Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology

Type Article
Date 2020-04
Language English
Author(s) Demuynck Pieter1, Tyrrell Toby1, Garabato Alberto NaveiraORCID1, Moore Mark Christopher1, Martin Adrian Peter2
Affiliation(s) 1 : Univ Southampton, Ocean & Earth Sci, Southampton SO14 3ZH, Hants, England.
2 : Natl Oceanog Ctr, Southampton SO14 3ZH, Hants, England.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2020-04 , Vol. 17 , N. 8 , P. 2289-2314
DOI 10.5194/bg-17-2289-2020
WOS© Times Cited 1
Abstract

The nutrient composition (high in nitrate but low in silicate) of Subantarctic Mode Water (SAMW) forces diatom scarcity across much of the global surface ocean. This is because diatoms cannot grow without silicate. After formation and downwelling at the Southern Ocean's northern edge, SAMW re-emerges into the surface layers of the mid- and low-latitude oceans, providing a major nutrient source to primary producers in those regions. The distinctive nutrient composition of SAMW originates in the surface waters of the Southern Ocean, from which SAMW is formed. These waters are observed to transition from being rich in both silicate and nitrate in high-latitude areas of the Southern Ocean to being nitrate-rich but silicate-depleted at SAMW formation sites further north. Here we investigate the key controls of this change in nutrient composition with an idealised model, consisting of a chain of boxes linked by a residual (Ekman- and eddy-induced) overturning circulation. Biological processes are modelled on the basis of seasonal plankton bloom dynamics, and physical processes are modelled using a synthesis of outputs from the data-assimilative Southern Ocean State Estimate. Thus, as surface water flows northward across the Southern Ocean toward sites of SAMW formation, it is exposed in the model (as in reality) to seasonal cycles of both biology and physics. Our results challenge previous characterisations of the abrupt northward reduction in silicate-to-nitrate ratios in Southern Ocean surface waters as being predominantly driven by biological processes. Instead, our model indicates that, over shorter timescales (years to decades), physical processes connecting the deep and surface waters of the Southern Ocean (i.e. upwelling and entrainment) exert the primary control on the spatial distribution of surface nutrient ratios.

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Demuynck Pieter, Tyrrell Toby, Garabato Alberto Naveira, Moore Mark Christopher, Martin Adrian Peter (2020). Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology. Biogeosciences, 17(8), 2289-2314. Publisher's official version : https://doi.org/10.5194/bg-17-2289-2020 , Open Access version : https://archimer.ifremer.fr/doc/00676/78840/