Web of Life
Scientists Explore Changes in Great Lakes Aquatic Food Web

Imagine a three-dimensional spider web, its structure held together by a complex network of connecting threads. The Great Lakes aquatic food web is similar. Each aquatic species - from microscopic organisms to fish - supports one or more other species by providing food, or energy. The result is a complex web of connections upon which life in the Great Lakes depends.

It's important to understand these natural connections, says ecologist Dr. Doran Mason of NOAA's Great Lakes Environmental Research Laboratory (GLERL), so that scientists can better understand unnatural changes.

These changes are what scientists refer to as "food web disruptions" - some of which may be impacting fish.

"When you start breaking these connections that have evolved naturally in the Great Lakes," explains Mason, "the question becomes ‘how can we predict the consequences to fish communities?"

A key species
Consider the case of Diporeia - a shrimp-like organism that burrows in the upper layers of lake sediment. Diporeia has been a component of the Great Lakes ecosystem since glaciers retreated thousands of years ago. The organism spends most of its time in the sediment, feeding on organic material that settles to the lake bottom from the water column. In turn, Diporeia provides a nutritious meal for several Great Lakes fish including whitefish.

In routine sampling, GLERL biologist Tom Nalepa has documented that Diporeia began disappearing from the sediment in parts of southern Lake Michigan since about 1992. Numbers of Diporeia in some locations have declined from 10,000 per square meter to virtually none. More recently, says Nalepa, the organism has declined in northern Lake Michigan as well. "Its disappearance is very significant," says Nalepa. "Diporeia makes up about 70 percent of the biomass of benthic, or bottom-dwelling, organisms in deeper waters of Lake Michigan. It's a key species in the lake's food web."

According to GLERL, the organism's disappearance has also been reported in parts of Lakes Erie and Ontario. Scientists suspect that the loss of Diporeia may be affecting the condition of yearling smelt in Lake Erie and populations of slimy sculpin and young lake trout in Lake Ontario. In Lake Michigan, the loss of Diporeia may be affecting lake whitefish, one of Michigan's most important commercial fish species.

Diporeia