PHYTOPLANKTON SPATIAL AND TEMPORAL DISTRIBUTIONS IN GREEN-BAY, LAKE MICHIGAN, PRIOR TO COLONIZATION BY THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA)

Spatial and seasonal patterns of change in phytoplankton composition, size structure, and standing stock along a trophic gradient in Green B ay, Lake Michigan, prior to the colonization of the bay by the zebra m ussel (Dreissena polymorpha) are documented. Based on particle size di stributions and microscope analysis of epilimnetic samples collected b etween June and October 1981 throughout Green Bay, at least two distin ct regions can be identified. Lower bay waters are hypereutrophic and differ from all other regions throughout summer and fall. Upper bay re gions are meso-oligotrophic and similar to Lake Michigan proper. These analyses demonstrate that the lower bay and upper bay regions undergo seasonal changes in epilimnetic algal communities similar to those wh ich occur in separate lakes with differing trophic status. Recent redu ctions in nutrient loading to the lower bay region should produce a cl ear improvement in water quality, but the impact of colonization of th e bay by zebra mussels is less clear given the abundance of filamentou s and colonial phytoplankton in lower Green Bay.