Silicon cycle in the tropical South Pacific: contribution to the global Si cycle and evidence for an active pico-sized siliceous plankton

Karine Leblanc,Véronique Cornet,Hervé Claustre,Olivier Grosso,Peggy Rimmelin-Maury,Camille Brunet,Bernard Quéguiner,Nathalie Leblond,Joséphine Ras,Sandra Hélias-Nunige

doi:10.5194/bg-15-5595-2018

Abstract

Abstract. This article presents data regarding the Si biogeochemical cycle during two oceanographic cruises conducted in the tropical South Pacific (BIOSOPE and OUTPACE cruises) in 2005 and 2015. It involves the first Si stock measurements in this understudied region, encompassing various oceanic systems from New Caledonia to the Chilean upwelling between 8 and 34∘ S. Some of the lowest levels of biogenic silica standing stocks ever measured were found in this area, notably in the southern Pacific gyre, where Chlorophyll a concentrations are the most depleted worldwide. Integrated biogenic silica stocks are as low as 1.08±0.95 mmol m−2 and are the lowest stocks measured in the South Pacific. Size-fractionated biogenic silica concentrations revealed a non-negligible contribution of the pico-sized fraction (<2–3 µm) to biogenic silica standing stocks, representing 26%±12% of total biogenic silica during the OUTPACE cruise and 11%±9% during the BIOSOPE cruise. These results indicate significant accumulation in this size class, which was undocumented for 2005, but has since then been related to Si uptake by Synechococcus cells. Si uptake measurements carried out during BIOSOPE confirmed biological Si uptake by this size fraction. We further present diatoms community structure associated with the stock measurements for a global overview of the Si cycle in the tropical South Pacific.

Highlights

Siliceous phytoplankton, especially diatoms, are often associated with nutrient-rich eutrophic ecosystems
This article presents the first set of field results from the South Pacific Ocean between 8 and 34◦ S, spanning from New Caledonia over to the Chilean upwelling, and notably, from the most Chlorophyll a-depleted region at a worldwide scale (Ras et al, 2008): the South Pacific gyre (SPG)
The 1 μM isoline was centered at ∼ 100 m in the western part of the MA and deepened to ∼ 200 m in the SPG

Summary

Introduction

Especially diatoms, are often associated with nutrient-rich eutrophic ecosystems. Diatoms are known to contribute more importantly to primary production in meso- to eutrophic systems, yet several studies have emphasized that even if they are not dominant in oligotrophic regions, they may still contribute up to 10 %–20 % of C primary production in the equatorial 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 upward revision of the contribution of oligotrophic gyres to global Si budgets (Nelson and Brzezinski, 1997).

Results

Discussion

Conclusion