Are eulittoral assemblages suitable for detecting the effects of sewage discharges in Atlantic and Mediterranean coastal areas?

The search for cost-effective indicators able to detect anthropogenic impacts is an essential prerequisite to plan mitigation interventions, which can prevent further deterioration of coastal areas. Eulittoral assemblages are accessible for sampling, and their dominant species are easy to identify and widely distributed. This study assessed the potential of eulittoral assemblages to reflect environmental perturbations related to sewage discharge across different environmental and geographical locations. Sampling occurred in impacted (near sewage discharge) and control areas, on both Atlantic and Mediterranean shores. Multivariate analyses showed that the structure of eulittoral invertebrate assemblages differed significantly between control and impacted areas for the Mediterranean, but not for the Atlantic case study. Macroalgae responses to sewage discharge were found on the Mediterranean shore with a significantly higher abundance of Chlorophyta near the outfall. Limpet populations of the genus Patella appear to be affected by sewage discharge. The response of limpet populations differed between Atlantic and Mediterranean shores, probably due to among-species differences of ecological traits. Our results suggest that the response of eulittoral assemblages and limpets to sewage discharge might be strictly context-dependent, thus limiting their potential to serve as general indicators in monitoring the effects of this source of human impact over large spatial scales.