Changes in epilithic diatom communities and periphytic biomass downstream of a reservoir on a Mediterranean river (Calabria region, S. Italy)

Diatom community structure and morphotype classification were studied and periphyton biomass, course particulate organicmatter (CPOM), hydrogeomorphological parameters, and nutrient concentrations were measured at 4 stations downstream of theCecita dam (Mucone River) and at 3 control sites located on different tributaries of the Mucone River in May, August, and November2005. The lowest values of current velocities and the highest amounts of sand deposition and CPOM accumulation were detected at thestations closest to the dam. Downstream, the confluence with the first permanent tributary (Cerreto River) of the Mucone showed a“rhythral” morphology, similar to that of the control sites. Multivariate analysis showed that the community was constantly segregatedinto 2 groups: G1 (stations upstream of the Cerreto inflow), dominated by early colonizers, adapted to both high and low currentvelocity and tolerating burial and light deficiencies; and G2 (all other stations), where the prevailing taxa were species typical of stableenvironments with high values of flow and abundance of coarse substrates. Genus-based morphotype classification failed to detect anydifferences between the 2 groups. Biomass levels were higher at station G2 than G1, while accumulation of CPOM downstream of thereservoir promoted fine particulate organic matter production and a switch from autotroph to heterotroph dominated biofilms.