Tamborski, Joseph , Beek, Pieter, Rodellas, Valentí, Monnin, Christophe, Bergsma, Erwin, Stieglitz, Thomas, Heilbrun, Christina, Cochran, J. Kirk, Charbonnier, Céline, Anschutz, Pierre, Bejannin, Simon and Beck, Aaron (2019) Temporal variability of lagoon–sea water exchange and seawater circulation through a Mediterranean barrier beach. Limnology and Oceanography, 64 (5). pp. 2059-2080. DOI 10.1002/lno.11169.

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Abstract

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 Ra-224(ex)/Ra-223 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 m(3) 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 m(3) 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.

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