I stepped into the water at the lake’s edge, and only the thin plastic soles of my water shoes protected my feet from the razor edges of the mussels clustered on the rocky bottom of the lake – I could feel hundreds of shells crunching beneath my heels with each step. It was 1993, the year zebra mussels invaded Lake Champlain, Vt., and the striped shells of the mussels were everywhere, coating dock pilings, moorings, and rocks, sharp enough to leave nasty cuts on bare feet. Zebra mussels are small, less than two inches across, and native to the Black and Caspian Seas and the Sea of Azov in Eastern Europe. They’ve now spread throughout much of the Great Lakes and Mississippi River drainages, and where they’ve gone, they’ve taken over.
Zebra mussels were first detected in Oneida Lake, N.Y., about 200 miles southwest of Lake Champlain, in 1991. As reported recently in the journal PLoS ONE, a team of scientists from Cornell University and SUNY Buffalo State conducted a survey of the molluscs – mostly gastropods and bivalves – in Oneida Lake, which they compared to historical surveys conducted every few decades beginning in 1915.
The lake experienced more than one drastic change due to human activity during that time – in particular, the researchers note a period of high nutrient levels and low water clarity, the height of which was in 1967, and the zebra mussel invasion in 1991.
In one bay, the scientists found that by 2012, the number of bivalves – mussels and clams – was ten times as great as it had been in 1915-17. Not surprisingly, almost all of those bivalves were exotic, or non-native, and at least one family of native freshwater mussels appeared to be gone completely – no individuals from that family were found in the 2012 surveys.
Zebra mussels are filter feeders: they eat algae from the lake water around them, and, in the process, they increase water clarity. The researchers suggest that this is the mechanism behind the recovery of underwater gastropods – snails and slugs – that they found; after the number of gastropods in Oneida Lake declined in the 1960s, when water clarity was at its worst, gastropod numbers and diversity rebounded to close to 1917 levels by 2012. Poor water quality during the 1960s, the authors argue, limited the growth of algae on the bottom of the lake, the main source of food for gastropods. When zebra mussels invaded, they ate the algae suspended in the lake, which allowed more sunlight to reach the lake bottom. The scientists suspect the increase in light led to an algae boom on the lake bed, and the subsequent revitalization of the gastropod population of Oneida Lake.
Zebra mussels don't just make life more difficult for bare-foot swimmers – they also clog water pipes, corrode underwater pilings, and choke boat engines. By altering the ecosystems they invade, they may also, at least in the case of Oneida Lake, contribute to the recovery of other organisms.