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Silicon Cycle in the Tropical South Pacific: Contribution to the Global Si

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Silicon Cycle in the Tropical South Pacific: Contribution to the Global Si Biogeosciences, 15, 5595–5620, 2018 https://doi.org/10.5194/bg-15-5595-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Silicon cycle in the tropical South Pacific: contribution to the global Si cycle and evidence for an active pico-sized siliceous plankton Karine Leblanc1, Véronique Cornet1, Peggy Rimmelin-Maury2, Olivier Grosso1, Sandra Hélias-Nunige1, Camille Brunet1, Hervé Claustre3, Joséphine Ras3, Nathalie Leblond3, and Bernard Quéguiner1 1Aix-Marseille Univ., Université de Toulon, CNRS, IRD, MIO, UM110, Marseille, 13288, France 2UMR 6539 LEMAR and UMS OSU IUEM – UBO, Université Européenne de Bretagne, Brest, France 3UPMC Univ Paris 06, UMR 7093, LOV, 06230 Villefranche-sur-mer, France Correspondence: Karine Leblanc ([email protected]) Received: 13 April 2018 – Discussion started: 15 May 2018 Revised: 26 July 2018 – Accepted: 27 August 2018 – Published: 20 September 2018 Abstract. This article presents data regarding the Si bio- 1 Introduction geochemical cycle during two oceanographic cruises con- ducted in the tropical South Pacific (BIOSOPE and OUT- Siliceous phytoplankton, especially diatoms, are often asso- PACE cruises) in 2005 and 2015. It involves the first Si ciated with nutrient-rich eutrophic ecosystems. However, the stock measurements in this understudied region, encompass- global budget of biogenic silica production by Nelson et al. ing various oceanic systems from New Caledonia to the (1995) already pointed out the importance of these organ- Chilean upwelling between 8 and 34◦ S. Some of the low- isms in oligotrophic areas where, despite their low concen- est levels of biogenic silica standing stocks ever measured tration and due to the geographical extension of these sys- were found in this area, notably in the southern Pacific tems, their silica production would be comparable to that of gyre, where Chlorophyll a concentrations are the most de- areas overlying major diatomaceous sediment accumulation pleted worldwide. Integrated biogenic silica stocks are as zones. However, studies that have documented the Si cycle in low as 1:08±0:95 mmol m−2 and are the lowest stocks mea- the Pacific Ocean, the largest oligotrophic area of the World sured in the South Pacific. Size-fractionated biogenic sil- Ocean, mainly focused on the equatorial region, and the ica concentrations revealed a non-negligible contribution of northern subtropical gyre. This article presents the first set the pico-sized fraction (<2–3 µm) to biogenic silica stand- of field results from the South Pacific Ocean between 8 and ◦ ing stocks, representing 26% ± 12% of total biogenic sil- 34 S, spanning from New Caledonia over to the Chilean up- ica during the OUTPACE cruise and 11% ± 9% during the welling, and notably, from the most Chlorophyll a-depleted BIOSOPE cruise. These results indicate significant accumu- region at a worldwide scale (Ras et al., 2008): the South Pa- lation in this size class, which was undocumented for 2005, cific gyre (SPG). but has since then been related to Si uptake by Synechococcus Diatoms are known to contribute more importantly to pri- cells. Si uptake measurements carried out during BIOSOPE mary production in meso- to eutrophic systems, yet sev- confirmed biological Si uptake by this size fraction. We fur- eral studies have emphasized that even if they are not dom- ther present diatoms community structure associated with the inant in oligotrophic regions, they may still contribute up stock measurements for a global overview of the Si cycle in to 10 %–20 % of C primary production in the equatorial the tropical South Pacific. Pacific (Blain et al., 1997). In the oligotrophic Sargasso Sea (BATS station), their contribution was estimated to be as high as 26 %–48 % of new annual primary production (Brzezinski and Nelson, 1995) and to represent up to 30 % of particulate organic carbon (POC) export, leading to an up- ward revision of the contribution of oligotrophic gyres to global Si budgets (Nelson and Brzezinski, 1997). Similar Published by Copernicus Publications on behalf of the European Geosciences Union. 5596 K. Leblanc et al.: Silicon cycle in the tropical South Pacific studies carried out in the North Pacific (HOT station) led to region. We detail size-fractionated BSi inventories in the wa- new estimates, as diatoms were found to be less important ter column, Si export fluxes, associated diatom community contributors to primary production. A combination of both structure composition and Si uptake and kinetic rates in the Atlantic and North Pacific oligotrophic gyre budgets led to South Pacific. Our key results show some of the lowest BSi a revised contribution of 13 Tmol Si yr−1, a 51 % diminution stocks ever measured, which may warrant for a new revision of the previous estimate (Brzezinski et al., 2011). of the contribution of oligotrophic areas to the global