Succession and collapse of macrozoobenthos in a subtropical hypertrophic lake under restoration (Lake Rodó, Uruguay)

Abstract

We studied the succession patterns of the benthic community following a whole-lake restoration experiment in a subtropical hypertrophic lake (Lake Rodo´ ,3 4� 55¢ S5 6� 10¢ W, Montevideo, Uruguay). The restoration measures involved diversion of the main inlet and removal of upper 1-m sediment and biomanipulation of the fish community. Between January 1997 and November 1999, we sampled sediments seasonally to analyse changes in benthos in relation to other abiotic and biotic characteristics of the system. The benthic community of the lake was composed of three families and nine genera. The maximum density (646 ind m )2 ), as well as the maximum taxonomic richness (six), were observed 1 month after the lake was refilled. Since 1998, the benthic abundance decreased considerably and continuously and a total absence of benthic organisms was registered by the end of the year. The low abundance of macroinvertebrates during 1997 could be explained by the food preferences of the dominant fish species, and the high fish biomass at the beginning of the biomanipulation process. However, the most relevant physico-chemical temporal patterns were the increase of organic matter and nutrients in the sediment and the fluctuations of oxygen and nitrate in the deepest layer of the water column. The disappearance of benthos was related to these temporal changes. These results stress the importance of the increase of organic matter for the changes in the physicochemical environment, and its importance in the benthic succession and possible collapse. We suggest that in hypertrophic lakes, the effects of organic matter enrichment in the sediment can be even more relevant than fish predation in shaping the zoobenthos.