Impact of Submarine Groundwater Discharge on Microbial Plankton in a Tidal Crystalline Coastal System
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Submarine groundwater discharge (SGD) has been largely ignored in the tidal crystalline coast of the Atlantic arc of the Iberian Peninsula despite it can contribute to eutrophication of coastal waters. The Ría de Vigo is a productive coastal inlet, located in the northwest of the Iberian Peninsula. Coastal upwelling is considered the main allochthonous nutrient source to the Ría de Vigo, while SGD may represent up to 23% of riverine discharge to the system. Regardless of the magnitude of SGD to this coastal inlet, its impact on marine microbial dynamics remains unknown. Within this context, we assessed the response of marine microbial plankton growth (based on changes in phytoplankton and bacterial biomass) and taxonomic community composition (assessed using 16S and 18S rRNA gene metabarcoding) to SGD inputs of different magnitude and nature during the dry and the wet seasons by mixing seawater with different proportions (5%, 10%, and 20%) of porewater obtained from three subterranean estuaries in the Ría de Vigo (Cesantes, Panxón, and O Vao). Seawater and porewater controls were included to compare growth and community structure in the endmembers and in the mixed treatments. Phytoplankton growth was stimulated by porewater addition up to ca. 4-fold compared to the seawater control, while bacterial responses ranged from 0.8 to 1.4-fold. There was a clear spatial and temporal variability in the response patterns, related to the chemical composition of the porewaters. In the dry period, phytoplankton response to SGD was proportional to total dissolved nitrogen. During the wet period, prokaryote response to SGD was inversely proportional to nitrite concentration, suggesting a potential inhibitory effect. Our results highlight the variability of the impacts of SDG on coastal waters as well as the differential impact on phytoplankton and bacteria which might alter the services provided by the microbial community in this productive ecosystem.