Biological Data from GLNPO's Great Lakes Open Water Surveillance Program 2001-2006

Abstract

The United States Environmental Protection Agency’s Great Lakes National Program Office conducts annual assessments of the aquatic communities of the Great Lakes each spring and summer. This report summarizes the results of biological analyses of the 2001 through 2006 chlorophyll, phytoplankton, zooplankton and benthic macroinvertebrate samples that were conducted by the staff of the University of Wisconsin-Superior’s Lake Superior Research Institute.

Lake Superior displayed the classic characteristics of a cold-water, oligotrophic lake, with spring chlorophyll concentrations averaging less than 1 ug/L. Chlorophyll levels increased slightly during the summer months. Spring phytoplankton biomass during this study period averaged only 33 mg/m³ with diatoms contributing 36% of the biomass. Phytoplankton biomass was slightly higher in the summer than in spring. Although diatoms were still the dominant taxa found in August, seasonal changes in species composition were noted. In 2004 the composition of the summer phytoplankton community changed dramatically due to increased density and biomass of cryptophytes, cyanophytes, and dinoflagellates. Lake Superior’s zooplankton community was dominated by the large cold-water calanoid copepods Limnocalanus macrurus, and Diaptomus sicilis. While the smaller cyclopoid copepod Diacyclops thomasi was commonly found, it added little to total biomass. Crustacean zooplankton density and biomass were generally greater in summer than in the spring. Few changes were noted in zooplankton species composition or biomass during the six-year study period. The benthic invertebrate community of Lake Superior was dominated by the amphipod Diporeia, which was generally found at densities of less than 1000/m² at the deeper offshore stations. Oligochaete worms, primarily Stylodrilus heringianus, and fingernail clams in the genus Pisidium were also found in moderate densities, especially at stations in the eastern basin of the lake.