The University of Southampton
University of Southampton Institutional Repository

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

Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology
Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology
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.
1726-4170
2289-2314
Demuynck, Pieter
7d463a1f-b669-4879-a13e-240f51065549
Tyrrell, Toby
6808411d-c9cf-47a3-88b6-c7c294f2d114
Naveira Garabato, Alberto
97c0e923-f076-4b38-b89b-938e11cea7a6
Moore, Mark Christopher
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Martin, Adrian Peter
9d0d480d-9b3c-44c2-aafe-bb980ed98a6d
Demuynck, Pieter
7d463a1f-b669-4879-a13e-240f51065549
Tyrrell, Toby
6808411d-c9cf-47a3-88b6-c7c294f2d114
Naveira Garabato, Alberto
97c0e923-f076-4b38-b89b-938e11cea7a6
Moore, Mark Christopher
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Martin, Adrian Peter
9d0d480d-9b3c-44c2-aafe-bb980ed98a6d

Demuynck, Pieter, Tyrrell, Toby, Naveira Garabato, Alberto, Moore, Mark Christopher and 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. (doi:10.5194/bg-17-2289-2020).

Record type: Article

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.

Text
bg-17-2289-2020 - Version of Record
Available under License Creative Commons Attribution.
Download (8MB)

More information

Accepted/In Press date: 21 February 2020
Published date: 22 April 2020

Identifiers

Local EPrints ID: 444450
URI: http://eprints.soton.ac.uk/id/eprint/444450
ISSN: 1726-4170
PURE UUID: 44b38949-40f8-46a4-9892-fc928d185d47
ORCID for Toby Tyrrell: ORCID iD orcid.org/0000-0002-1002-1716

Catalogue record

Date deposited: 20 Oct 2020 16:30
Last modified: 18 Feb 2021 16:46

Export record

Altmetrics

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×