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

• Published in: Biogeosciences
• Main Authors: Dehairs, F, Fripiat, F, Cavagna, AJ, Trull, TW, Fernandez, C, Davies, D, Roukaerts, A, Fonseca Batista, D, Planchon, F, Elskens, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2015
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Kerguelen; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-12-1459-2015; http://ecite.utas.edu.au/100095

This paper presents whole water column data for nitrate N, O isotopiccomposition for the Kerguelen Plateau area and the basin extending east ofHeard Island, aiming at understanding the N-cycling in this naturally ironfertilized area that is characterized by large re-current phytoplanktonblooms. The KEOPS 2 expedition (OctoberNovember 2011) took place in springseason and complements knowledge gathered during an earlier summer expeditionto the same area (KEOPS 1, FebruaryMarch 2005). As noted by others aremarkable condition of the system is the moderate consumption of nitrateover the season (nitrate remains >20 μM) while silicic acidbecomes depleted, suggesting significant recycling of nitrogen. Nitrateisotopic signatures in the upper water column do mimic this condition, withsurprising overlap of spring and summer regressions of δ 18 O NO 3 vs. δ 15 N NO 3 isotopiccompositions. These regressions obey rather closely the 18 ϵ/ 15 ϵ discrimination expected for nitrate uptake( 18 ϵ/ 15 ϵ = 1), but regression slopes aslarge as 1.6 were observed for the mixed layer above the Kerguelen Plateau. Apreliminarily mass balance calculation for the early bloom period pointstoward significant nitrification occurring in the mixed layer and which maybe equivalent to up to 47% of nitrate uptake above the KerguelenPlateau. A further finding concerns deep ocean low δ 18 O NO 3 values (<2) underlying highchlorophyll waters at the Polar Front Zone and which cannot be explained byremineralization and nitrification of the local particulate nitrogen flux,which is too small in magnitude. However, the studied area is characterizedby a complex recirculation pattern that would keep deep waters in the areaand could impose a seasonally integrated signature of surface water processeson the deep waters.