Macroinvertebrates in neotropical streams: richness patterns along a catchment and assemblage structure between 2 seasons

We investigated macroinvertebrate richness in 10 streams of different sizes within the Carmo River catchment in Brazil. Specifically, we tested 2 predictions of the river continuum concept (RCC): 1) within the catchment, mid-sized streams (orders 3-4) have the richest biota, and 2) macroinvertebrate assemblage structure is more stable during the dry season than during the rainy season when natural spates are frequent. We sampled the streams using individual stones as sampling units. Observed and estimated values of richness did not follow the hump-shaped pattern of richness along a gradient of stream size as predicted by the RCC; the richest streams were smaller than those predicted. No difference in assemblage structure between seasons was found on the basis of observed and estimated richness or abundance. The similarity in assemblage structure between the rainy and dry seasons was also supported by multivariate analysis. Observed richness and species composition (reflected in multivariate analysis) were strongly correlated to stream size and the presence of fine sediments over rocks. Assemblage structure in these streams seems to be deterministic, in that richness and species composition are related to physical habitat characteristics.