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Zebra Mussels Contribute to Increase in Toxic Algae

Michigan State University researchers Stephen Hamilton and Orlando Sarnelle discovered during a long-term study that zebra mussels can actually increase Microcystis, a type of phytoplankton known as “blue-green algae” or cyanobacteria, that forms harmful floating blooms. 

The study, titled Cascading effects: Insights from the U.S. Long Term Ecological Research Network, is one of five projects recently highlighted in a special feature in the Ecological Society of America’s journal, Ecosphere.

“Microcystis literally means small cell, but numerous cells cluster together in colonies that can float to the surface to form scums,” said Sarnelle, professor emeritus with the Department of Fisheries and Wildlife in the College of Agriculture and Natural Resources. “It is one of the most common causes of nuisance algal blooms in nutrient-enriched waters, including Lake Erie where it is a concern for municipal water supplies.”

“Lakes colonized by zebra mussels tend to have about three times more Microcystis,” added Hamilton, a professor in the Department of Integrative Biology and at W.K. Kellogg Biological Station.

Sarnelle collaborated with Hamilton on a multiyear study that was part of the National Science Foundation's Long-Term Ecological Research Network (LTER). Forty years ago, the NSF recognized the need for research studies that lasted more than a few years and launched the LTER Network.

The researchers suspected the zebra mussels were consuming competitors of Microcystis, which paved the way for the cyanobacteria to flourish under lower nutrient availability than it usually needs. In 2010, an unexpected summer die-off of zebra mussels in Gull Lake during prolonged warm temperatures provided a whole-lake test of the relationship, an opportunity that scientists sometimes call a “natural experiment.”

“Normally, Microcystis thrives in warmer water,” said Jeffrey White, who was a graduate student advised by Sarnelle at the time and is now a faculty member at Framingham State University in Framingham, Massachusetts. “Instead, we saw an 80 percent decrease in the Gull Lake Microcystis population when the zebra mussels died despite optimal temperatures for its growth.”

Stephen said, “This fortuitous observation following years of sampling strengthens the argument that there is a cause-and-effect relationship, and not just a correlation, between zebra mussels and increased Microcystis. Multiyear studies can catch slow, unusual or extreme events that could be making important changes resulting in long-term lasting effects in the ecosystems."