Submarine Groundwater Discharge (SGD) supplies large amounts of solutes to the ocean. In coastal areas, the confluence of fresh and saline groundwater results in subterranean estuaries (STEs), biogeochemically active sites in which biogeochemical transformations mediated by microbes can control SGD-driven nutrient fluxes to the ocean. However, very little is known about microbial communities in STEs and their drivers. Our aim was to explore the spatio-temporal variations in prokaryotic abundance and heterotrophic activity along a STE in a Mediterranean alluvial aquifer. Groundwater was collected from several piezometers (depths 6-22m) along the entire salinity gradient in two different hydrological seasons. Prokaryote abundance and activity (inferred by single-cell activity measurements and radioactive leucine incorporation) were compared with different physicochemical parameters. Results show large spatial variations in prokaryotic abundance and activity following physicochemical gradients, but little seasonal variability. Prokaryote abundance was maximum at the deepest saline layers and lowest in intermediate fresh groundwater. More active cells in saline samples were observed, which coincided with increases in nitrate and ammonium, suggesting microbial transformations of groundwater along STEs. Our results indicate that prokaryotic communities in coastal aquifers are highly heterogeneous in terms of biomass and activity, implying that their role in nutrient transformations will vary throughout STEs and calling for detailed spatial explorations of these key ecosystems

ASLO Aquatic Sciences Meeting, Resilience and Recovery in Aquatic Systems, 4-9 June 2023, Palma de Mallorca, Spain