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Changes in water quality variables at a mid-depth site after proliferation of dreissenid mussels in southeastern Lake Michigan
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    Fundamental and Applied Limnology, 188(3), 233-244.

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  • Description:
    Studies evaluating the impacts of dreissenid mussels in Lake Michigan have largely focused on changes in phytoplankton dynamics in the offshore region (i.e., >100 m depth) even though mussel biomass is actually highest in mid-depth coastal regions of Lake Michigan (i.e., 30-50 m). Here we report on changes at the base of the food web during 1995-2014 at a mid-depth site located in southeastern Lake Michigan. Specifically, we evaluated trends in Secchi depth, surface mixed layer chlorophyll-a and total phosphorus (TP), sub-epilimnetic deep chlorophyll layer concentrations, and near bottom chlorophyll-a concentrations and whether there have been shifts in the seasonal patterns of these variables. Median chlorophyll-a concentrations declined over 63 % during the spring isothermal period following the sharp increase in mussel abundance between 1996-2002 and 2007-2014. Chlorophyll-a concentrations in the spring were generally between 2 and 3 mg m(-3) in 1996-2002, but almost never exceeded 1 mg m(-3) in 2007-2014. Secchi depths increased in all months between 1996-2002 and 2007-2014, with the greatest changes being observed in the spring. Total phosphorus in the surface mixed layer declined over the study period, but not at as fast a rate as chlorophyll-a, a change consistent with mussel invasions. There was a 90 % decline in the median depth integrated deep chlorophyll-a concentration between 1995-2000 and 2007-2014 in June when this feature was at its peak. Chlorophyll-a concentrations in the near bottom zone also decreased over time, likely due to their constant contact with dreissenid mussels. The declines in chlorophyll-a and changes in nutrient dynamics at the mid-depth site are consistent with dreissenid induced impacts that have also been documented at deeper, offshore sites in Lake Michigan.
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