Lake Michigan after dreissenid mussel invasion: Water quality and food web changes during the late winter/spring isothermal period

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© 2014 by Taylor & Francis Group, LLC. This chapter documents water quality and lower foodweb changes in the offshore region of southern Lake Michigan during the late winter/spring isothermal mixing period during three distinct decades: 1983-1989 (pre-dreissenids), 1995-1998 (post-zebra mussel/pre-quagga mussel), and 2007-2010 (post-quagga mussel). Total phosphorus, chlorophyll a, and phytoplankton primary productivity did not change or decreased minimally ( < 22% decrease) between 1983-1989 and 1995-1998 but decreased substantially between 1995-1998 and 2007-2009 (34%, 68%, and 71% decrease, respectively). Secchi depth transparency increased from 6 m in 1985-1989 and 7 m in 1995-1998 to 18 m in 2007-2010. These pronounced changes in water column properties during the isothermal period in 2007-2010 compared to earlier periods were primarily attributed to the filtering activities of the quagga mussel (Dreissena rostriformis bugensis) and to a lesser extent to phosphorus load reductions. In contrast to substantial changes in phytoplankton, total zooplankton biomass did not differ between time periods (1986-1988, 1994-1998, and 2007-2009). However, biomass of cyclopoid copepods declined to nearly negligible levels in 2007-2009, perhaps reflecting a more oligotrophic system that favored calanoid copepods. Trends in biomass of calanoid copepods were not readily apparent or possibly increased in 2007-2009. Densities of Mysis diluviana declined 60% between 1996-2000 and 2007-2010. These large changes in limnological parameters and the food web have contributed to significant changes in traditional nutrient stoichiometry: Increases in total phosphorus to chlorophyll ratios and decreases in phytoplankton carbon to chlorophyll ratios were noted in 2007-2009 as compared to 1983-1987 and 1995-1998. Our results show that the recent expansion of quagga mussels in Lake Michigan has affected almost all aspects of water column nutrient and foodweb dynamics during the late winter/spring isothermal mixing period. We believe that future actions to manage the Lake Michigan ecosystem must consider these unique impacts of dreissenids and such impacts will affect management actions elsewhere in the Great Lakes.

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Quagga and Zebra Mussels: Biology, Impacts, and Control, Second Edition