Biomass and composition of macroinvertebrate communities associated with different types of macrophyte architectures and habitats in a large fluvial lake

The influence of macrophyte habitat and architecture on macroinvertebrate biomass, abundance, and richness was investigated in Lake St. Pierre, a large fluvial lake of the St. Lawrence River (Quebec, Canada). A lake-wide estimate of macroinvertebrate biomass associated with different macrophyte habitats was also calculated in order to assess the quantitative effects of vegetation changes on macroinvertebrate communities. For two years during the ice-free period, phytophilous macroinvertebrates were sampled in macrophyte beds comprising more than ten species of plants and three habitats (emergent, floating-leaved, submerged), and in three submerged macrophyte architectures based on plant morphology (simple, intermediate, and complex). Invertebrate sub-samples were classified into four functional groups (detritivore, grazer, crawling predator, diving predator). Biomass and density of invertebrates were expressed per unit of plant dry weight. The main findings are that macroinvertebrate biomass, abundance and richness were significantly greater in submerged than in emergent and floating-leaved habitats. However, macrophytes with a complex architecture did not host significantly greater macro invertebrate biomass than plants with a simpler architecture. This could be related to substrate preferences of herbivores (mostly Gastropoda) toward the tape grass Vallisneria americana. Differences in macroinvertebrate abundance and biomass were found between the two years associated with variations in the river water level. During the year with average water level, total macroinvertebrate biomass was 16 % greater than in the year with a lower water level. We conclude that a reduction in the water level of Lake St. Pierre, predicted to occur with climate change, could lead to a decrease in benthos biomass which constitutes a crucial food source for fish.