Polychaetes from a southwestern shallow shelf Atlantic area (Argentina, 38degrees S) affected by sewage discharge

A benthos-samp ling program was carried out in the coastal area of Mar del Plata city in order to know the composition and distribution of infaunal po lychaetes living on a subtidal soft-bottom substrate affected by the sewage discharge. Sandy bottoms showed heterogeneous sediment composition and ver y low organic matter content. Thirty three polychaete species were identifi ed in a range depth between 6.5 to 12.5 m. A comparison with data from 30 y ears ago from adjacent areas deeper than 12 m is herein presented. Polychae te distribution was related both to the type of sediment and to the content of organic matter. Owenia fusiformis Delle Chiaje was dominant and peaked at stations near the sewage effluent. Small-sized polychaetes were abundant in the sampling area. A possible explanation to the faunal composition of sites near sewage effluents is that hydrodynamic forces frequently disturb the area, and benthic fauna is permanently in an early stage of development . The effect of sewage appeared to be restricted to the shallower subtidal area around the outfall.