Effects of hydrological regime, landscape features, and environment on macroinvertebrates in St. Lawrence River wetlands

Wetlands of a large fluvial lake in the St. Lawrence River (Qu,bec, Canada) were visited during 3 years (2004-2006) to collect macroinvertebrates across the belt of emergent vegetation. We tested the hypothesis that hydrology, river landscape, and local environment would explain variations in macroinvertebrates. The 66 taxa collected lake-wide comprised a few abundant but widespread groups (Malacostraca, Oligochaeta, Chironomidae, and Mollusca). Between 2004 and 2006, total abundance at 5 sites monitored annually fell by one order of magnitude, and taxa richness decreased from 19 to 12 taxa. Proportion of amphipods (Gammaridae) dropped sixfold whereas proportion of annelids (Oligochaetes) rose ninefold. The impoverishment of macroinvertebrates coincided with low summer water levels in 2005 and 2006, resulting in the periodic emersion of up-slope sites. Spatial differences in macroinvertebrate communities were less important than inter-annual differences, owing to large variability among sites. Patterns in macroinvertebrate communities were related to water depth, vegetation, and local changes in sediments (31% of variance explained). Gammaridae (Malacostraca) were strongly associated with down-slope sites, whereas Oligochaeta (Annelida) dominated in up-slope sites. Inter-annual changes in water level had major effects on macroinvertebrate communities in Lake Saint-Pierre, above and beyond other environmental variables.