Size structure of benthic invertebrate assemblages in a Mediterranean river

We examined the temporal, longitudinal, and among-substrate variation of the size structure of the invertebrate assemblage in a Mediterranenan river (Aniene River, central Italy). Sampling was conducted with quantitative methods on different substrates every 2 mo at 2 sites for 1 y and at 9 additional sites in spring 1997. Water-quality variables, periphyton standing crop, and physical characteristics of sites were also recorded. The Aniene River showed large longitudinal changes in overall physical features, periphyton biomass, and water quality that were reflected in dramatic changes of the taxonomic composition of the invertebrate assemblage. Insects were numerically dominant in the upper reach and codominant with gastropods in the middle reach, whereas oligochaetes, crustaceans, and chironomids were predominant in the lower and polluted reach. Despite these changes, the shape of the size structure of the invertebrate assemblage was similar among sites, dates, and substrates, with limited departures from the average size spectrum. Body mass alone accounted for the largest part of the variance of the abundance per size class (48%) whereas date, site, and substrate accounted for an additional 13%. These data support early observations of size spectrum invariance from North American streams and rivers, reinforcing the view that size-dependent processes may structure lotic benthic assemblages.