POLYCHAETE ASSEMBLAGES IN A SOUTHERN SHALLOW SHELF AREA AFFECTED BY SEWAGE DISCHARGE
Elías, R.1 & Bremec, C. S.2
1Departamento de Ciencias Marinas, Universidad Nacional de Mar del Plata.Dean Funes 3350 (7600) Mar del Plata, Argentina.
2CONICET- Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP). c.c. 175 (7600) Mar del Plata, Argentina
A benthos sampling program was carried out on the coast of the Mar del Plata city area to assess the structure of macrobenthic communities of subtidal soft-bottoms and their relationship with sewage effluent. Polychaetes and molluscs were dominant in species number, while crustaceans were dominant in abundance, followed by polychaetes and molluscs. Sandy bottoms showed great heterogeneity in sediment composition but were uniformly low in organic matter content. Thirty-three polychaete species were identified in a depth range between 6 to 11 m. Although there is little information about polychaete composition in the area, a qualitative comparison with data from 30 years ago of adjacent areas is herein presented. The dominance of Owenia fusiformis was detected during this study, reaching 38.7 % in the sampling area, peaking in the stations in front of the sewage effluent. Although the area is enriched by suspended flocks from domestic sewage, the organic content of the sediment is very low (0.21 to 0.48 %). In spite of this enrichment, the polychaete fauna seems to be small in size. One possible explanation for the small size of local fauna is that the area is frequently disturbed by hydrodynamic forces, and the benthic fauna is permanently in an early stage of development. Recolonization following a complete defaunation due to disturbances caused by winds and storms is widely referred in the literature on benthic environments. In the shallow shelf of southern Brazil, it is suggested that after the winter defaunation, recolonization by opportunistic species takes place. Affinities with other environments (i.e. Chilean and Peruvian shallow shelves) was found, in relation to the composition and small size of the fauna.