A polychaete-dominated community in the NW Mediterranean Sea, 20 years after cessation of sewage discharges.

Abstract

Benthic recovery following cessation of wastewater discharges in shallow soft-bottom environments off Barcelona was assessed by revisiting an old sampling site in 2008 that had been studied when the impacts of such discharges were more acute (1987–88). In 1987–88, sediments were highly polluted by organic matter and inorganic contaminants. Although the amount of silt–clay in the sediments diminished from 95 to 25% during the intervening 20-year period and significant improvements were observed in the content of polychlorinated biphenyls and polycyclic aromatic hydrocarbons, their metal content was still very high. However, the benthic community currently showed a clear increase in complexity and diversity. It changed from an assemblage that was 98%-dominated by the Capitella capitata complex to an assemblage dominated by Mediomastus fragilis, Capitella capitata and Ophryotrocha hartmanni. By 2008, more species were found and there was increased representation from different trophic groups, whereas the abundance and biomass values were clearly reduced by almost two and one order of magnitude, respectively. Mean annual density evolved from 385,261 ind. m–2 with a mean annual biomass of 12.75 g dry wt m–2 in 1987–88 to 8155 ind. m–2 and 0.94 g dry wt m–2 in 2008. Using a regression model that allowed comparability between both sets of data, secondary production of the community was reduced from 207.7 g dry wt m–2 year–1 in 1987–88 to 8.0 g dry wt m–2 year–1 in 2008. The organic input decreased, but the metal concentration present in the sediments may inhibit their full recovery to normal conditions.