Enhanced growth in juvenile Nereis diversicolor after its exposure to anaerobic polluted sediments

Organic enrichment and anaerobic sediment conditions are the dominant anthropogenic influences on the Bilbao Estuary, resulting in the elimination of all benthic aerobic activity along 3/4 parts of its watercourse bed. To simulate the recolonization of azoic sediments by the native polychaete Nereis diversicolor, once water-oxic conditions in the estuary improve, two series of juvenile survival and growth assays were conducted, both, respectively, with and without the addition of exogenous food. Long-term exposures (about 30 days) of juveniles to azoic sediments caused no mortalities. On the contrary, an increased growth occurred in both types of tests, indicating the existence of a higher nutritional quality in the anaerobic sediments of the estuary. The ecological implications of our findings are clear; azoic sediments may be re-populated by juvenile invaders of N. diversicolor, provided that water-oxic conditions improve, through the implementation of a sewerage treatment plan promoted by the local Water Authority.