A comparison between water circulation and terrestrially-driven dissolved silica fluxes to the Mediterranean Sea traced using radium isotopes

Abstract

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.

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

“…DSi transects were similar to DIC, reaching a relative maximum (100 μ mol L −1 in August) in the center of the beach (Fig. ), indicating that dissolution of Si minerals takes place within the sand barrier and releases DSi into the pore water (Anschutz et al ; Tamborski et al ). Vertical DIC and DSi profiles sampled closest to the Mediterranean Sea increased in concentration with increasing depth, similar to 222 Rn and 223,224 Ra, for all six sampling seasons (Fig.…”

“…During November 2016, SGD was estimated to supply a DSi flux of 2.4 ± 1.4 mol Si d −1 m −1 of shoreline for La Franqui based on offshore transects that were conducted in the coastal seas (Tamborski et al ). This DSi flux was hypothesized to be driven by both lagoon–sea water exchange (relatively high water levels) and seawater circulation (Tamborski et al ); this flux is 2–3 orders of magnitude larger than the DSi fluxes calculated here for the swash‐zone (Table ). The DSi flux of Tamborski et al () may include additional, unaccounted for flow paths between the swash‐zone (this study) and 300 m offshore, where the surface water transects to quantify SGD originated (fig.…”

“…Tides are minor along the French Mediterranean Sea, such that wave setup and water level fluctuations are hypothesized to drive a net seaward flowing circulation cell beneath the swash‐zone (Sous et al ). The temporal variability of seawater circulation as a vector for solute transport to the French Mediterranean Sea is largely unknown (Tamborski et al ).…”

“…This DSi flux was hypothesized to be driven by both lagoon-sea water exchange (relatively high water levels) and seawater circulation ); this flux is 2-3 orders of magnitude larger than the DSi fluxes calculated here for the swashzone ( Table 7). The DSi flux of Tamborski et al (2018) may include additional, unaccounted for flow paths between the swash-zone (this study) and 300 m offshore, where the surface water transects to quantify SGD originated ( fig. 6 of Tamborski et al 2018).…”

“…The DSi flux of Tamborski et al (2018) may include additional, unaccounted for flow paths between the swash-zone (this study) and 300 m offshore, where the surface water transects to quantify SGD originated ( fig. 6 of Tamborski et al 2018). Water depth is ≤ 5 m over 300 m from shore; seawater circulation through this shallow zone, over a large area (~3 km 2 ), may thus be an adequate source of DSi (and Ra) offshore of La Franqui.…”