CONICET | Buscador de Institutos y Recursos Humanos

The Bahía Blanca Estuary receives untreated sewage from a growing urban population. To investigate whether the contribution of allochthonous organic matter affects the mesozooplankton community, surface tows were made in the channel which receives the sewage input (Canal Vieja, CV) and in a control site (Bahía del Medio, BM). Sampling was conducted during summer and winter of 2009. As expected, the disturbed site (CV) presented significantly higher values of nutrients (except silicates), ammonium and temperature and lower values of pH, dissolved oxygen and salinity. Phaeopigments were consistently higher at BM. Although both sites presented similar zooplankton species composition, dominance was different. The native copepod Acartia tonsa dominated during summer at BM, showing its maximum abundance there (9,610 ind. m-3) coinciding with historical data. At CV, A. tonsa codominated with the crab Neohelice granulata zoeae, the latter with a maximum abundance of 1,383 ind. m-3. Observations of the microheterotrophic community yielded information on typical assemblages, and indicated higher abundances at BM. Extremely high abundance of aloricate ciliates, especially Lohmmanniella sp., was observed at CV. High turbidity, coupled with the contributions of the vast expanse of surrounding salt marshes, contribute to the elevated basal eutrophication level of this well mixed system. Zooplankton in this region are thus likely to be adapted to high organic matter content. Nevertheless, some differences in planktonic variables between the sites were observed. These may be the first biological signs of an allochthonous eutrophication process. However, the geomorphology and circulation patterns of this estuary are likely to minimize the effect of the untreated sewage discharge.