Primary production in coastal lagoons supported by groundwater discharge and porewater fluxes inferred from nitrogen and carbon isotope signatures

The role of groundwater and porewater fluxes as a source of nutrients to coastal ecosystems and their ecological implications are increasingly being recognized. In this study, the role of groundwater discharge and porewater fluxes in supporting primary production of coastal lagoons was evaluated by stable nitrogen and carbon isotope signature tracing of the transfer of nitrogen and carbon to primary producers. We determined the isotope signatures in macrophytes and phytoplankton (via particulate organic matter) and in the potential sources of inorganic nitrogen and carbon in two coastal lagoons on the French Mediterranean coastline. The delta N-15 of both macrophytes and phytoplankton reflect predominantly the nitrogen isotopic signatures of the karstic ground-water and porewater sources, indicating that they must have sourced the majority of their nitrogen from these sources, with punctual contributions from sewage input characterised by a significantly different isotopic signature. The delta C-13 signature of primary producers also shows the contribution of karstic groundwater discharge and porewater exchange as significant sources of dissolved inorganic carbon to primary production. This study links physical (nutrient delivery) and ecological (nutrient uptake) processes associated with primary production, thereby providing direct evidence for the ecological role groundwater discharge and porewater fluxes can play in coastal ecosystems.