Submarine groundwater discharges in the Creus Cape (NW Mediterranean): new data for an hydrodynamical and biochemical sea model

Abstract

Eutrophication is a process driven by enrichment of water by nutrients, especially compounds of nitrogen and phosphorus, leading to: increased primary production, changes in the balance of organisms, and water quality degradation. It occurs naturally; however, human activities have accelerated its rate and extent through both point-source discharges and non-point loadings of limiting nutrients into aquatic ecosystems. Since the beginning of the XXIst century, concern regarding eutrophication led to the adoption of policies aimed at combating eutrophication, such as the Water Framework Directive and the Marine Strategy Framework Directive. To implement these policies and achieve their goals, the proper comprehension of aquatic ecosystems structure and dynamics and the exchanges occurring among them are essential; in other words, to provide an integrated vision of the hydrosphere is key. Under the umbrella of the AquaINFRA project (https://aquainfra.eu/), we are developing a new set of hydrodynamical and biogeochemical simulations of our case study, the Catalan coast (NW Mediterranean), to assess the risks and hazards to its coastal ecosystems. A regional circulation model (BFMcoupler; see Galiana, S. et al. at Session OS2.1) that takes into account continental inputs from both point and non-point source discharges is used, including from riverine to submarine groundwater discharges (SGD). This work aims to focus on the relevance of SGD and their seasonal pattern in the north of the Creus Cape, the northernmost part of the Catalan coast. The coastline of this zone is characterized by Mediterranean dry fields, fragmented  forests, few small urban areas and small catchment zones of temporal streams. It shows a 591 mm mean annual precipitation and 14.8 ºC mean annual air temperature. Data, collected from 2011 to 2016 (following Directives requirements) in five sampling stations located at 1m from the coast, reveal lower mean salinity (36.24) and higher mean nitrate (8.63 µM) and silicate (8.10 µM) concentrations compared with zones of similar characteristics, such as the Montgrí coast located further south (three sampling stations; 37.61; 3.18 µM; 2.23 µM), which indicate the presence of SDG. However, their mean chlorophyll-a concentrations are similar, 1.06 and 1.11 µg/l, which implies that primary production is not enhanced by nutrients. The seasonal pattern of this zone is similar to that observed at open and surface Mediterranean waters, except for the decrease in salinity and the increase in nitrate during autumn and winter, typically rain and wet periods, respectively. Therefore and according to Garcia-Solsona, E., et al. (2010), SDG are significant in coastal waters of the Mediterranean Sea, where their influence is more prominent in coastal zones without the influences of urban areas or rivers. Currently, these SDG data are being included in the hydrodynamical and biochemical model of the AquaINFRA project for a better understanding of the eutrophication process in coastal waters. Thereby, they will provide information for the European Directives and, thus, improve the integrated management of aquatic ecosystems under the ecosystem approach. ------------------------------------ (This project has received funding from the European Commission’s Horizon Europe Research and Innovation programme under grant agreement No 101094434)