Influence of riparian forests on fish assemblages in temperate lowland streams

Abstract

The characteristics of riparian vegetation along streams vary with natural and anthropogenic factors. Deforestation for agricultural purposes has consequences for the physical in-stream structure and function, such as the predominance of autotrophic or heterotrophic stream metabolism. Open canopy lowland streams are often dominated by macrophytes, with potential direct and indirect effects on the fish community. We tested for possible differences in the structure (relative abundance of species, mean body size, and density) and composition (species richness, species identity, and different trophic groups) of fish assemblages between open canopy streams (OCS) and riparian forest streams (RFS), including pool and riffle habitats, in temperate lowland Denmark. OCS reaches exhibited higher alpha and beta diversity and frequently hosted rare species. Almost 50 % of the recorded species appeared only in OCS. OCS also had smaller mean body size of fish and tended to have higher fish densities. The relative abundance of the different trophic groups did not differ between the two streams types, but the RFS had a higher abundance and occurrence frequency of intolerant salmonids. Our results suggest that modification of riparian habitats can affect richness patterns and that strong functional changes may occur as a consequence of forest clearance through changes in the relative importance of a keystone species, trout (Salmo trutta).