Temporal variability of lagoon–sea water exchange and seawater circulation through a Mediterranean barrier beach

The subterranean flow of water through sand barriers between coastal lagoons and the sea, driven by a positive hydraulic gradient, is a net new pathway for solute transfer to the sea. On the sea side of sand barriers, seawater circulation in the swash-zone generates a flux of recycled and new solutes. The significance and temporal variability of these vectors to the French Mediterranean Sea is unknown, despite lagoons constituting ~ 50% of the coastline. A one-dimensional ²²⁴Ra_ex/²²³Ra reactive-transport model was used to quantify water flow between a coastal lagoon (La Palme) and the sea over a 6-month period. Horizontal flow between the lagoon and sea decreased from ~ 85 cm d⁻¹ during May 2017 (0.3 m³ d⁻¹ m⁻¹ of shoreline) to ~ 20 cm d⁻¹ in July and was negligible in the summer months thereafter due to a decreasing hydraulic gradient. Seawater circulation in the swash-zone varied from 10 to 52 cm d⁻¹ (0.4–2.1 m³ d⁻¹ m⁻¹), driven by short-term changes in the prevailing wind and wave regimes. Both flow paths supply minor dissolved silica fluxes on the order of ~ 3–10 mmol Si d⁻¹ m⁻¹. Lagoon–sea water exchange supplies a net dissolved inorganic carbon (DIC) flux (320–1100 mmol C d⁻¹ m⁻¹) two orders of magnitude greater than seawater circulation and may impact coastal ocean acidification. The subterranean flow of water through sand barriers represents a significant source of new DIC, and potentially other solutes, to the Mediterranean Sea during high lagoon water-level periods and should be considered in seasonal element budgets.