Coastal wetland insect communities along a trophic gradient in Green Bay, Lake Michigan

Abstract

Insects of Great Lakes coastal wetlands have received little attention in spite of their importance in food webs and sensitivity to anthropogenic stressors. We characterized insect communities from four coastal wetlands that spanned the length of a trophic gradient in Green Bay during spring and summer of 1995. We sampled flying insects using sticky traps in dense emergent, sparse emergent, and open water—submergent vegetation zones within each wetland and estimated numerical abundance, biomass (mg dry weight) and taxonomic composition. We found that insect abundance and biomass were distributed differently among vegetation zones within wetlands along the gradient during both spring and summer. Insect abundance was highest at oligotrophic Portage Marsh during spring and lowest in wetlands toward the lower (southern), eutrophic end of the bay. Biomass did not differ consistently along the trophic gradient but increased with increasing emergent vegetation cover in 3 of 4 wetlands during both seasons. Ordination revealed distinct gradients in community structure on both regional (i.e., upper, middle, and lower Green Bay) and local (vegetation zones within wetlands) scales. Wetlands sorted in order of trophic status during both seasons, primarily due to abundant small Chironomidae, such as trophic-sensitiveHeterotrissocladius changi, in middle and upper bay wetlands. Chironomidae also were a dominant component of open water-submergent assemblages in all wetlands. Lower bay wetlands were characterized by fewer but larger Chironomidae (e.g.,Chironomus spp.), as well as Ceratopogonidac, Calliphoridae, and Ephydridae, which were most abundant in stands of emergent vegetation. Our results suggest that eutrophy in the lower bay may contribute to relatively poor foraging conditions for insectivorous fish and young waterfowl during spring, and they demonstrate the utility of using insect communities to assess environmental degradation, such as excessive nutrient loading, in coastal wetlands of the Great Lakes.