Efficiency of the Silicate Pump at a Coastal Oligotrophic Site in The

Biogeosciences, 2, 219–229, 2005 www.biogeosciences.net/bg/2/219/ Biogeosciences SRef-ID: 1726-4189/bg/2005-2-219 European Geosciences Union Efficiency of the silicate pump at a coastal oligotrophic site in the Mediterranean Sea K. Leblanc1, B. Queguiner´ 2, P. Raimbault2, and N. Garcia2 1College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA 2Laboratoire d’Oceanographie´ et de Biogeochimie,´ CNRS, University of Aix-Marseille II, Campus de Luminy, 13288 Marseille Cedex 9, France Received: 31 March 2005 – Published in Biogeosciences Discussions: 9 May 2005 Revised: 15 August 2005 – Accepted: 16 August 2005 – Published: 29 August 2005 Abstract. A one-year survey of the sedimenting particulate depletion of silicic acid over the entire water column which material was conducted at the SOFi site located on the edge was observed by mid-April in a companion study (Leblanc et of the continental slope in the Gulf of Lions (Northwest- al., 2003) and may cause a severe Si limitation of the diatom ern Mediterranean) between September 1999 and December spring bloom in this oligotrophic region of the Gulf of Lions. 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 1 Introduction sedimentation. The pattern of the downward flux of BSi was partly typical with increased daily sedimentation rates during Even though continental margins represent only 8% of the spring followed by smaller peaks during summer. However, ocean surface, they are the sites of high marine organic pro- an unusual sedimentation event was recorded in the bottom ductivity and contribute to between 18 and 33% of total trap in February before the spring thermal stratification oc- oceanic C production (Wollast, 1991). Due to their shal- curred, and represented 37% of the annual BSi mass flux. low depths, they are also the sites of enhanced C burial in The total annual BSi flux at the SOFi site was estimated from the sediments with a burial rate 10 to 100 times higher than the bottom trap and amounted to 86.8 mmol Si m−2 y−1, the oceanic domain (Berger et al., 1989). Liu et al. (2001) estimated the total C burial rate in the continental margins while the organic carbon flux amounted to 154.9 mmol C 15 −1 m−2 y−1. The overall efficiency of C export to depth com- to be 0.15 x 10 g C y ,which would represent two-thirds pared to surface primary production budgets was low (3%) of the total oceanic burial. Understanding the biological and and similar to values found for oligotrophic regions of the physical processes driving the export of organic material in Eastern Mediterranean. On the other hand, we documented the coastal zones is thus crucial to a better characterization a very high efficiency of the Si export at the SOFi site, with of these areas as sinks or sources of C. 62% of the Si produced in the surface layer exported to the A number of studies have focused on the C, N, P budgets bottom trap. The excellent correlation between BSi and par- of these coastal zones while much fewer have addressed the ticulate organic C (POC) in the bottom trap further indicates Si cycle. Diatoms are known to dominate in nutrient rich that the bulk of the organic matter is transferred to depth in and highly turbulent areas and are therefore largely repre- association with diatoms. However, the main fraction of the sented in coastal environments (Margalef, 1978). They are C produced by phytoplankton is lost in the water column often associated with massive spring blooms followed by through oxidation or metabolisation, while biogenic silica is subsequent rapid sedimentation, resulting in stronger C ex- more efficiently transferred to depth. This strong Si/C de- port to depth as compared to the development of a microbial coupling with depth is likely to be the cause of the spring loop, which favors C recycling within the photic layer (Leg- endre and Lefevre,` 1989). The opal burial was first thought Correspondence to: K. Leblanc to occur primarily in the Southern Ocean, below the Polar ([email protected]) Front belt, where sediments are almost entirely composed of © 2005 Author(s). This work is licensed under a Creative Commons License. Figure 1 220 K. Leblanc et al.: Efficiency of the silicate pump at a coastal oligotrophic site 44°N a b Mooring line Marseille 43.5°N Buoy PPS 5/2 sediment trap SOFi 56 m 43°N ANDERAA current meter Gulf of Lions 42.5°N PPS 5/2 sediment trap 142 m ANDERAA current meter 42°N Accoustic release 162 m Bottom ballast 3°E4°E5°E6°E Fig. 1. (a) Study site location, (b) Schematic of the mooring line. diatomaceous opal, and led to a previous