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Climate and Lake Superior’s Crunchy Creatures

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 under the lens of the proverbial microscope are copepods – crustacean zooplankton that are a favorite food of fish and the most prevalent type of animals in the lake.

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 an order of magnitude 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 supports existing evidence that these zooplankters have become much more numerous than they were 30 years ago and that the relative dominance of the three most abundant species has shifted towards the most diminutive of the lot.

Copepods are relatives to a diverse group of shrimp-and-lobster kin whose worldwide abundance and diversity have led to people calling them the insects of the sea. Plus, they are indeed members of the Phylum Arthropoda, having an exoskeleton, antenna, and several sets of legs more than you.

Most copepods roam the oceans, but about 50 copepod species live in Lake Superior. Of these, Megard and his colleagues scrutinize three abundant species (Diacyclops thomasi, Leptodiaptomus sicilis, and Limnocalanus macrurus) that comprise more than 90 percent of the copepod biomass.

Their objective was to describe the population patterns of copepods in the western arm of Lake Superior with satellite and sonar data coordinated with conventional plankton tows. Their innovative use of technology provides a way to understand copepod population distribution, which is not uniform, in a clearer, data-rich way.

Copepod abundance in a slice of Lake Superior from Two Harbors, Minn., to Port Wing, Wisc., during an upwelling event. Adapted from Megard et al 2009.

Upwelling of cold, deep water along Minnesota’s North Shore was detected by satellites and temperatures recorded at water intakes of municipal water treatment plants. This upwelling of deep water occurs along the northwestern coast of Lake Superior when northwest winds push warmer surface water eastward. Coastal upwellings can make the surface water temperatures along Minnesota’s coast over 25º F (14º C) 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."

Many studies have shown that copepod abundance depends on midsummer water temperatures. Knowing this, the science team boarded the R/V Noodin, the research vessel formerly owned and operated by University of Minnesota Duluth's Large Lakes Observatory. Their mission was to map copepod numbers with sonar technology during two daytime cruises and collect reference samples with a plankton net.

The copepods stacked up as the researchers expected. The three species that dominated the net tows contributed unequally to the acoustic backscattering because they were not the same size.

The tiniest, Diacyclops, was barely detected by sonar compared to the two larger species. However, the picture the sonar backscattering painted confirmed what the satellite imagery suggested. When upwelling occurs along Minnesota’s North Shore, copepods can be tens of times more abundant in the warmer water near the Wisconsin coast.

Although copepods can swim with astonishing rapidity (no bigger than the head of a pin, they can move about 45,000 times their body length per hour, equal to about 50 mph for 6-foot tall human), their abundance toward eastern shores during upwelling events is a product of drifting on wind-driven currents more than acts of free will. Despite the much warmer surface water, their growth and reproductive rates are not fast enough to account for the observed change in distribution.

Moving the discussion from a timeframe of a week to that of decades, Megard and his colleagues suggest that a small increase of water temperature leads to large increases in the abundance of planktonic crustaceans. Other studies have documented an increase in Lake Superior's surface temperature, and that copepod abundance can double if water temperature increases only 5º F (about 2.4º C). Like Megard's data, other research suggests that copepod abundance in Lake Superior's western arm has doubled, and that there has been a species shift favoring the small Diacyclops over the last 30 years.

Previously published research disagrees with their assertion that temperature is the primary driving force. In a peer reviewed article based on data collected at a similar time and place but in a different way*, scientists attributed observed changes in copepod abundance and species proportions to very real changes in the population sizes of copepod predators: fish, and in particular cisco (formerly called lake herring).

Still, Megard maintains "You don't necessarily need to invoke predation to explain the changes in Lake Superior's copepod population over the last 30 years."

If you would like to read Effects of wind and temperature on Lake Superior copepods, the article by Robert Megard, Elise Ralph, and Michelle Marko published in January 2009, contact Minnesota Sea Grant and request JR 547.

* Johnson, T. B., M. H. Hoff, A. S. Trebitz, C. R. Bronte, T. D. Corry, J. F. Kitchell, S. J. Lozano, D. M. Mason, J. V. Scharold, S. T. Schram, and D. R. Schreiner. 2004. Spatial patterns in assemblage structures of pelagic forage fish and zooplankton in western Lake Superior. J. Great Lakes Res. 30 (Supplement 1): 395-406.

By Sharon Moen
April 2009

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