Changing Nutrient Dynamics in the Great Lakes: Testing Assumptions of the Nearshore Phosphorus Shunt Hypothesis Using Long-Term Data Sets

The Great Lakes have undergone rapid ecological changes due to invasive species establishment in the region. Here, we analyze long-term nutrient dynamics using change point analysis to test some key assumptions of the ‘near shore phosphorous shunt’ hypothesis.  The EPA-GLNPO nutrient time series provides broad spatial and seasonal resolution across the five lakes.  Our statistical approach makes no a priori assumptions about Dreissenia invasion period, and instead, uses non-parametric sorting for neutral identification of change points.  Our results provide support for the near shore shunt illustrated by the dynamics of chlorophyll, total phosphorous, silica, and secchi depth.  Chlorophyll concentrations in western Lake Erie declined from the late 1980’s to the early 1990’s with subsequent declines in total phosphorous during the 1990’s and 2008. Lake Ontario summer silica declined across the late 1990’s into 2004, suggesting silica replenishment amidst declining productivity. Lastly, chlorophyll concentrations in Lake Michigan’s southern basin declined in the late 2000’s and water clarity increased after 2002 and 2005. Results from the southern lakes conform to expectation of rapid nutrient shunt effects in higher productivity systems.  The application of the EPA-GLNPO data set to detect multi-decadal ecosystem changes supports the case for continued investment in long-term monitoring programs.