As compared to natural areas, benthic communities exposed to organic pollution are characterized by low diversity and high variability. In Mar del Plata area the intertidal community developed around the sewage effluent is dominated by the mytilid Brachidontes rodriguezii. A quantitative sampling were performed in two tidal levels in both impacted and control sites, during Spring 1997, Summer, Autumn, Winter and Spring 1998, and Summer 1999, with environmental variables of water and sediments. The non parametric analysis showed highly significant differences among stations and between tidal levels due to the dilution gradient, but in the most impacted site the organic pollution minimized differences between levels. In addition, the classical reduction in diversity was not evident close to the effluent, and the impacted areas showed more diversity than natural areas. The control area and the most impacted station showed a low dispersion index (a measure of variability), while the intermediate stations showed the highest dispersion index. The analyses also showed that this bizarre behaviour occurred in time, and the control site evidenced more variability than impacted areas. This spatial and temporal paradox in the B. rodriguezii intertidal community can be explained by a high richness in the impacted areas due to the organic enrichment that produces the presence of species that tolerate organic pollution and a decrease in the mussel dominance. Seasonal variation was also minimized in the impacted area, except in summer.