Silica cycling in the ultra-oligotrophic eastern Mediterranean Sea

Abstract

Abstract. Although silica is a key plant nutrient, there have been few studies aimed at understanding the Si cycle in the eastern Mediterranean Sea (EMS). Here we use a combination of new measurements and literature values to explain the silicic acid distribution across the basin and to calculate a silica budget to identify the key controlling processes. The surface water concentration of ∼ 1 µM, which is unchanging seasonally across the basin, was due to the inflow of western Mediterranean Sea (WMS) water at the Straits of Sicily. It does not change seasonally because there is only a sparse population of diatoms due to the low nutrient (N and P) supply to the photic zone in the EMS. The concentration of silicic acid in the deep water of the western Ionian Sea (6.3 µM) close to the S Adriatic are an of formation was due to the preformed silicic acid (3 µM) plus biogenic silica (BSi) from the dissolution of diatoms from the winter phytoplankton bloom (3.2 µM). The increase of 4.4 µM across the deep water of the EMS was due to silicic acid formed from in situ diagenetic weathering of aluminosilicate minerals fluxing out of the sediment. The major inputs to the EMS are silicic acid and BSi inflowing from the western Mediterranean (121 × 10 9 mol Si yr −1 silicic acid and 16 × 10 9 mol Si yr −1 BSi), silicic acid fluxing from the sediment (54 × 10 9 mol Si yr −1 ) and riverine (27 × 10 9 mol Si yr −1 ) and subterranean groundwater (9.7 × 10 9 mol Si yr −1 ) inputs, with only a minor direct input from dissolution of dust in the water column (1 × 10 9 mol Si yr −1 ). This budget shows the importance of rapidly dissolving BSi and in situ weathering of aluminosilicate minerals as sources of silica to balance the net export of silicic acid at the Straits of Sicily. Future measurements to improve the accuracy of this preliminary budget have been identified.

P Fluxes and Prokaryotic Cycling at Benthic Boundary Layer in the Deep Southeastern Mediterranean Sea

“…Silicic acid is a conservative measure of nutrient flux as it follows a net dissolution which result in efflux to the overlying water, especially in oxic and carbon‐poor sediments (e.g., Katz et al., 2020). As expected, the efflux of silicic acid in our incubations were moderate (66–241 μmole·m −2 d −1 , Figure ), in accordance with the very low flux of biogenic silica (Krom et al., 2014).…”

Distribution and budget of dissolved and biogenic silica in the Bohai Sea and Yellow Sea

“…7), and its contribution to primary production of 0.06 % is less than the global contribution of wet and dry deposition (0.21 %; Tre ´guer and De La Rocha 2013), but exceeds previous estimates in the East China Sea (0.03 %, Liu et al 2005). However, the estimate of F A is based on limited data, and ignores the potential contribution to the available Si budget of lithogenic silica in dust (Tre ´guer and De La Rocha 2013; Krom et al 2014). Therefore, our estimate for F A is probably conservative.…”

Climatological distribution of dissolved inorganic nutrients in the Western Mediterranean Sea (1981–2017)

“…The BGC-WMED product capture details in Fig. 15 about the longitudinal gradient in nitrate and phosphate, along the water column where nutrient sink deeper from west to east as previously seen in Pujo-Pay et al (2011) and Krom et al (2014), an increased oligotrophy from west to east with higher concentrations in the two nutrients in the western side of the section and a more oligotrophic character toward east.…”

“…From the surface to 150-200 m, Lc is rather constant, while from 200 to 600 m, the horizontal Lc increases for all nutrients. The vertical Lc behaves similarly, for nitrate and phosphate, due to the homogeneity of the intermediate water mass, as explained also by Troupin et al (2010) of the biogenic silica is slower than that of the other nutrients (DeMaster, 2002) which explain its progressive increase towards deeper layers (Krom et al, 2014).…”

“…In the easternmost areas, the surface depletion in nutrients (Van Cappellen et al, 2014) is attributed to the variation in the thermohaline properties that has impacted primary production (Ozer et al, 2017) The deficiency of inorganic nutrients is explained by the effect of the anti-estuarine circulation, with the IW coming from the EMED, which is known to be poor in nutrients (Krom et al, 2014;, accumulates nutrient along its path. Thus, this relative nutrient-rich Mediterranean outflow is lost to the Atlantic Ocean.…”