Efficiency of the silicate pump at a coastal oligotrophic site in the Mediterranean Sea

Abstract

Abstract.A one-year survey of the sedimenting particulate material was conducted at the SOFi site located on the edge of the continental slope in the Gulf of Lions (Northwestern Mediterranean) between September 1999 and December 2000. The main focus of this paper was to establish an annual budget of biogenic silica export, to describe the seasonnal pattern of Si fluxes in relation to the organic and mineral C fluxes and to quantify the Si/C decoupling processes during sedimentation. The pattern of the downward flux of BSi was partly typical with increased daily sedimentation rates during spring followed by smaller peaks during summer. However, an unusual sedimentation event was recorded in the bottom trap in February before the spring thermal stratification occurred, and represented 37% of the annual BSi mass flux. The total annual BSi flux at the SOFi site was estimated from the bottom trap and amounted to 86.8 mmol Si m −2 y −1 , while the organic carbon flux amounted to 154.9 mmol C m −2 y −1 . The overall efficiency of C export to depth compared to surface primary production budgets was low (3%) and similar to values found for oligotrophic regions of the Eastern Mediterranean. On the other hand, we documented a very high efficiency of the Si export at the SOFi site, with 62% of the Si produced in the surface layer exported to the bottom trap. The excellent correlation between BSi and particulate organic C (POC) in the bottom trap further indicates that the bulk of the organic matter is transferred to depth in association with diatoms. However, the main fraction of the C produced by phytoplankton is lost in the water column through oxidation or metabolisation, while biogenic silica is more efficiently transferred to depth. This strong Si/C decoupling with depth is likely to be the cause of the spring Correspondence to: K. Leblanc (leblanc@com.univ-mrs.fr) depletion of silicic acid over the entire water column which was observed by mid-April in a companion study (Leblanc et al., 2003) and may cause a severe Si limitation of the diatom spring bloom in this oligotrophic region of the Gulf of Lions.

Environmental controls on particle fluxes in the Gulf of Valencia, NW Mediterranean.

“…As expected, this signal was clearer at intermediate water depth in the central part of the Gulf in trap Ct, where the OM and bSi average contents were the highest among the four traps (Table 2). The same temporal pattern was observed in the Gulf of Lions for downward fluxes of organic carbon, biogenic silica, diatoms, silicoflagellates, coccoliths and foraminifera, which showed the highest annual values during the winter-spring transition (Leblanc et al, 2005;Rigual-Hernández et al, 2010.…”

“…Such convective upwelling makes the NW Mediterranean a highly productive area relative to the oligotrophic eastern basin of the Mediterranean Sea (Bosc et al, 2004). Downward fluxes of diatoms, silicoflagellates, coccoliths and foraminifera show the seasonal trend of convective cells, with annual maxima during the winter-spring transition (Rigual-Hernández et al, 2010, which is also reflected in the organic carbon and biogenic silica fluxes (Leblanc et al, 2005). However, at the surface layer, advection may displace phytoplanktonrich water out of the convective cells, favoring blooms in areas where stratification prevails (Estrada, 1996;Sánchez-Vidal et al, 2004;Severin et al, 2014).…”

Possible impacts of volcanic ash emissions of Mount Etna on the primary productivity in the oligotrophic Mediterranean Sea: Results from nutrient-release experiments in seawater

“…The ultimate limiting nutrient for the phytoplankton growth in the MedSea has long been considered to be P (Bonnet et al, 2005;Guerzoni et al, 1999;Thingstad and Rassoulzadegan, 1995;Thingstad et al, 1998), with an increasing trend from west to east (Krom et al, 1991), while recent studies suggested limitation of nitrogen (N) in the western and central MedSea and co-limitation of N and P in the eastern basin (Tanaka et al, 2011). Depending on the time of the year the region limitation may shift to another macro-nutrient such as silica (Si) associated particularly to diatom production (Leblanc et al, 2005). Fertilizing potential of Etna ash fallout has been evidenced by the two to three times increased chlorophyll-a concentrations which were correlated with increased Fe-concentrations in the ash fallout region in the Ionian Sea during July 2001 eruption (Censi et al, 2010).…”

“…4) (Bonnet et al, 2005;Schink and Kingston, 1967). However, severe Si-limitation of diatom spring blooms has been observed at least for one coastal zone, in the Gulf of Lion, where the Si concentrations were lowered down to 120 nM, indicating the significance of Si especially after the spring bloom period (Leblanc et al, 2005), which is an ultimate nutrient required by the diatoms, dominating the new production in the MedSea (Socal et al, 1999). Furthermore, recent biological experiments have shown that, when grown in ash-fertilized seawater, diatoms (e.g., Thalassiosira pseudonana) benefit more from volcanic ash compared to coccolithopores (e.g., Emiliania huxleyi) (Hoffmann et al, 2012).…”

Composition and variability in the export of biogenic silica in the Changjiang River and the effect of Three Gorges Reservoir

“…Silicon (Si) is an essential element in aquatic ecosystems. Dissolved silicate (DSi) is typically in the form of monosilicic acid (H 4 SiO 4 ) and required for the growth of diatoms, which represents a major portion of planktonic primary production in the ocean (Nelson et al, 1995;Leblanc et al, 2005). Biogenic silica (BSi) is generated when diatoms, plants and other organisms uptake DSi during photosynthesis; it is an important source of DSi due to its typically amorphous form and thus plays an important role in the Earth's Si cycle (Conley, 2002;Laruelle et al, 2009).…”