In an article recently published by the International Society of Limnology, Minnesota Sea Grant researchers report that some of the smallest, crunchiest creatures in Lake Superior are pushed around by summer winds and are responding to warming surface waters. The creatures are copepods -- crustacean zooplankton that are the most prevalent type of animals in the lake and a favorite food of ciscoes (formerly termed lake herring) and other fish.
Robert (Bob) Megard, professor emeritus of Ecology, Evolution, and Behavior at the University of Minnesota Twin Cities, and University of Minnesota Duluth colleagues at the Large Lakes Observatory published evidence that copepods can be 10 times more abundant in the warmer waters pushed toward Wisconsin than they are on the colder side of the lake when wind-driven upwellings occur near Minnesota's North Shore. Their data also supports evidence that copepods have become much more numerous than they were 30 years ago.
Most copepods roam the oceans, but about 50 copepod species live in Lake Superior. Of these, Megard and his colleagues scrutinized three that comprise more than 90 percent of the copepod biomass: Diacyclops thomasi, Leptodiaptomus sicilis, and Limnocalanus macrurus. The scientists' innovative use of sonar and satellite technology provided a way to understand copepod population distribution, which is not uniform, in a clearer, data-rich way.
Upwelling of cold, deep water along Minnesota's North Shore occurs along the northwestern coast of Lake Superior when northwest to southwest winds push warmer surface water eastward. Coastal upwellings can make the surface water temperatures along Minnesota's coast at least 25 degrees F colder than the water temperatures on the Wisconsin side of the lake.
"It is striking how prevalent summer upwelling is along the North Shore," said Megard. "Upwelling not only transports water and all that it contains from bottom to surface, but also shifts it laterally across the lake. The underlying physics and the consequences of these events are fascinating."
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Other studies have documented an increase in Lake Superior's surface temperature, and that copepod abundance can double if water temperature increases only 5 degress F. Megard attributes the doubling of copepod abundance in Lake Superior's western arm over the last 30 years, as well as a species shift favoring the small Diacyclops, to an increase in summer surface water temperature.
"You don't necessarily need to invoke predation to explain the changes in Lake Superior's copepod population over the last 30 years," said Megard.
Other scientists ascribe these observed changes in Lake Superior's copepod populations to changes in the population dynamics of Lake Superior's fish, and in particular ciscoes, which have become far less numerous.
The three most abundant copepods
The three most abundant copepods in Lake Superior. Imagining that they are cars: (pictured below, from left) the sporty hotrods (Diacyclops thomasi) are best at zipping around the shallower warmer waters near Wisconsin. Mid-sized sedans (Leptodiaptomus sicilis) mingle with the hotrods but also operate well across the lake's surface. Limnocalanus macrurus, the SUV, roughs it in the colder middle of the lake at greater depths.
Source: www.glsc.usgs.gov/greatlakes
copepods/
