With their permeable sandy substrates, beaches represent potential hot spots of subterranean mixing between fresh groundwater and intruding seawater, resulting in the formation of subterranean estuaries (STE) that discharge into coastal waters. Current scientific challenges consist in characterizing microbially mediated biogeochemical transformations occurring in STE and their spatiotemporal variability, as well as evaluating the potential of such nutrient-rich groundwaters to contribute to coastal food web functioning in case of ground water discharge. The hydrological and biogeochemical spatiotemporal variability of a French Mediterranean superficial STE was evaluated through nine sampling campaigns conducted in 2023 and 2024. Salinity ranged from 1 to 40, with a diversity of situations suggesting sea water intrusion as well as of discharge into coastal seawater. Based on salinity in nearby seawater, discharge of groundwater was estimated to represent up to 8.5% of coastal water. Along with these variable conditions, water residence time in the STE was estimated to range between 1 and 20 days thanks to the analysis of radium isotopes. Oxygenation levels varied from near saturation to anoxia. Macronutrients concentrations were very high compared to sea water, especially in ammonium and phosphates (up to 220 and 7µM, respectively), as well as in dissolved organic and inorganic carbon (up to 1.5 and 33mM, respectively). The sensitivity of the microbial basis of the coastal planktonic foodweb to representative groundwater discharges representing observations at our study site were evaluated experimentally. For that, coastal seawater and its natural microbial community was exposed to 0-20% of groundwater. Dose-response modelling revealed lowest observed effect concentrations down to 0.1% after 24h for phytoplankton and nearly linear responses to the proportion of groundwater. Heterotrophic prokaryotes also appeared stimulated within 2 days for more than 5% of groundwater added. The coastal microbial community thus appeared highly sensitive to estimated groundwater discharge levels.