Diporeia site preference in Lake Superior: Food or physical factors?

Vital to the Lake Superior food web, the amphipod Diporeia remains the dominant macroinvertebrate in Lake Superior despite drastic population declines throughout the rest of the Laurentian Great Lakes. Diporeia is most abundant in the slope region of the lake at water depths between 30 and 125 m. It has been hypothesized that this depth range is preferred because of elevated primary production and deposition within this zone. This hypothesis of food driving habitat preference has not been directly tested. Here we used 120-hour preference-avoidance trials to record Diporeia choice of sediments from different water depths, seasons, and other treatments. Most preferences were weak to absent; however, Diporeia strongly preferred sediment from 30- and 60-m water depths over deeper or shallower sites. Contrary to the hypothesis about food driving habitat choice, chemical characteristics did not explain this strong preference. Grain size variation was the only measured variable that was consistent between the sites preferred by Diporeia and different from unpreferred sites. Both the 30- and 60-meter sites contained predominantly medium silt but had a wider range in grain sizes. These results indicate that physical habitat characteristics may have a stronger bearing on Diporeia habitat preference than food availability and may account for their distribution in the lake. The results also may imply that the role of dreissenid mussels as ecosystem engineers altering sediment physical characteristics may be important where they are abundant.