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Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions

Citation

Dehairs, F and Fripiat, F and Cavagna, AJ and Trull, TW and Fernandez, C and Davies, D and Roukaerts, A and Fonseca Batista, D and Planchon, F and Elskens, M, Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions, Biogeosciences, 12, (5) pp. 1459-1482. ISSN 1726-4170 (2015) [Refereed Article]


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Copyright 2015 The Author

DOI: doi:10.5194/bg-12-1459-2015

Abstract

This paper presents whole water column data for nitrate N, O isotopic composition for the Kerguelen Plateau area and the basin extending east of Heard Island, aiming at understanding the N-cycling in this naturally iron fertilized area that is characterized by large re-current phytoplankton blooms. The KEOPS 2 expedition (October–November 2011) took place in spring season and complements knowledge gathered during an earlier summer expedition to the same area (KEOPS 1, February–March 2005). As noted by others a remarkable condition of the system is the moderate consumption of nitrate over the season (nitrate remains >20 μM) while silicic acid becomes depleted, suggesting significant recycling of nitrogen. Nitrate isotopic signatures in the upper water column do mimic this condition, with surprising overlap of spring and summer regressions of δ18ONO3 vs. δ15NNO3 isotopic compositions. These regressions obey rather closely the 18ϵ/15ϵ discrimination expected for nitrate uptake (18ϵ/15ϵ = 1), but regression slopes as large as 1.6 were observed for the mixed layer above the Kerguelen Plateau. A preliminarily mass balance calculation for the early bloom period points toward significant nitrification occurring in the mixed layer and which may be equivalent to up to 47% of nitrate uptake above the Kerguelen Plateau. A further finding concerns deep ocean low δ18ONO3 values (<2‰) underlying high chlorophyll waters at the Polar Front Zone and which cannot be explained by remineralization and nitrification of the local particulate nitrogen flux, which is too small in magnitude. However, the studied area is characterized by a complex recirculation pattern that would keep deep waters in the area and could impose a seasonally integrated signature of surface water processes on the deep waters.

Item Details

Item Type:Refereed Article
Keywords:nitrogen cycling, Southern Ocean, Kerguelen Plateau, nitrate isotopic compositions
Research Division:Earth Sciences
Research Group:Oceanography
Research Field:Chemical Oceanography
Objective Division:Environment
Objective Group:Physical and Chemical Conditions of Water
Objective Field:Physical and Chemical Conditions of Water in Marine Environments
UTAS Author:Trull, TW (Professor Thomas Trull)
UTAS Author:Davies, D (Ms Diana Davies)
ID Code:100095
Year Published:2015
Web of Science® Times Cited:10
Deposited By:CRC-Antarctic Climate & Ecosystems
Deposited On:2015-05-01
Last Modified:2017-10-31
Downloads:287 View Download Statistics

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