DOWNSTREAM EFFECTS OF A NEW HYDROPOWER IMPOUNDMENT ON MACROPHYTE, MACROINVERTEBRATE AND FISH COMMUNITIES

The downstream effects of Valparaiso Reservoir (Rio Tera, north-wester n Spain) on macrophyte, macroinvertebrate and fish communities are exa mined by comparing their respective structures before (1986) and after (1990-1991) the creation of this hydropower impoundment. A single sam pling station was selected 2.4 km below the dam. Macrophytes significa ntly (p < 0.05) decreased their total biomass, although the species co mposition remained dominated by two species (Myriophyllum verticillatu m and Ranunculus fluitans). The macroinvertebrate community, on the ot her hand, exhibited a significant (p < 0.05) decrease in taxonomic ric hness, total density and total biomass. In general, planarians, epheme ropterans, coleopterans, plecopterans and trichopterans disappeared or decreased their abundances. Scrapers (as relative biomass) were the f unctional feeding group most adversely affected by the new flow regula tion. With regard to the fish community, the most significant change w as the absence of all resident coarse fishes (cyprinids, primarily) at the sampling site during the 1990 and 1991 sampling surveys. Converse ly, salmonids (Salmo trutta) persisted; the trout population was domin ated by individuals of the 0+ age group (less-than-or-equal-to 1 year old) both before and after the construction of Valparaiso Dam. It is c oncluded that short-term flow fluctuations induced by hydroelectric po wer generation were the main factor causing the observed adverse effec ts. In this respect, some management measures to minimize these effect s are recommended.