefulness of a tool such as our bioenerge tics model for partitioning the relative impacts of densities and size on a variety of factors such as consumption and pseudofeces productio n.