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 × 109 mol Si yr−1 silicic acid and 16 × 109 mol Si yr−1 BSi), silicic acid fluxing from the sediment (54 × 109 mol Si yr−1) and riverine (27 × 109 mol Si yr−1) and subterranean groundwater (9.7 × 109 mol Si yr−1) inputs, with only a minor direct input from dissolution of dust in the water column (1 × 109 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.
Highlights
Silicon is a key plant nutrient in the global ocean
A total silica budget for the eastern Mediterranean Sea (EMS) basin including both silicic acid and silica derived from particulate matter (BSi and chemical weathering of aluminosilicate minerals (LSi)) is calculated. Using this silica budget we identify the most important processes affecting the silica cycle and suggest where detailed measurements are required to improve the accuracy of the silica budget
The Eastern Mediterranean Transient (EMT) event, which changed the deep circulation of the EMS, altered the depth distribution of silicic acid
Summary
Silicon is a key plant nutrient in the global ocean. There have been many studies describing the oceanic silica cycle (e.g. Benitez-Nelson et al, 2007; Brzezinski et al, 2011; DeMaster, 1981; Dugdale and Wilkerson, 1998; Nelson et al, 1995; Ragueneau et al, 2000). The depth distribution of silicic acid (SA) in the global ocean is typically low in the photic zone with an increase in concentration with depth. There are problems with this simple explanation in the EMS because over large areas there are very low diatom numbers (Ignatiades et al, 2009; Psarra et al, 2000), assumed to be because the nutrient supply to the photic zone is below the threshold required for diatom growth. While Crombet et al (2011) found evidence for what they describe as a “deep glass forest” of diatoms at the bottom of the deep chlorophyll maximum/upper nutricline across the Western Mediterranean Sea (WMS), they only found diatoms in the EMS at locations where there was a locally increased nutrient supply to the photic zone (Straits of Sicily, Mediterranean front and Cyprus eddy).
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