Phytoplankton Spatial and Temporal Distributions in Green Bay, Lake Michigan, Prior to Colonization by the Zebra Mussel ( Dreissena polymorpha)

Abstract

Spatial and seasonal patterns of change in phytoplankton composition, size structure, and standing stock along a trophic gradient in Green Bay, Lake Michigan, prior to the colonization of the bay by the zebra mussel ( Dreissena polymorpha) are documented. Based on particle size distributions and microscope analysis of epilimnetic samples collected between June and October 1981 throughout Green Bay, at least two distinct regions can be identified. Lower bay waters are hypereutrophic and differ from all other regions throughout summer and fall. Upper bay regions 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 which occur in separate lakes with differing trophic status. Recent reductions in nutrient loading to the lower bay region should produce a clear improvement in water quality, but the impact of colonization of the bay by zebra mussels is less clear given the abundance of filamentous and colonial phytoplankton in lower Green Bay.