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

F Dehairs,M Elskens,A Roukaerts,F Planchon,D Davies,C Fernandez,D Fonseca Batista,A.-J Cavagna,T W Trull,F Fripiat

doi:10.5194/bg-12-1459-2015

Abstract

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 18&amp;varepsilon;/15&amp;varepsilon; discrimination expected for nitrate uptake (18&amp;varepsilon;/15&amp;varepsilon; = 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.

Highlights

The Kerguelen Plateau and leeward off-shelf areas are characterized by intense seasonal phytoplankton blooms, which are sustained by enhanced iron supply from deep water (Blain et al, 2007, 2008)
The present data set adds to the existing data set on dual nitrate isotopic composition for the seasonally ice covered Polar Antarctic Zone (DiFiore et al, 2009) and a meridional section in the Pacific sector (Rafter et al, 2013)
It adds information on the seasonal evolution of nitrate isotopic composition in the iron-fertilized Kerguelen area, by complementing an earlier study that was conducted during summer in the same area (Trull et al, 2008)

Summary

Introduction

The Kerguelen Plateau and leeward off-shelf areas are characterized by intense seasonal phytoplankton blooms, which are sustained by enhanced iron supply from deep water (Blain et al, 2007, 2008). Mosseri et al (2008) report that despite silicic acid and nitrate uptake ratios being close to 1, the apparent nitrate consumption over the season was much lower than the silicic acid consumption, implying significant shallow remineralization of N, as evidenced by substantial subsurface ammonium concentrations, reaching up to 2 μM (Mosseri et al, 2008). It is likely that such conditions would favour a surface ocean development of nitrifying Bacteria and Archaea, with some members of the latter group known to have affinities for ammonium equaling and even exceeding those of diatoms (Martens-Habbena et al, 2009; Stahl and de la Torre, 2012)

Methods

Results

Discussion

Conclusion