estimates that 2/3 2 Material and methods of the global burial of biogenic silica took place in this region (DeMaster, 1981; Treguer´ et al., 1995). Recent studies The study site (depth 162 m) was located in the Gulf of evidenced that the opal burial rate in the Southern Ocean has Lions in the North Western Mediterranean Sea (5◦07060 E; been largely overestimated due to sediment focusing, and re- 43◦04000 N) on the edge of the continental slope 20 miles established the budget of this region to a 17 to 37% contribu- off Marseille (Fig. 1a). The sedimenting particulate mat- tion to the global opal burial (Pondaven et al., 2000; Rague- ter was collected between September 1999 and Novem- neau et al., 2000). The “lost” Si burial in the Southern Ocean ber 2000 by a mooring line equipped with two sediment is thought to be balanced by the a higher export rate on con- traps (Technicap®PPS 5/2, collection area 1 m2, 24 receiv- tinental margins. New estimates indicate that the Si burial ing cups), located at 56 m and 142 m depth, 20 m above bot- for the continental margins amounts to 32 to 39% of the total tom (Fig. 1b). The cups were filled before deployment with burial, leading to a reasonable balance of the global Si budget filtered seawater (0.45 µm) containing 5% (v/v) buffered (Ragueneau et al., 2000). formaldehyde for sample preservation. The sediment trap In the Mediterranean Sea, the Si cycle is overall poorly sampling interval was set at 3–4 days in order to catch any documented compared to the available studies on C, N and P. short scale sedimentation pulse, while trap contents were re- trieved every three months. The samples were stored in the In the present study, we have focused on the Si cycle and on ◦ opal fluxes throughout a yearly survey in the Gulf of Lions, dark at 4 C and were processed at the laboratory a day after located on the French Mediterranean coast. The long-term recovery for total mass flux and particulate carbon. observation site SOFi (Site d’Observation FIxe) has been the Large swimmers were removed using a 200 µm mesh, and object of a seasonal study since 1997. From September 1999 any remaining ones were removed manually. Sub-samples to November 2000, we conducted additional measurements were then filtered onto GF/F Whatman filters (0.2 µm), ◦ describing the silicon cycle. A companion paper published rinsed with deionised water and oven-dried at 60 C. Total in a special issue dedicated to the Gulf of Lions describes carbon was analyzed by combustion on a CHN carbon an- the seasonal pattern of diatom distribution and biogenic sil- alyzer (LECO 800) according to Raimbault et al. (1999). A ica production in the water column (Leblanc et al., 2003). replicate sample was then calcinated to remove the organic C In the present paper, complementary data characterizing the content and analyzed for particulate mineral carbon (CaCO3) downward fluxes of particulate matter and biogenic silica in on the CHN. Particulate organic carbon (POC) was calcu- sediment traps are further presented. The aim of this study lated by substraction of the two measurements (POC=total was to establish an annual budget of biogenic silica export C–mineral C). Calcium carbonate was calculated by using at the SOFi site and in particular: to describe the annual Si the inorganic carbon (CaCO3=inorganic carbon×100/12). flux patterns, to compare them with the other constituents of A known volume was sub-sampled for biogenic silica the downward flux such as particulate organic and inorganic (BSi) determination, and the sediment material was freeze- carbon (POC and calcium carbonate, CaCO3) and finally to dried after centrifugation and removal of the supernatant. An address the Si/C decoupling processes during the sedimenta- optimal quantity of 5 mg dry sediment was determined for tion. the BSi measurement. Pre-treatment assays showed a better Biogeosciences, 2, 219–229, 2005 www.biogeosciences.net/bg/2/219/ Figure 3 Figure 2 K. Leblanc et al.: Efficiency of the silicate pump at a coastal oligotrophic site 221 100 4000 Surface trap (56 m) 80 Bottom trap (142 m) ) -1 3000 d -2 60 2000 40 % dissolution 20 Mass flux (in mg m 1000 0 0246810 0 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Amount of biogenic silica in each cup (in mmol Si) Fig. 2. Percentage of the dissolved silicon fraction (from silicic acid Fig. 3. Total mass flux in mg m−2 d−1 of dry weight in the sur- measurements) in each sediment cup compared to the total particu- face trap (56 m) and the bottom trap (142 m) throughout the survey late amount of biogenic silica.

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