Taken from August/September 1997 Twine Line

Thirteen researchers and their graduate students gathered on the campus of Ohio State University this July to discuss the mysteries of the cyanobacterium Microcystis, the culprit in a surprise late-summer bloom two years ago that coated Lake Erie in what looked like a slick of green paint (see TL, October 1995). The lake hadn't experienced such a bloom of nuisance cyanobacteria (formerly called blue-green algae) since the '80s, shortly after federal water regulations cut back on pollution loads, particularly phosphorus. (In Lake Erie, phosphorus is the limiting factor for algae growth.) The bloom was even more surprising, considering that phosphorus removal plus the activities of zebra mussels, a decade-old nonindigenous aquatic pest of the Great Lakes, had notably returned Lake Erie's water to a crystal clarity that it hadn't seen in decades. Although Microcystis always inhabits the lake at some low concentration, this sudden bloom was decidedly out of the blue.

It's not so much the occasional or even surprising bloom, though, that makes Microcystis so interesting and worth the $220,000 that the Lake Erie Protection Fund (LEPF) is spending to support this research. It's the fact that Microcystis naturally produces a toxin called microcystin. This toxin has been known to kill wildlife and is known in rats to be a hepatotoxin, that is, a chemical that is toxic to the liver. That's the concern of Dr. Jeff Bush, director of LEPF. When all is said and done, what he wants to know is this: are blooms like this going to make people swimming in Lake Erie sick? And, can we anticipate the next one?

And the answer is...the researchers don't know yet.

That's the answer that no one likes to hear, especially if they're the ones putting cold cash on the line to get it. But the thing about levels of microcystin in Lake Erie during blooms is that they may not be high enough to make swimmers sick immediately but that even at low levels, their effects may still be harmful. Think of cigarettes: one won't hurt you, two won't hurt you, a whole pack won't hurt you, but in the long term if you smoke a pack a day, there's a good chance you'll get sick. That sort of exposure--to a little bit over and over again for a long time--is called chronic exposure, and research with rats shows that chronic exposure to low levels of microcystin can promote tumors in the liver.

Whether that occurs in humans is not yet known, although according to this research team's Microcystis expert, Dr. Wayne Carmichael of Wright State University, scientists in China are now investigating a link between a region in China with a high incidence of liver cancer and the local drinking water known to contain microcystins.

Nor do scientists know whether bloom levels of microcystin in Lake Erie can cause vomiting and diarrhea in swimmers as can the intestinal bacterium E. coli after a storm (when combined storm and sanitary sewers overflow, sending untreated sewage into the lake). Experience shows that Microcystis blooms occur under unusual conditions, so that a swimmer getting sick during a bloom may just as well be responding to some other toxin or pollutant.

But if these results seem a bit less than satisfying, consider the sober discovery made by Lisa Babcock-Jackson, one of the team's graduate students at Ohio State. Small crustacea called gammarids that live among zebra mussels on the lake bottom happily devour the mussels' feces and pseudofeces (food that is taken in by the mussels and consolidated but ejected rather than being allowed to pass through their gut). When Microcystis was present in the feces and pseudofeces, the gammarids ingested it as well, thereby building up microcystin in their own tissues. The bad news is that lots of different fish eat gammarids, including the fish beloved by Lake Erie anglers, particularly small mouth bass, according to Dr. David Culver of Ohio State and the research team's organizer. Thus by eating gammarids during Microcystis blooms, fish concentrate microcystin in their tissues. That in the long run may prove unhappy news for the fish and fish eaters.

[Perhaps even more sobering was Babcock-Jackson's related discovery that a much greater variety of fish than expected, including yellow perch, are eating zebra mussels. These also contain microcystin and concentrate man-made toxic pollutants in their tissues.]

The researchers continue to examine various bloom-causing conditions such as nutrients, or temperature, or light levels that might make the next bloom predictable. They suspect, however, that zebra mussels had a role, although precisely how they contributed is being discussed. The videos of Dr. Hank Vanderploeg of NOAA's Great Lakes Environmental Research Laboratory (GLERL) in Ann Arbor show that zebra mussels eat other phytoplankton but consolidate and spit out larger Microcystis colonies in the form of pseudofeces. They do, however, ingest small colonies and single cells. Thus, Microcystis gets concentrated in the lake water, while other palatable phytoplankters are removed. Whether this activity could cause a bloom is unknown.

The lab experiments of team-member Dr. Robert Heath of Kent State University, also show zebra mussels rejecting Microcystis. In addition, his work suggests that zebra mussels contribute phosphorus to the immediate environment in their wastes, recycling phosphorus from algae. Heath suggests that this contribution of phosphorus, the limiting factor of algae growth, may contribute to the growth of many types of algae in the vicinity potentially changing the competitive balance for nutrients that allows Microcystis to bloom.

Microcystis did bloom again in Lake Erie in 1996 and 1997, although not to the news-making extent of 1995. Still, if low levels of microcystin are a possible health hazard to humans, then Lake Erie may not need that thick slick of green before we think twice about diving in.

Other Related Links
Aquatic Nuisance Species
Zebra Mussels

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