Influence of sewage pollution on a rocky intertidal community dominated by the mytilid Brachidontes rodriguezi

Abstract

The characteristic Brachidontes rodriguezi community was absent from the highly polluted intertidal area surrounding a sewage outfall at Quequen (Argentina). It began to recover in lower tidal levels with better water renewal and shorter dessication periods. The chlorophyte Ulva lactuca was the most abundant organism at intermediate pollution levels, but reached its highest abundance in association with B. rodriguezi. The pulmonate limpet Siphonaria lessoni, the chlorophytes Cladophora sp. and Enteromorpha compressa, and blue-green algae increased in the vicinity of the outfall. A decrease in epilithic community biomass in the highly polluted area could be inferred from changes in percent cover, stratification and bare substratum proportion. Highest diversity was reached at intermediate pollution levels. Diversity decreased at higher levels due to low specific r~chness and also in relatively unpolluted areas due to space monopolization by B. rodriguezi. Therefore, sewage pollution could be regarded as a disturbance factor preventing space monopolization by the most successful competitor Sewage effects on the benthic community were similar to the decrease in abundance observed towards upper intertidal levels. Community structure proved to be a better pollution indicator than seawater variables, since it can be considered as reflecting an average of varying environmental conditions.