The introduction of the invasive freshwater bivalve Dreissena polymorpha altered the
lower trophic levels of many North American aquatic ecosystems. In Saginaw Bay, zebra
mussels became established during the late summer and fall of 1991, causing
environmental changes and economic losses. Seven years of monitoring data
characterizing the lower trophic levels of Saginaw Bay before, during, and after the
zebra mussel invasion were collected between 1990-1996 by the National Oceanic and
Atmospheric Administration Great Lakes Environmental Research Laboratory. In this
study, I investigated shifts in the phytoplankton community composition over the sevenyear
period. Using multivariate statistics and a clustering analysis, five distinct
phytoplankton assemblages were identified. Major shifts in community composition were
identified in 1) the fall of 1991, 2) 1992-1993, and 3) the summer of 1994. A dynamic
ecosystem model coupled to a zebra mussel bioenergetics model was used to analyze the
forces driving these changes. After successfully calibrating the model to 1991 conditions,
test scenarios were run to identify important zebra mussel mediated alterations to the
phytoplankton community of inner Saginaw Bay. In addition to the direct filtration of
phytoplankton, clearing of the water column and recycle of phosphorus were identified as
causal mechanisms in the observed changes in the phytoplankton community
composition. This study suggests that both direct (filtration) and indirect (nutrient
cycling) mechanisms are important in understanding the long-term changes in the
phytoplankton of Saginaw Bay induced by zebra mussels. This work describing the
changes in an aquatic ecosystem resulting from the introduction of an invasive species is
important for both ecosystem management and advancing the basic understanding of
ecosystem response to disturbance.Master of ScienceNatural Resources and EnvironmentUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/60336/1/Fishman_Thesis_Final.pd
