Abundance and Distribution of Profundal Zoobenthos in Spanish Reservoirs: Differences between 1973/75 and 1987/88 Surveys

Abstract

This paper deals with the abundance and distribution patterns of profundal zoobenthos from 81 Spanish reservoirs which were sampled during mixing 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 benthic groups were considered: Oligochaeta, Tanypodinae and other Chironomidae (basically Chironominae with some Orthocladiinae). While differences between reservoirs were highly significant for all three groups, differences between basins were only reflected by chironomids. Differences between surveys depended on the benthic group and the season. Between 1973/75 and 1987/88, Oligochaeta densities halved significantly in winter (all basins), tanipodins increased 1.7-fold in summer (northern and central Spain), while other chironomids significantly increased 1.8-fold in summer (northern and central Spain) and 1.6-fold in winter (all basins). The increase in chironomids accompanied by the decrease in oligochaetes was partly attributed to better oxygen conditions in the last survey.