Submarine groundwater discharge (SGD) contributes significant nutrient fluxes to the coastal ocean, which are controlled by inputs to coastal aquifers and geochemical processes that alter aquifer fluid composition in the freshwater-seawater mixing zone (recently termed the âsubterranean estuaryâ, STE). The diversity of STEs in hydrogeologic form and biogeochemical function remains largely unexplored, especially in the tropics and karstic regions. The Yucatán Peninsula is a low-relief carbonate platform with coastal mangrove forests and karst geology that permits fast rainfall infiltration, minimal surface flow, and high SGD. Field sampling conducted in May 2007 revealed the geochemical composition of surface water and porewater in two coastal lagoons and aquifer fluids in cenotes, wells, and submarine springs. Measurements indicate that nutrient (N, P, Si) biogeochemistry in this coastal aquifer is governed by anthropogenic inputs to the thin freshwater lens, sorption onto aquifer solids, and mixing with saline water in the underlying seawater intrusion and in STEs. We report groundwater-derived nutrient fluxes to the coastal ocean estimated using radiogenic tracers of SGD and compare our findings in this region with other hydrogeologic settings.