This recommendation is especially relevant in the context of the current review of the Great Lakes Water Quality Agreement by the International Joint Commission, Scavia said. Through the IJC, the United States and Canada jointly manage the Great Lakes.
Though the zebra mussel is better known to the public, over the past decade it has largely been overshadowed by the quagga mussel, which can thrive far from shore in deep, mud-bottomed waters. Each of the fingernail-size quagga mussels filter about a quart of water a day, and billions of them now blanket the bottoms of lakes Michigan and Huron down to depths of nearly 400 feet.
They feed on algae, including single-celled plants called diatoms that are encased in glass-like shells made of silica, which the diatoms extract from lake water. Until recently, the diatoms "bloomed" each spring in the Great Lakes, and the level of silica in upper lake waters dropped as diatoms built their protective shells, then sank to the lake bottom, taking the silica with them.
The drop in silica levels due to the spring diatom bloom, known as the seasonal drawdown, has long been used as an indicator of overall algal production in the Great Lakes.
Reviewing records of silica levels in lakes Michigan and Huron collected over the past 30 years by the Environmental Protection Agency, Evans and her colleagues found that algal production throughout the two lakes was about 80 percent lower in 2008 than it had been in the 1980s.
In Lake Michigan, the decrease in the seasonal drawdown coincided with an explosion in the quagga mussel population and its expansion to greater depths, which began in 2004. The same changes occurred a few years earlier in Lake Huron, where quagga mussels greatly increased in abundance between 2000 and 2003.
"For years, all the talk was about the zebra mussels. And then its close cousin comes in, the little quagga m
|Contact: Jim Erickson|
University of Michigan