Changes in a hydropsychid guild downstream from a eutrophic impoundment

Abstract

The structural changes induced by a small (0.015 km3) eutrophic hydropower impoundment, located in the middle Rio Duraton (northern Spain, Duero basin), on the downstream hydropsychid guild were studied by comparing physicochemical and biological characteristics of an upstream site with those of three downstream stations placed 0.2, 2.5 and 7.6 km below the dam. Species richness, species diversity and concentration of dominance were depressed downstream from the impoundment. Total biomass and density were also depressed at the station immediately below the dam. However, total density and biomass values were higher at sites further downstream than at the upstream reference site, presumably due to improved food supply and more available habitat. Abundances of Cheumatopsyche lepida, Hydropsyche sp. and H. pellucidula decreased significantly or completely disappeared at downstream sites. On the contrary, abundances of H. siltalai, H. exocellata and H. bulbifera increased downstream except at the station just below the dam. H. lobata was present at low densities at all sampling sites. H. pellucidula was the dominant species upstream, whereas H. siltalai became dominant downstream. The size structure of H. bulbifera, H. exocellata and H. siltalai experienced a modification below the impoundment as a probable consequence of the reduction in the number of competing hydropsychid species. It is concluded that the main physicochemical factors involved in the spatial variation of hydropsychid species were dissolved oxygen deficit and short-term flow fluctuations.