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The Utility of a Broad-Based Approach in Assessing Ecosystem Changes in the Laurentian Great Lakes

Paul J. Horvatin 1 and Richard P. Barbiero 2

1 Great Lakes National Program Office, U.S. Environmental Protection Agency, Chicago IL
2 CSC, 1359 W. Elmdale Ave., Chicago IL

The Great Lakes National Program Office (GLNPO) of the United States Environmental Protection Agency (USEPA) has primary responsibility within the U.S. monitoring the offshore waters of the Great Lakes. Two of the main goals of the monitoring program are tracking recovery from eutrophication and assessing the impacts of a growing number of invasive species. These large systems are still imperfectly understood, and the variables constituting useful 'signals' can often be unexpected. Therefore the GLNPO monitoring program has adopted an approach that tracks a broad range of limnological variables, and also employs detailed taxonomic analyses of planktonic communities. Two recent examples of trends detected in the lakes illustrate the utility of this approach.

Although phosphorus loading to Lake Michigan has been greatly reduced in the past 30 years, corresponding trends in in-lake phosphorus concentrations have been obscured by high variability. Recovery from eutrophication in the lake has been more apparent from changes in silica, due to the nature of phosphorus-silica dynamics in the lake. Recent increases in silica concentrations, therefore, have provided the most compelling evidence to date that phosphorus load reductions are impacting the Lake Michigan ecosystem.

Bythotrephes longimanus, a predatory cladoceran from northern Europe, invaded the Great Lakes in the 1980s. Vulnerability to Bythotrephes predation is apparently species-specific, and the organism has altered cladoceran community structure in the lakes, without changing total cladoceran numbers. While changes to the cladoceran community have been dramatic, they would have gone undetected with a less detailed taxonomic monitoring program.

Keywords: eutrophication, invasive species, water quality.

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