The effect of regulation caused by a dam on the distribution of the functional feeding groups of the benthos in the sub basin of the Grande River (san Luis, Argentina).

The construction of small dams in principal streams is one of the most common forms of regulation in the province of San Luis since they cause changes of physical, chemical and biological nature downstream. The purpose of this study was to analyze the short-term modifications in the food organization of benthic macroinvertebrates communities due to the construction of a dam in the Grande River (San Luis, Argentina). Two sampling sites were established: one before the dam and another one after it. The samplings were carried out with Surber sampler and during an annual cycle extending from April 1997 to March 1998, and two complementary samplings were done in low and high waters. The field sampling design was stratified randomly, and 3 pseudo replicas were taken in a transect seasonally and monthly and were then averaged. Macroinvertebrates were classified in different functional feeding groups. The comparisons at the level of physical and chemical variables and absolute abundances of the functional groups were carried out by means of the Wilcoxon test for two related samples. The collector-filterers, scrapers and predators increase whereas the collector-gatherers and shredders decreased. There were significant differences at the level of gatherers and shredders.