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

摘要

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 (October–November 2011) took place in springseason and complements knowledge gathered during an earlier summer expeditionto the same area (KEOPS 1, February–March 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 δ18O_NO3 vs. δ15N_NO3 isotopiccompositions. These regressions obey rather closely the 18ϵ/15ϵ discrimination expected for nitrate uptake(18ϵ/15ϵ = 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 δ18O_NO3 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.