Epiphytic algae and invertebrates on aquatic mosses in a Quebec stream

We sampled algae and invertebrates growing on aquatic mosses in a Quebec tr out stream to a) assess their temporal and spatial variations in biomass, t axonomy, and size, and b) to compare the communities on mosses to those on nearby rocks. The biomass of epiphytic algae and invertebrates was remarkab ly stable during the summer despite a strong mid-summer flood. The inverteb rate community was dominated by chironomids (especially Orthocladiinae), Ep hemeroptera and Coleoptera. The relative importance of these taxa changed o ver 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 biomas s: dense vegetation had less epiphyton per unit of plants than sparse stand s. With increasing moss biomass, invertebrates tended to become more abunda nt, but smaller, resulting in an almost constant biomass across the moss bi omass gradient. Algal biomass and invertebrate density were much larger (5- fold and 10-fold, respectively) on mosses than on nearby rocks whereas inve rtebrate biomass was similar on the two substrata, because of a striking di fference in size distribution between the two communities. Algal and invert ebrate communities living on aquatic mosses are temporally stable and relat ed to the density of moss patches. Epiphytic invertebrate communities diffe r in density, size, taxonomy, but not necessarily in biomass, from communit ies on other substrata. The extent of moss cover can, therefore, alter stre am metabolism and trophic interactions.