Increased Chlorophyll a, phytoplankton abundance, and cyanobacteria occurrence following invasion of Green Bay, Lake Michigan by dreissenid mussels

Abstract

The invasion of freshwater ecosystems in North America by the zebra mussel, Dreissena polymorpha (Pallas, 1769), has repeatedly led to rapid changes in water clarity and phytoplankton abundance. Examining cases where a successful invasion has occurred but where rapid changes do not occur can help us understand the full range of responses of ecosystems to invasions. We re-sampled established sampling locations in Green Bay, Lake Michigan during four years of the post-invasion period. We determined Secchi depths, Chlorophyll-a concentrations, and phytoplankton composition and biovolume at five stations along a previously documented trophic gradient. Our data support the continued existence of a strong trophic gradient between lower and middle bay areas of Green Bay. Secchi depth did not significantly change following invasion at either lower bay or middle bay locations. Responses to invasion were stronger at middle bay than lower bay for both Chlorophyll-a and phytoplankton biovolume. Chlorophyll-a was significantly greater in the post-invasion period at both locations, while phytoplankton biovolume was significantly higher following invasion in middle bay. Along with these increases in abundance there was a significant shift to higher and more frequent dominance of the phytoplankton community by cyanobacteria. The increased phytoplankton abundance during the post-invasion period is likely explained by selective feeding and increased nutrient recycling by zebra mussels under high turbidity conditions in this highly productive embayment of Lake Michigan.