September 27, 2010

The complex network of predators and prey that inhabit Lake Michigan has changed so drastically in recent decades that future trends for the food web are murky, according to scientists at the NOAA Great Lakes Environmental Research Lab (GLERL), the NOAA Cooperative Institute for Limnology and Ecosystems Research (CILER), and other academic partners. These trends are documented in a special issue of the Journal of Great Lakes Research.

The changes in this network, a system that biologists call the food web, pose an uncertain future for both water quality and fisheries management. Several of their studies show that these trends are driven by non-native mussels that invaded Lake Michigan beginning in the late 1980s. The studies are online now and coming out in print this month.

“We do not know what the future holds,” said Gary Fahnenstiel, Ph.D., of GLERL, located in Ann Arbor. “We need to continue monitoring mussel populations, particularly in the cold, offshore regions of the lake, in order to develop realistic and sustainable management goals.”

Among the research findings:

• A paper by GLERL researchers Thomas Nalepa, Steven Pothoven and David Fanslow compares trends of zebra and quagga mussel populations from the 1990s through 2008. Zebra mussels reached a peak in the early 2000s and then declined. Quagga mussels were first found in the southern portion of Lake Michigan in 2001 and have continued to increase ever since.

• In another study, GLERL researchers found that the spring diatom bloom has declined 87 percent since 1983 in the southeastern portion of Lake Michigan. Diatoms are a calorie-rich phytoplankton group that serves as an important food resource for many invertebrates in the lake. Researchers attributed the steep decline to invasive mussels that consume this phytoplankton as they filter water. GLERL researchers Henry Vanderploeg, Fahnenstiel, Pothoven and Nalepa worked with David Klarer of the Ohio Department of Natural Resources and Don Scavia of CILER at the University of Michigan on this study.

• Steve Pothoven (left) and Gary Fahnenstiel, researchers at NOAA's Great Lakes Environmental Lab, at the Muskegon Lake Field Station.

  High resolution (Credit: NOAA).

  Researchers calculated the rates at which quagga mussels consume phytoplankton at different depths. They conclude that the 2003-2004 expansion of quagga mussels into deeper water explains the disappearance of the spring phytoplankton bloom. GLERL researchers James Liebig, Vanderploeg, Fahnenstiel, Nalepa and Pothoven conducted this study.

• A study by Pothoven, Fahnenstiel and Vanderploeg measured declines of up to 81 percent in the abundance of a tiny freshwater shrimp. The scientists noted that the observed decrease in the shrimp population size could be influenced by declining phytoplankton populations that reduce the shrimp’s food supply. They also noted that fish may be grazing on shrimp at higher rates due to declining zooplankton, which are typically a staple of the fish diet.

The future of the Lake Michigan ecosystem will ultimately depend on the eventual stabilization of the invasive quagga mussel population. These mussels are rapidly expanding into deeper colder waters, but are expected to decline and reach a stable level in the future.

“The Lake Michigan analyses, carefully documented in this special issue, should make people sit up and take notice, once the ecosystem impacts of the invasive mussels are seen together in this comprehensive sense,” said Marie Colton, Ph.D., director of GLERL.

With much unknown about how quickly and how far the mussels will expand their range, the scientists say they need to continue long-term monitoring and research efforts, coupled with active involvement of resource managers to develop appropriate management actions.

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