Analysis of the effects of wet and dry seasons on a Mediterranean river basin: Consequences for coastal waters and its quality management

•Phytoplankton and nutrients were studied in the receiving waters of a Mediterranean river.•Dry and wet scenarios were compared in order to characterize the nutrient sources.•During storm events wastewater discharge was a source due to combined sewer overflow.•During summer untreated effluent was directly discharged due to the increased population.•WFD monitoring programs should take into account combined sewer systems location.

Rivers play a major role in the delivery of nutrients to coastal ecosystems which are essential for ecosystem productivity. However, the increase of nutrients due to anthropogenic activities can cause eutrophication problems. This study analyzes the seasonal variation of phytoplankton communities in the coastal receiving waters of a Mediterranean river. Two scenarios are compared: the wet and the dry season with distinctive characteristics. During the wet season agricultural runoff and combined sewer overflows (CSO) were responsible for nutrient discharges, while during the dry season partially treated effluent from wastewater was the main nutrient source. In the receiving waters, diatoms typical seasonal cycle was modified by CSO discharges during rain episodes, while dinoflagellate abundance was higher in the dry season due to partially treated effluents discharges and low turbulence. We recommend that the design of the Water Framework Directive monitoring programs should take into account wastewater treatment plants and combined sewer systems located near the coast. Management decisions should take into account that only reductions in CSO and partially treated summer effluent are likely to be efficient in the short term. Analyzing the corrective measures cost through a cost–benefit analysis would help to determine whether the costs are excessive or not.