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- Title
Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions.
- Authors
Dehairs, F.; Fripiat, F.; Cavagna, A.-J.; Trull, T. W.; Fernandez, C.; Davies, D.; Roukaerts, A.; Fonseca Batista, D.; Planchon, F.; Elskens, M.
- 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 the 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 ... vs. ... 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 preliminary mass balance calculation for the early bloom period points toward significant nitrification occurring in the mixed layer and which could account for up to 80% of nitrate uptake above the Kerguelen Plateau. A further finding concerns deep ocean low ... values (< 2%0) underlying high chlorophyll waters at the Polar Front Zone and which cannot be explained by remineralisation and nitrification of the local particulate nitrogen flux, which is too small in magnitude. However, the studied area is characterised 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.
- Subjects
KERGUELEN Plateau; NITROGEN cycle; NITRIFICATION; PHYTOPLANKTON; REGRESSION analysis
- Publication
Biogeosciences Discussions, 2014, Vol 11, Issue 9, p13905
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bgd-11-13905-2014