Water chemistry and macroinvertebrates were studied in the River Tera (north‐western Spain) to assess the effects on the river of two chained reservoirs with hypolimnetic release during the stratification period in 1991. The river‐water variables most affected by the dam in this study are temperature, pH, conductivity, silicate, ammonia, nitrate and phosphate. Of these, temperature, with more constant values in the river after the dams, in contrast with diel and seasonal cycles, generates a special environment (short thermal amplitude) several kilometres downstream. On the other hand, ammonia, owing to the magnitude of its changes, seems to be the main factor affected by the dams, and a modifier of river water quality, although its values do not seem to be critical. Although dissolved oxygen is greatly affected by the dams, particularly during stratification conditions, bottom release by hollow‐cone solves the problem of reoxygenation downstream from the dam. Unregulated stations presented the most diverse macroinvertebrate communities and also the highest values of the biotic index BMWP', typical of very clean waters. The stations influenced by the dams showed lower values, but with a gradual trend towards recovery downstream from the dam. At 10 km below the reservoir system, benthic communities showed an important recovery. Some management tools related to the ecological problems caused by this type of reservoir are discussed. © 1997 John Wiley & Sons, Ltd.

TROPHIC STATE EVALUATION AND COMPARISON OF MODELS RELATED TO PHOSPHORUS IN CERNADlLLA AND VALPARAISO RESERVOIRS (TERA RIVER, ZAMORA)Cernadilla (1965) and Valparaíso (1988) reservoirs, located in the river Tera which is a tributary within the Duero basin (Zamora), are used to produce electric energy and provide irrigation. The trophic state of the headwater reservoir (Cernadilla) has became worse since Valparaíso construction. In 1991, the interna1 phosphorus loading was 120,97 mg TP.m summer in Cernadilla, and 30,92 mg TP.m2 summer in Valparaíso. These estimates were calculated from the increased mass of hypolimnetic phosphorus divided by the surface area of the reservoir. The rates of phosphorus release from anoxic sediment surfaces were 14,40 and 6,2 mg TP.m2 d ', in Cernadilla and Valparaíso respectively. Both reservoirs were classified as mesotrophic in 1991.

Monitoring of the quality of a drinking water supply reservoir according to the Water Framework Directive (A ñarbe reservoir, North basin)The Añarbe reservoir is located in the Iberic-Macaronesian region and is a water body provisionally identified as heavily modified, comparable to 'lake', that according to the "A" classification system of the WFD is "siliceous, lowlands, big and very deep", and according to the "B" system is ¨lowlands, northern, warm monomictic and of acidic waters". Two annual cycles of studies have been completed (from May of 2004 to January of 2006) looking to the main biological, physicochemical and hydromorphological parameters. The trophic state is oligotrophic for most of the parameters, except for the total phosphorus, that classifies it as mesotrophic. It does not present algal blooms and the average oxygen concentration in the hypolimnion in the zone of the dam during the stratification is 4.5 mg O 2 /l. The percentage of variation of the water volume of the reservoir respective to the maximum volume varies between 40 and 33 % for the years 2004 and 2005, respectively. It shows a good ecological potential and could be considered as a reference within its group.

No abstract