Variação da diversidade e composição de macroinvertebrados em riachos da bacia hidrográfica do Rio dos Sinos, RS, Brasil

Abstract

The aquatic macroinvertebrate community can be structured by processes in multiple spatial scales. Reduction or substitution of riparian forest by different land uses and the impacts on the physical and chemical conditions of streams can negatively affect the macroinvertebrate community causing homogenization of composition. Therefore, we assessed the following hypotheses. Article I) Macroinvertebrate richness and EPT (Ephemeroptera, Plecoptera and Trichoptera) richness decrease with reduced riparian zone width; macroinvertebrate and functional feeding group (FFG) composition vary with a reduced riparian zone width; and reduced riparian zone width similarly influence the macroinvertebrate community in different stream substrates. Article II) Land use (forest, agriculture, pasture and urbanization) alters physical and chemical variables in streams; and β diversity decreases, or the macroinvertebrate composition is more homogeneous, in stream reaches impacted by land use. Article III) γ diversity is not homogenously distributed among scales, with differences between the pattern observed in winter and summer; and β diversity depends on different substrate types, but most composition dissimilarity occurs in larger spatial scales, regardless of the season analyzed. In order to obtain these responses we selected reaches of three headwater streams in the Sinos River watershed, northeastern southern Brazil. The macroinvertebrate composition on litter substrate differed from that on riffle (stone and gravel) substrata, showing proportionally lower diversity within the lowest scale (α1) and higher diversity at the highest scale (β4). We found differences in the composition (β diversity) between the reach scale and the stream scale in both winter and summer. There were significant differences in macroinvertebrate composition among riparian zone widths. The macroinvertebrate composition and FFG differed among substrates, independent of riparian zone width. The physical and chemical stream variables varied according to the land use intensity. A homogenization of the community, or reduced β diversity, occurred with increased stream velocity, ammoniacal nitrogen and total phosphorous, and higher heterogeneity occurred with more shade and higher pH. Our results revealed that streams in southern Brazil support a very heterogeneous macroinvertebrate community and the role of spatial configuration in these streams strongly influences macroinvertebrate diversity. A riparian width greater than 15 m is necessary to maintain composition and trophic conditions of macroinvertebrate families found in more pristine states of conservation. Furthermore, the results emphasize the importance of β diversity studies for evaluating the homogenization of macroinvertebrate community in streams impacted by different land uses in southern Brazil.