Deep silicon maxima in the stratified oligotrophic Mediterranean Sea

Abstract

Abstract. The silicon biogeochemical cycle has been studied in the Mediterranean Sea during late summer/early autumn 1999 and summer 2008. The distribution of nutrients, particulate carbon and silicon, fucoxanthin (Fuco), and total chlorophyll-a (TChl-a) were investigated along an eastward gradient of oligotrophy during two cruises (PROSOPE and BOUM) encompassing the entire Mediterranean Sea during the stratified period. At both seasons, surface waters were depleted in nutrients and the nutriclines gradually deepened towards the East, the phosphacline being the deepest in the easternmost Levantine basin. Following the nutriclines, parallel deep maxima of biogenic silica (DSM), fucoxanthin (DFM) and TChl-a (DCM) were evidenced during both seasons with maximal concentrations of 0.45 µmol L −1 for BSi, 0.26 µg L −1 for Fuco, and 1.70 µg L −1 for TChl-a, all measured during summer. Contrary to the DCM which was a persistent feature in the Mediterranean Sea, the DSM and DFMs were observed in discrete areas of the Alboran Sea, the Algero-Provencal basin, the Ionian sea and the Levantine basin, indicating that diatoms were able to grow at depth and dominate the DCM under specific conditions. Diatom assemblages were dominated by Chaetoceros spp., Leptocylindrus spp., Pseudonitzschia spp. and the association between large centric diatoms (Hemiaulus hauckii and Rhizosolenia styliformis) and the cyanobacterium Richelia intracellularis Correspondence to: Y. Crombet (yann.crombet@univmed.fr) was observed at nearly all sites. The diatom's ability to grow at depth is commonly observed in other oligotrophic regions and could play a major role in ecosystem productivity and carbon export to depth. Contrary to the common view that Si and siliceous phytoplankton are not major components of the Mediterranean biogeochemistry, we suggest here that diatoms, by persisting at depth during the stratified period, could contribute to a large part of the marine primary production as observed in other oligotrophic areas.

Silica cycling in the ultra-oligotrophic eastern Mediterranean Sea

“…which is not altered by uptake due to diatoms in the surface waters because there are insignificant numbers of diatoms (Ignatiades et al, 2009;Psarra et al, 2000). In those locations where diatoms are known to exist, such as at the Mediterranean front (Crombet et al, 2011), it is possible to see decreases in SA due to diatom uptake. The concentration of silica in the 200-500 m layer, which is the intermediate water exported from EMS, was 1-8 µmol Si kg −1 in summer (September-October) and somewhat higher in winter (1-10 µmol Si kg −1 , Fig.…”

“…However 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). Elsewhere, small non-silicate 4212 M. D. Krom et al: Silica cycling in the ultra-oligotrophic eastern Mediterranean Sea phytoplankton, more efficient in utilising nutrients in limited supply, dominate (Huete-Ortega et al, 2011;Siokou-Frangou et al, 2010).…”

“…Stations 6, 7 and 8 have a relatively shallow and constant gradient which was rather different from the profile from station 10. These stations are from the deep EMS in areas where Crombet et al (2011) found no evidence of increased diatom presence in the overlying water column. The upper porewater SA from cores 6, 7 and 8 have been used to calculate an average SA flux from the sediment in the basin to EMS deep waters.…”

“…Benitez- Nelson et al, 2007;Brzezinski et al, 2011;DeMaster, 1981;Dugdale and Wilkerson, 1998;Nelson et al, 1995;Ragueneau et al, 2000). However, there have been few studies examining the unusual distribution of silica in the eastern Mediterranean Sea (EMS) (Crombet et al, 2011;Kress and Herut, 2001;Ribera d'Alcalà et al, 2003;Schink, 1967). 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.…”

Diatom Phytochromes Reveal the Existence of Far-Red-Light-Based Sensing in the Ocean

“…Regarding the vertical distribution of DPHs in the water column, there are several records of diatoms actively growing in the subsurface layer (Crombet et al, 2011) or in highly mixed water columns (Backhaus et al, 2003;Smetacek et al, 2012) where cells may be displaced at depth and remain there for long periods of (A) Scheme of the translation products of Pt-DPH in the wild-type and transformation control (Tc), and in two knockout lines (KO1 and 2). Sequences retrieved from KO1 and KO2 lines shown in red, for the different alleles, the mutations in the Pt-DPH locus compared with the wild type, and on the right the nucleotide insertions (+), deletions (2), and/or transversions.…”