In the early 1990s, Saginaw Bay experienced an invasion of zebra mussels, which caused a significant perturbation to the ecosystem and shifted energy flow, potentially affecting every component of the food web from bacteria to fish. LimnoTech performed a study that greatly expanded scientific knowledge and understanding of ecosystem dynamics in response to multiple stressors.
Blooms of Mycrocystis, a toxin-producing blue-green alga, had been all but eliminated in the bay in the 1980s with controls on external phosphorus loadings. The algae became re-established following the zebra mussel invasion, but without increases in external phosphorus loadings. Concern was also raised about the potential impacts of zebra mussels on toxic chemical dynamics in the bay, especially PCBs.
LimnoTech scientists developed an ecosystem mass balance model to test various hypotheses on the relative impacts of phosphorus loadings and zebra mussel dynamics on phytoplankton production, Microcystis blooms, benthic-pelagic coupling, and PCB transport, fate, and bioavailability.
Significant findings were that the zebra mussel invasion of Saginaw Bay has caused a redirection of nutrients and PCBs from the water column to the sediments, and that bioaccumulation of PCBs in the lower food chain can be strongly influenced by zebra mussels. Enhancement of blue-green algae production in the presence of zebra mussels appears to depend on recycle of external phosphorus loads, and on selective rejection of the algae by the mussels. These findings are being used to inform the re-evaluation of the target phosphorus load to Saginaw Bay in the1978 Water Quality Agreement between the U.S. and Canada.