Groundwater-Driven Nutrient Inputs To Mar Menor Coastal Lagoon: An Overlooked Driver Of Ecosystem Degradation
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Coastal lagoons are systems with great ecological importance that support valuable ecosystem services. However, their restricted connection to the sea favors the accumulation of pollutants, making them vulnerable to external inputs. Mar Menor (SE Spain), the largest hypersaline lagoon in southern Europe, has recently suffered several eutrophication events linked to nutrient inputs. The lagoon is connected to an unconfined Quaternary aquifer, artificially recharged by irrigation from agriculture. Submarine Groundwater Discharge (SGD) flows through this nutrient-rich aquifer, acting as a transfer mechanism of solutes from land to sea. However, its contribution to the Mar Menor nutrient budget remains unknown. SGD comprises different pathways: fresh terrestrial discharge, long-scale seawater recirculation through the sediments, and short-scale porewater exchange (PEX). The chemical composition of each one is strongly shaped by its origin, groundwater transit time, and biogeochemical reactions occurring in the coastal aquifer before discharging into the ocean. In this study, we aim to quantify the contribution of each SGD pathway to the nutrient budget of the lagoon. We performed 3 intensive campaigns in March, July, and November 2021, where we estimated the magnitude of each SGD pathway and the associated nutrient fluxes (NO3−, NO2−, NH4+, Si(OH)4, PO43−). Our results suggest that recirculated SGD represents up to 99% of total groundwater inputs to the lagoon, and short-scale flows significantly increase in summer. A preliminary calculation of the PEX-derived nutrient release to the water column is estimated to be comparable to or exceed all other nutrient sources, emerging as the main source of these pollutants. This can be particularly important in summer, coinciding with eutrophication events. In addition, it seems to be predominantly enriched in reduced species such as NH4+, increasing oxygen demand in the lagoon. Resolving the SGD contribution to the nutrient budget is critical for predicting water quality in lagoonal ecosystems.