Submarine groundwater discharge to the coastal environment of a Mediterranean island (Majorca, Spain): Ecosystem and biogeochemical significance

This article reports the results of a study of submarine groundwater discharge (SGD) to coastal waters of Majorca (NW Mediterranean). The overall aim is to evaluate the relevance of SGD of the island and chemically characterize the components that are supplied to the coastal waters through this pathway. Although other discharge areas are identified, we particularly focus on SGD in bays and areas of increased sea water residence time where effects of the discharges are expected to be most notable. Analysis at four selected embayments with different land-use characteristics indicated a link between human activities (mainly agriculture and urban) and compounds arriving to the coast. A pathway for these elements is the diffuse discharge along the shoreline, as suggested by the inverse relationship between salinity and nutrients in nearshore porewaters. A general survey was conducted at 46 sites around the island, and used dissolved radium as a qualitative indicator of SGD. Measurements of nutrients (P and N), pCO₂ and TOC were performed to characterize the elements delivered to the coastal environment. Most nearshore samples showed ²²⁴Ra enrichment (mean ± SE, 7.0 ± 0.6 dpm 100 l^-1) with respect to offshore waters (1. 1 ± 0.2 dpm 100 l^-1); however, ²²⁴Ra measurements along the coast were highly variable (1.0-38.1 dpm 100 l^-1). Coastal samples with enhanced radium levels showed elevated pCO₂ with respect to atmospheric concentrations, which together with high pCO₂ in groundwater (>5,000 ppm) indicates that SGD is an important vector of CO₂ to coastal waters. Moreover, a relationship between ²²⁴Ra and phytoplankton biomass was established, suggesting an important impact of SGD on coastal productivity. The results presented here provide a first approximation of the SGD effect in the coastal waters of Majorca, and indicate that SGD could be an important source of nutrients and CO₂ to the coast, strongly influencing the productivity and biogeochemical cycling of the coastal waters of Majorca.