Mollusc communities along upstream–downstream gradients in small coastal basins of the south-western Iberian Peninsula

Abstract

Freshwater mollusc communities readily respond to various human-induced stressors, and thus are appropriate models for studying the effects of such stressors on the structure and dynamics of stream macroinvertebrate communities. This paper examined the distribution of freshwater molluscs in 35 stream reaches of 18 small coastal basins in the south-western Iberian Peninsula. Using several multivariate techniques, I showed that mollusc distribution mainly responded to gradients in drainage area, water availability, pollution and salinity. Upstream and downstream communities were clearly differentiated, with the former dominated by freshwater species (Bithynia tentaculata, Galba truncatula, Radix balthica, Ancylus fluviatilis and Planorbarius metidjensis) and the latter by species typical of brackish waters (Peringia ulvae and Myosotellamyosotis). There was a clear decrease in species richness from upper to lower reaches. The conservation of mollusc communities of these small basins requires a deeper understanding of their ecological requirements, effective control of urban discharges and an analysis of their interaction with invasive species.