Evaluating the sources of nutrient inputs to coastal lagoons is required to understand the functioning of these ecosystems and their vulnerability to eutrophication. Whereas terrestrial groundwater processes are increasingly recognized as relevant sources of nutrients to coastal lagoons, there are still limited studies evaluating separately nutrient fluxes driven by terrestrial groundwater discharge and lagoon water recirculation through sediments. In this study, we assess the relative significance of these sources in conveying dissolved inorganic nutrients (NO, NH and PO) to a coastal lagoon (La Palme lagoon; France, Mediterranean Sea) using concurrent water and radon mass balances. The recirculation of lagoon water through sediments represents a source of NH (1900-5500 mol d) and PO (22-71 mol d), but acts as a sink of NO. Estimated karstic groundwater-driven inputs of NO, NH and PO to the lagoon are on the order of 200-1200, 1-12 and 1.5-8.7 mol d, respectively. A comparison between the main nutrient sources to the lagoon (karstic groundwater, recirculation, diffusion from sediments, inputs from a sewage treatment plant and atmospheric deposition) reveals that the recirculation of lagoon water through sediments is the main source of both dissolved inorganic nitrogen (DIN) and phosphorous (DIP) to La Palme lagoon. These results are in contrast with several studies conducted in systems influenced by terrestrial groundwater inputs, where groundwater is often assumed to be the main pathway for dissolved inorganic nutrient loads. This work highlights the important role of lagoon water recirculation through permeable sediments as a major conveyor of dissolved nutrients to coastal lagoons and, thus, the need for a sound understanding of the recirculation-driven nutrient fluxes and their ecological implications to sustainably manage lagoonal ecosystems.

The recirculation of seawater through permeable coastal sediments is increasingly recognized as an important source of nutrients, including dissolved silica (DSi), to the coastal ocean. Here, we utilized a Ra isotope (223 Ra, 224 Raex, 228 Ra) mass balance to quantify DSi fluxes driven by water recirculation to a small shallow coastal lagoon (La Palme; French Mediterranean) during June 2016, as compared to karstic groundwater spring inputs. The DSi flux driven by lagoon water recirculation (derived from 224 Raex) was approximately one order of magnitude greater (1,930 ± 1,650 mol d-1) than the DSi load of the karstic groundwater spring (250 ± 50 mol d-1) and greater than molecular diffusion (970 ± 750 mol d-1). Lagoon water recirculation was a negligible source of 228 Ra, indicating that recirculation-driven DSi inputs occur over a timescale of days. Offshore transects were studied to quantify fluxes of marinederived submarine groundwater discharge (SGD) from the permeable sandy coastline adjacent to the lagoon, into the Mediterranean Sea. Surface water transects revealed near-shore enrichments of Ra and DSi, attributed to wave-setup and water exchange through the permeable beach between the lagoon and the sea. Upscaling over the 9.5 km stretch of sandy beaches results in a marine SGD-driven DSi flux of 2.3 ± 1.3 *10 4 mol d-1 , similar in magnitude to the Têt river during November 2016 (3.3 ± 2.4 *10 4 mol d-1), the largest river in the region. A positive relationship between DSi and 224 Raex in lagoon water and seawater, but not 228 Ra, suggests that 224 Raex and DSi enrichments are derived from a similar source, the sediment (i.e. lithogenic particle dissolution), operating on short timescales. A marine SGD-driven DSi flux to the Gulf of Lions (3.8 ± 2.2 *10 5 mol d-1) is likely continuous over time. The relatively constant DSi inputs from water recirculation for the shallow lagoons and beaches along the French Mediterranean Sea may sustain primary production in the coastal zone. In comparison, terrestrial 3 groundwater and rivers supply temporally variable nutrient (N, P, Si) inputs via changes in regional precipitation, runoff and aquifer storage.