The downstream effects of Valparaíso Reservoir (Río Tera, north-western Spain) on macrophyte, macroinvertebrate and fish communities are examined by comparing their respective structures before (1986) and after (1990-1991) the creation of this hydropower impoundment. A single sampling station was selected 2.4 km below the dam. Macrophytes significantly (p < 0.05) decreased their total biomass, although the species composition remained dominated by two species (Myriophyllum verticillatum and Ranunculus fluitans). The macroinvertebrate community, on the other hand, exhibited a significant (p < 0.05) decrease in taxonomic richness, total density and total biomass. In general, planarians, ephemer-opterans, coleopterans, plecopterans and trichopterans disappeared or decreased their abundances. Scrapers (as relative biomass) were the functional feeding group most adversely affected by the new flow regulation. With regard to the fish community, the most significant change was the absence of all resident coarse fishes (cyprinids, primarily) at the sampling site during the 1990 and 1991 sampling surveys. Conversely, salmonids (Salmo trutta) persisted; the trout population was dominated by individuals of the 0+ age group (⩽ 1 year old) both before and after the construction of Valparaíso Dam. It is concluded that short-term flow fluctuations induced by hydroelectric power generation were the main factor causing the observed adverse effects. In this respect, some management measures to minimize these effects are recommended.
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