Increased emphasis on protection of recreational water quality has led to extensive use of fecal indicator bacteria monitoring of coastal swimming waters in recent years, allowing for long-term, widespread retrospective studies. These studies are especially important for tracking environmental changes and perturbations in regional waters. We show that E. coli concentrations (EC) have decreased in Lake Michigan over the last 15years, coincident with the rapid invasion of Eurasian quagga mussels (Dreissenidae). While median water clarity in Lake Michigan increased by 32% from 2000 to 2014, median EC decreased by 34.9%. Of the 45 Lake Michigan beaches studied, 42 (93.3%) showed a relative decrease (76% significantly, p<0.05), in mean log E. coli between pre- and post-2007. As a result, Lake Michigan beach advisory frequency decreased by 40.0% (p<0.001) from 19.9% in 2000-2007 to 11.9% in 2008-2014. Finite Volume Coastal Ocean Model simulations at Ogden Dunes beach confirm that EC would decrease in response to the observed changes in water clarity (predicted=4.3%, actual=2.3%). In contrast, mean EC in western Lake Erie showed the opposite trend, with 17 of 19 (89.5%) beaches increasing in mean EC after 2007 (p<0.001). We explore plausible explanatory influences on lakewide EC and conclude that bacterial photoinactivation due to increased water clarity is an important contributing factor explaining the general decrease of E. coli densities in Lake Michigan. The trends and explanatory factors reported here may have important public health, management and ecological implications.
Keywords: E. coli; Great Lakes; Photoinactivation; Transparency; Water quality; Zebra and quagga mussels.
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