Abstract
Abstract. The biological composition of the material exported to a moored sediment trap located under the winter mixed layer of the naturally fertilized Kerguelen Plateau in the Southern Ocean was studied over an annual cycle. Despite iron availability in spring, the annual particulate organic carbon (POC) export (98.2 mmol m−2) at 289 m was low, but annual biogenic silica export was significant (114 mmol m−2). This feature was related to the abundance of empty diatom cells and the ratio of full to empty cells exerted a first-order control in BSi : POC export stoichiometry of the biological pump. Chaetoceros Hyalochaete spp. and Thalassiosira antarctica resting spores were responsible for more than 60% of the annual POC flux that occurred during two very short export events of < 14 days in spring–summer. Relatively low diatom fluxes were observed over the remainder of the year. Faecal pellet contribution to annual carbon flux was lower (34%) and reached its seasonal maximum in autumn and winter (> 80%). The seasonal progression of faecal pellet types revealed a clear transition from small spherical shapes (small copepods) in spring, to larger cylindrical and ellipsoid shapes in summer (euphausiids and large copepods) and finally to large tabular shapes (salps) in autumn and winter. We propose in this high-biomass, low-export (HBLE) environment that small but highly silicified and fast-sinking resting spores are able to bypass the intense grazing pressure and efficient carbon transfer to higher trophic levels that are responsible for the low fluxes observed the during the remainder of the year. More generally our study also provides a statistical framework linking the ecological succession of diatom and zooplankton communities to the seasonality of carbon and silicon export within an iron-fertilized bloom region in the Southern Ocean.
Highlights
The Southern Ocean is the place of exposure of old upwelled waters to the atmosphere and the formation of mode waters, thereby ventilating an important part of the global ocean and playing a central role in distributing heat, carbon and nutrients in the global ocean (Sarmiento et al, 2004; Takahashi et al, 2012; Sallée et al, 2012)
The biogenic silica (BSi) : particulate organic carbon (POC) molar ratio was highest at the beginning of the season and dropped to 0.64 ± 0.06 in cup #5, following the first export event
We suggest that diatom resting spores gather three essential characteristics for effective POC export to the deep ocean: (1) they efficiently bypass the grazing pressure near the mixed layer due to their morphological characteristics such as very robust frustules (CRS) or numerous spines (TRS; high export efficiency), (2) they are efficiently transferred to depth due to the thick and dense frustule increasing sinking velocity and (3) their high carbon content is protected from microbial degradation by the thick frustules
Summary
The Southern Ocean is the place of exposure of old upwelled waters to the atmosphere and the formation of mode waters, thereby ventilating an important part of the global ocean and playing a central role in distributing heat, carbon and nutrients in the global ocean (Sarmiento et al, 2004; Takahashi et al, 2012; Sallée et al, 2012). It is generally acknowledged that regional variations in plankton community structure are responsible for variations in nutrient stoichiometry in the Southern Ocean (Jin et al, 2006; Weber and Deutsch, 2010) and that the biological pump is a central process regulating this stoichiometry (Ragueneau et al, 2006; Salter et al, 2012; Primeau et al, 2013) These characteristics emphasize the importance of biological processes in the Southern Ocean waters for the availability of silicic acid and nitrate (Sarmiento et al, 2004; Dutkiewicz et al, 2005) as well as Published by Copernicus Publications on behalf of the European Geosciences Union. It has been proposed that change in the uptake ratio of silicate and nitrate by Southern Ocean phytoplankton in response to increased iron availability during the Last Glacial Maximum could have played a substantial role in varying atmospheric CO2 (Brzezinski et al, 2002; Matsumoto et al, 2002)
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