Immediate effect of sewerage improvement on the phytoplankton and physicochemical conditions in the Urdaibai estuary (southeastern Bay of Biscay)

Abstract

Anthropic nutrient enrichment has become a major environmental issue in estuaries around the world, being the effluents of wastewater treatment plants (WWTPs) one of the main causes. Phytoplankton is, together with nutrients, a useful indicator for the ecological status assessment of estuaries due to its basal position in the food chain. The present study aims to evaluate the immediate effect of the cessation of the wastewater discharges on the environmental conditions and phytoplankton community along the eutrophicated Urdaibai estuary (southeastern Bay of Biscay). Thus, a short-term comparison of the physicochemical conditions and the phytoplankton abundance and community composition (by pigment analysis and microscopy) was carried out before and after the sewerage works. Results confirmed that the cessation of wastewater discharges had an immediate effect in the estuary, mainly noticed in the inner part. The abrupt decrease of ammonium (from 72.3 to 18.2μmol/L) and phosphate (from 3.5 to 2μmol/L) concentrations led to the improvement of the water status, reaching values lower than the “good” status threshold along the whole estuary. The phytoplankton community composition also showed and immediate reaction in the surroundings of the WWTP, with the chlorophyll b becoming the dominant secondary pigment in the area after the diversion due to the immediate and rapid increase of Eutreptiella sp. However, since the sampling-period of this study did not exceed a month after the cessation, a longer study-period is necessary, including interannual studies, to test the real effect of the sanitation works in the Urdaibai estuary.