Distribution patterns and trophic structure of soft-bottom polychaete assemblages in a north-western Mediterranean shallow-water bay

This study describes the spatial distribution and trophic structure of the polychaetes inhabiting the soft-bottoms of Alfacs Bay, a semi-enclosed, shallow-water area (Ebro Delta, north-western Mediterranean). The hydrographic regime is basically marine, with a combination of seawater renewal and continental runoff causing permanent stratification. The Bay was affected by anthropogenic activities (viz. influx of irrigation waters, large-scale aquaculture, and periodical dredging at the mouth). 41 samples were taken during summer (July 1987 and September 1992). 101 polychaete species were identified, with an average density and biomass of 6370 (650-23700) ind..m(-2) and 7.52 (0,87-66) DW g.m(-2) respectively. Nine assemblages were identified: shelf (stressed, confined, typical), boundary deep central basin (inner, typical, seaward) and marine (typical, navigation channel). The trophic structure was dominated by deposit-feeding polychaetes (mainly Pseudomastus deltaicus, Heteromastus filiformis, Notomastus latericeus, Mastobranchus trinchesii, Euclymene oerstedii, Clymenura clypeata, Streblospio shrubsolii, Monticellina heterochaeta and Ampharete acutifrons). An increase in the ecological weight of carnivorous polychaetes (mostly Lumbrineris latreilli) from the shelf towards the marine assemblages was also noticed. The analysis of die polychaete taxocoenosis provides key items to understand the functioning of the semi-enclosed, shallow-water system under study