Dreissenid-induced changes in the Saginaw Bay phytoplankton community structure since the dreissenid invasion of Saginaw Bay, Lake Huron

Background/Question/Methods

Dreissena polymorpha (zebra mussel) was the first dreissenid mussel to establish in Saginaw Bay, and the only dreissenid species found in the bay between 1990 and 1996.  Following this time period the closely related D. bugensis (quagga mussel) progressively became the dominant Dreissenid mussel in Saginaw Bay.  Previous studies document the impacts of zebra mussels on the phytoplankton community during the initial years of the invasion; however, little information exists on the changes in the phytoplankton community since the species shift towards quagga mussels.  We document the recent changes (2008-2010) in phytoplankton community structure following the initial zebra mussel invasion (1990-1996) using a combination of indicators including chlorophyll a, phytoplankton identifications and biovolumes, and microcystin toxin concentrations. 

Results/Conclusions

The phytoplankton community in Saginaw Bay maintains a diverse fauna heavily dominated by diatoms.  However, changes in the phytoplankton community from 1991 to 1996 compared to 2009/2010 included an increase in the less desirable cyanobacteria abundances coinciding with the shift from a D. polymorpha dominated system to one dominated by the larger D. bugensis.  Total phytoplankton abundance decreased > 70% in 2008-2010 as compared to 1990-1996 while diatom and cryptophyte abundance decreased >80%.; conversely, cyanobacteria abundance increased over 200% during the same time period. These large changes in phytoplankton community structure are likely due to the strong selective force of intense filtering by Dreissenid mussels over the decade.  Large increases in cyanobacteria including the toxic Microcystis aeruginosa with associated high microcystin concentrations have resulted in further degradation of water quality in Saginaw Bay.  Although it is difficult to determine if the observed increase in cyanobacteria would have progressed had the system maintained a D. polymorpha dominance it is likely the increased filtering capacity of D. bugensis accelerated the facilitation of cyanobacteria, resulting in greater potential for harmful algal blooms.