Submarine groundwater discharge as a source of nutrients and trace metals in a Mediterranean bay (Palma Beach, Balearic Islands)

Submarine Groundwater Discharge (SGD) from a detrital coastal aquifer into the adjacent marine environment was investigated in a Mediterranean bay (Palma Beach, Balearic Islands). In this region, agriculture and tourism are potential sources of groundwater contamination. A survey in the Palma Beach revealed N, Fe and chlorophyll a enhancement associated to areas of preferential groundwater discharge from the nearby coastal aquifer. Groundwater sampling from wells and coastal piezometers indicated high concentrations of dissolved inorganic nitrogen and Fe (up to 2800 μmol L− 1 and 8100 nmol L− 1, respectively). Other nutrients, such as DIP, and trace elements were not particularly elevated, which is attributed to the adsorptive characteristics of the carbonated composition of this detrital aquifer and/or the lack of major sources. Cross-shore gradients of 223,224,226Ra isotopes indicated a diffusive shore-based source of these radionuclides and allowed estimates of a SGD flow of 56,000 ± 13,000 m3 d− 1. Our results show that SGD is a major pathway for delivering DIN (1900 mmol m− 1 d− 1), dissolved Fe (4.1 mmol m− 1 d− 1) and, to a lesser extent, DIP (16 mmol m− 1 d− 1) into the nearshore waters. This allochthonous input may sustain a substantial phytoplankton biomass resulting in an onshore-offshore gradient (4.7-7.1 mg m− 3 in nearshore seawater as compared with < 1 mg m− 3 in offshore stations). This work emphasizes the relevance of SGD-driven nutrient and trace metal inputs in the regulation of nearshore phytoplankton communities of oligotrophic areas.