MACROBENTHOS STRUCTURE IN THE WATERSHED OF A RIVER OF CENTRAL ITALY

A study was undertaken to define the benthic populations, the environm ental-ecological variability and trophic aspects according to the func tional feeding groups in the upper reaches of the Nera River, a tribut ary of the Tiber, in central Italy.; A total of 12 681 individuals was collected and 83 taxa were identified; 11 proved to be endemic for It aly, 7 of them typical of the central Apennines. The greatest number o f taxa (58), and the highest values of diversity were recorded at the lowest sampling station, 37 km from the source. A dendrogram construct ed by the UPGMA method based on values of similarity revealed two main clusters, one identified with crenon (at the source) and the other wi th rhithron (the downstream stations) fluvial typology. Correspondence analysis on the environmental matrix parameters confirmed this findin g. Taxa abundance was evaluated by grouping the individuals collected into classes using semi-quantitative sampling. The results of trophic- functional analysis, calculated on the basis of the estimated abundanc e and tested with statistical methods, agreed with fauna composition a nalysis clusters for the filtering collector and shredder categories. The zonal and clinal patterns reveal that there may be a weak upstream to downstream gradient within a single fluvial zone and almost discon tinuity in contiguous tracts when the abiotic gradient changes-suddenl y.