ABUNDANCE AND DISTRIBUTION OF PROFUNDAL ZOOBENTHOS IN SPANISH RESERVOIRS - DIFFERENCES BETWEEN 1973 75 AND 1987/88 SURVEYS/

This paper deals with the abundance and distribution patterns of profu ndal zoobenthos from 81 Spanish reservoirs which were sampled during m ixing and stratification periods in 1973/75 and again in 1987/88. Our main aims are to explore the regional variability and to compare these two surveys, separated by 14 years. The densities of three major bent hic groups were considered: Oligochaeta, Tanypodinae and other Chirono midae (basically Chironominae with some Orthocladiinae). While differe nces between reservoirs were highly significant for all three groups, differences between basins were only reflected by chironomids. Differe nces between surveys depended on the benthic group and the season. Bet ween 1973/75 and 1987/88, Oligochaeta densities halved significantly i n winter (all basins), tanipodins increased 1.7-fold in summer (northe rn and central Spain), while other chironomids significantly increased 1.8-fold in summer (northern and central Spain) and 1.6-fold in winte r (all basins). The increase in chironomids accompanied by the decreas e in oligochaetes was partly attributed to better oxygen conditions in the last survey.