Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions
|Author(s)||Dehairs F.1, Fripiat F.1, Cavagna A. -J.1, Trull T.W.2, 3, 4, Fernandez C.5, 6, 7, Davies D.2, 3, Roukaerts A.1, Fonseca Batista D.1, Planchon F.8, Elskens M.1|
|Affiliation(s)||1 : Vrije Univ Brussel, Analyt Environm & Geochem, Earth Syst Sci Res Grp, Brussels, Belgium.
2 : CSIRO Marine & Atmospher Res, Hobart, Tas, Australia.
3 : Antarctic Climate & Ecosyst Cooperat Res Ctr, Hobart, Tas, Australia.
4 : Univ Tasmania, Inst Marine & Antarct Studies, Hobart, Tas, Australia.
5 : Univ Paris 06, Univ Paris 04, Lab Oceanog Microbienne, Obser Oceanol,UMR7621, F-66650 Banyuls Sur Mer, France.
6 : Univ Concepcion, COPAS SurAustral Program, Dept Oceanog, Concepcion, Chile.
7 : Univ Concepcion, Interdisciplinary Ctr Aquaculture Res INCAR, Concepcion, Chile.
8 : Univ Bretagne Occidentale, Inst Europeen Mer, Lab Sci Environm Marin LEMAR, Brest, France.
|Source||Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2015-03-05 , Vol. 12 , N. 5 , P. 1459-1482|
|WOS© Times Cited||11|
|Note||Special issue KEOPS2: Kerguelen Ocean and Plateau Study 2|
|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 mu M) 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 delta O-18(NO3) vs. delta N-15(NO3) isotopic compositions. These regressions obey rather closely the (18)epsilon/(15)epsilon discrimination expected for nitrate uptake ((18)epsilon/(15)epsilon = 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 delta O-18(NO3) 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.|