The Blue-Green Blues

From May, 1996 (Update #27)

Offensive summer blooms of the potentially toxic blue-green algae, Mycrocystis, have returned to some Great Lakes waters, particularly Lake Huron's Saginaw Bay and Lake Erie. Saginaw Bay experienced blooms in both 1994 and 1995. In September 1995, Lake Erie's entire western basin was covered with what looked like "a thick slick of grass-green paint," according to the Ohio State University Stone Laboratory's John Hageman. Nuisance Microcystis blooms haven't occurred since the 1970s and early 1980s, before the United States and Canada lowered phosphorus inputs to the Great Lakes. The correspondence of the 1990s algal blooms with the arrival of the zebra mussel would seem entirely coincidental, given the mussel's reputation for filtering large quantities of plankton from the water column daily. "Since the zebra mussel's arrival, we hadn't seen, nor did we expect to see, any bloom of any kind of algae," said Alfred Beeton, director of the NOAA Great Lakes Environmental Research Laboratory (GLERL) in Ann Arbor, Mich. So the algal blooms are puzzling, given the zebra mussel's acknowledged role in producing the clearest water in decades in Lake Erie and Saginaw Bay. However, Henry Vanderploeg, also of GLERL, may have found a positive link between the zebra mussels and summer blooms of Microcystis. In his studies of the ecological effects of zebra mussels on Saginaw Bay, he has observed zebra mussels selectively filtering and rejecting phytoplankton in a way that could both promote and maintain Microcystis blooms. Using video equipment developed by J. Rudi Strickler of the Center for Great Lakes Studies at UW-Milwaukee, Vanderploeg made monthly observations of zebra mussel behavior during the algal blooms. Although the mussels remained open and siphoning, they exhibited a definite distaste for Microcystis, spitting BB-sized blobs of these algae back out into the water column, where they became resuspended. Vanderploeg speculates that these algal cells were uninjured and could continue to grow. While the mussels seldom slowed their pumping rates, their actual feeding rate declined due to the amount of Microcystis they spewed back into the water. In laboratory experiments using Microcystis from the Lake Erie 1995 bloom and a species of small laboratory-cultured algae (Rhodomonas), Vanderploeg confirmed that the mussels could continue to select smaller algae for normal digestion while expelling Microcystis. Because Microcystis may have a competitive advantage over other algae in conditions of high ammonium, the form of nitrogen excreted by the zebra mussels, it would be expected that fertilization of Saginaw Bay by zebra mussels would encourage Microcystis. But despite the high density of zebra mussels, Vanderploeg found nitrate concentrations (a form of nitrogen used by most algae) were 10-20 times higher than ammonium concentrations in the bay. Phosphorus, another element that might preferentially stimulate Microcystis, was excreted by the mussels at very low levels. Therefore, it appears that nutrient excretion by the mussels was not a major factor in promoting the Microcystis bloom. In addition to the aesthetic drawbacks of blue-green algal blooms and potential food chain disruptions, biologists are concerned about the potential toxicity of Microcystis. The algae is known to be responsible for some bird and fish kills and to cause gastrointestinal distress to humans. "In fact," Vanderploeg said, "the algal toxins may be what is causing the zebra mussels to reject Microcystis." Vanderploeg is continuing to examine this issue. In the meantime, scientists caution that the presence of toxins in the algae does not automatically make Microcystis harmful to fish and humans. According to Wayne Carmichael, an expert in algal toxins at Wright State University in Dayton, Ohio, a certain toxin level must be reached, which depends on the algae's growth conditions and on how fish or humans are exposed.

Martha Walter, Michigan Sea Grant.

ID: 199605-1.

The Zebra Mussel Update was a 4- to 8-page quarterly national newsletter published by the University of Wisconsin Sea Grant Institute from May 1990 through May 1997. The ZMU documented the spread of the zebra mussel -- an exotic nuisance mussel -- through North America's freshwater environments, especially the Great Lakes, and on efforts to control it.