Epiphytic algae and invertebrates on aquatic mosses in a Québec stream

Abstract

We sampled algae and invertebrates growing on aquatic mosses in a Quebec trout stream to a) assess their temporal and spatial variations in biomass, taxonomy, and size, and b) to compare the communities on mosses to those on nearby rocks. The biomass of epiphytic algae and invertebrates was remarkably stable during the summer despite a strong mid-summer flood. The invertebrate community was dominated by chironomids (especially Orthocladiinae), Ephemeroptera and Coleoptera. The relative importance of these taxa changed over the season as the size distribution of the entire community. Part (43 %) of the spatial variation in algal biomass was explained by moss biomass. However, epiphyton biomass did not increase proportionally with moss biomass: dense vegetation had less epiphyton per unit of plants than sparse stands. With increasing moss biomass, invertebrates tended to become more abundant, but smaller, resulting in an almost constant biomass across the moss biomass gradient. Algal biomass and invertebrate density were much larger (5-fold and 10-fold, respectively) on mosses than on nearby rocks whereas invertebrate biomass was similar on the two substrata, because of a striking difference in size distribution between the two communities. Algal and invertebrate communities living on aquatic mosses are temporally stable and related to the density of moss patches. Epiphytic invertebrate communities differ in density, size, taxonomy, but not necessarily in biomass, from communities on other substrata. The extent of moss cover can, therefore, alter stream metabolism and trophic interactions.