Si cy- In the eastern equatorial Pacific (EEP), it has been shown cle, and confirm recent findings of an active biological uptake that diatoms experience chronic Si limitation along the east- of Si in the pico-sized fraction. ern equatorial divergence in the so-called high-nutrient low- silicate low-chlorophyll (HNLSiLC) system (Dugdale and Wilkerson, 1998) as well as Si–Fe colimitation (Blain et al., 2 Material and methods 1997; Leynaert et al., 2001). Furthermore, oligotrophic re- 2.1 Sampling strategy gions are known to experience considerable variability in nutrient injections, leading to episodical blooms depending Results presented here encompass data from two French on the occurrence of internal waves (Wilson, 2011), meso- oceanographic cruises located in the South Pacific Ocean scale eddies (Krause et al., 2010), storms (Krause et al., (from 10 to 30◦ S), covering two transects that employed a 2009), or local upwellings or dust deposition events (Wilson, common sampling strategy of short- and long-duration sta- 2011; Calil et al., 2011). In nitrogen-depleted areas, punc- tions. The BIOSOPE (BIogeochemistry and Optics SOuth tual diatom blooms in the form of diatom–diazotroph asso- Pacific Experiment) cruise was undertaken in 2004, while the ciations (DDAs) are also known to occur and to contribute OUTPACE cruise took place in 2015, both aboard the R/V both to new primary production (Dore et al., 2008; Brzezin- L’Atalante. The BIOSOPE transect was sampled between the ski et al., 2011) and to non-diazotrophic diatoms through sec- Marquesas Islands (141◦ W, 8◦ S) and Concepción (Chile) ondary N release (Bonnet et al., 2016; Leblanc et al., 2016). (72◦ W, 35◦ S), between 24 October 12 and November 2004. While biogenic silica was classically associated with the The OUTPACE transect was sampled between New Caledo- largest size fractions, especially microplankton, a series of nia (159◦ W, 22◦ S) and Tahiti (160◦ W, 20◦ S) between 18 recent studies provide evidence for a role for picophyto- February and 3 April 2015 (Fig. 1). plankton such as Synechococcus in the Si cycle, showing that this ubiquitous lineage is able to take up and accumu- 2.2 Hydrology late Si (Ohnemus et al., 2016; Krause et al., 2017; Brzezinski et al., 2017). This was evidenced in the field in the equato- Water sampling and measurements of temperature and rial Pacific, the Sargasso Sea, and in culture work, suggest- salinity were performed using a SeaBird SBE 911plus ing a widespread diffuse role for this organism, which could CTD/Carousel system fitted with an in situ fluorometer and be more prominent in oligotrophic environments where di- 24 Niskin bottles. More details about the BIOSOPE cruise atoms are in low abundance. In the EEP, and despite very strategy are given in the Biogeosciences special issue intro- variable cellular Si content, Synechococcus represented for ductory article by Claustre et al.(2008) while the OUTPACE instance 40 % of water column biogenic silica (BSi) inven- cruise strategy is detailed in Moutin et al.(2017). Euphotic tory compared to diatoms in 2004, and twice that of diatoms layer depths (Ze) were calculated as described in Raimbault the following year (Baines et al., 2012). The role of small et al.(2008) and Moutin et al.(2017). Sampling depths were nano-sized diatoms has also probably been overlooked and adjusted to on-deck incubator screen attenuation using mea- we recently pointed out their general occurrence at the world- surements from an in situ PAR (photosynthetically active ra- wide scale and their occasional regional importance in di- diation) sensor (LI-COR instrument) mounted on the CTD atom blooms (Leblanc et al., 2018). (conductivity–temperature–depth) frame. Here we present the first set of field results from the South Pacific Ocean between 8 and 34◦ S, spanning from New 2.2.1 Inorganic nutrients Caledonia over to the Chilean upwelling and, notably, from the most depleted Chl a region worldwide (Ras et al., 2008), Nutrients were collected in 20 mL PE vials and analyzed the SPG. Results were obtained from two cruises carried out directly on-board on a SEAL Analytical auto-analyzer fol- a decade apart following longitudinal sections first in the lowing Aminot and Kérouel(2007) during BIOSOPE and at the laboratory during OUTPACE from frozen (0 ◦C) sam- southeastern Pacific (SEP) between the Marquesas Islands − and the Chilean upwelling, crossing the South Pacific gyre ples. During BIOSOPE, the nitrate (NO3 ) detection limit ± 3− (BIOSOPE cruise, October–December 2004), and then in was 0.05 µM (accuracy of 0:05 µM), phosphate (PO4 ) de- the southwestern Pacific (SWP) between New Caledonia and tection limit was 0.02 µM (accuracy of ±0:05 µM), and or- Tahiti (OUTPACE cruise, February–April 2015). Very simi- thosilicic acid (Si.OH/4) detection limit was 0.05 µM (ac- lar sampling strategies and analyses were conducted regard- curacy of ±0:05 µM). During OUTPACE the quantification ing the Si cycle and provide new data in this under-sampled limit was 0.05 µM for all nutrients.
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