Seasonal changes of abiotic and biotic gradients downstream a multiple use reservoir in a mediterranean river

The effect of regulated flow regimentation on hydrogeomorphological variables, autochthonous and allochthonous trophic sources and macroinvertebrates communities of the Mucone River, Calabria (Italy) were studied in May, August and November 2005 at three study sites. The first site (M1) was located close to the impoundment, the second one (M2) immediately upstream from the first permanent tributary (Cerreto stream) and the third one (M3) about 1 km downstream from the Cerreto confluence. Our results suggest that hydrogeomorphological gradients already exist in spring but their strongest development, with major consequences for the lotic biota, take place only with the beginning and ending of the dry season. Reduction of river width, shading due to well preserved riparian alder trees and marked sand deposition in November, were the variables negatively affecting epilithic microalgae biomass at site M1. These factors partially relieved at M2 site and became much less important at M3 resulting in the highest levels of autotrophic periphyton production.Coarse particulate organic matter accumulation exhibited an opposite trend due to the lack of discharge peaks at M1, whereas in November, litter fall removed most differences between stations. In May macrozoobenthic communities of the three stations were relatively similar (reophilous gatherer and scraper taxa). In November sand accumulation reduced spatial heterogeneity and habitats density close to the Cecita dam promoting settlement of shredder sand tolerant taxa (Plecoptera: Leuctridae) and a drop of community biodiversity. Flow reactivation, especially after Cerreto confluence, hindered sand accumulation, favoured autochthonous primary production and increased energy sources for macroinvertebrates.