Population dynamics and production of Scolelepis gaucha (Polychaeta: Spionidae) on the sandy beaches of Southern Brazil

Scolelepis gaucha (Orensanz & Gianuca, 1974) is the most abundant polychaete species on subtropical and temperate intertidal beaches along the Atlantic coast of South America. The population and production dynamics of this species were described from samples taken between May 1988 and July 1989 at each of 4 intertidal beach transects near Patos Lagoon in Southern Brazil. S. gaucha presented a multiannual life cycle (with 2 cohorts each year) which was related to the diffusion pattern necessary to maintain its endemic distribution on a coast with strong seasonal processes. The spatial differences observed in mortality rates (Zvarying from 3.24 to 11.35 yr-l) were associated with variations in intertidal stability as expressed by the different sedimentological zones found between transects. Differences in growth and mortality were observed between the cohorts, caused by the temporal modification of beach profile wh~ch benefitted the food uptake of the first cohort. Compared with other spionid species on sandy beaches, high values of mean annual biomass (between 2 and 31 g AFDW m-') and production (between 6 and 113 g AFDW m-' yr-') were found. Beach morphodynamics was the major factor controlling both the mortality and growth patterns of S. gaucha and thus its production on the Southern Brazil sandy beaches.