Implementation of a portable module for assessing the eutrophication risk: initial evaluation in the upwelling-driven bay of Ria de Arousa (NW-Iberian Peninsula)

Abstract This study presents the implementation of a portable module designed for autonomous analysis of sea-surface inorganic nutrients onboard vessels of opportunity, as an additional tool for assessing the levels of eutrophication risk. The study was carried out during August-mid-September 2019 in the Ria de Arousa and outer shelf area (NW-Iberian coastal upwelling system). During this period, the distributions of the measured sea-surface concentrations of nitrate and phosphate were compared according to three Oceanographic Environments (OEs). The OEs were defined according to the interplay between upwelling/downwelling events and river discharge on the coastal system. The nutrient measurements agreed well with the OEs, showing that the portable module is a useful tool for opportune measurements of sea-surface nutrients and can serve as a complement for the available monitoring networks. An initial evaluation of the eutrophication risk in this area indicated low risk levels (following the Environmental European Agency criteria) for most of the measured points in summer, except for some vulnerable areas under certain OEs. Nutrient concentrations are sensitive to periods of Sustained Upwelling events, reaching medium risk levels (7.14 - 9.05 µmol L-1 for nitrate and 0.39 - 0.64 µmol L-1 for phosphate) in inner parts of the Ria de Arousa. These areas are characterized by abrupt bathymetric changes that channel and intensify the upwelling processes, increasing sea-surface nutrient concentrations. High eutrophication risk levels of phosphate (1.53 µmol L-1) were detected close to the coastline during Upwelling Relaxation periods. Under these conditions, continental flows, previously retained by the upwelling, are able to expand. The location of these samples and the difference in concentration between phosphate and nitrate indicate a most likely source in wastewater outflows. Our results highlight the need for deeper studies on the synergy between upwelling/downwelling processes and the continental water discharges and its modulation of sea-surface nutrients.