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

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 (OctoberNovember 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 δ 18 O NO 3 vs. δ 15 N NO 3 isotopic compositions. These regressions obey rather closely the 18 ε / 15 ε discrimination expected for nitrate uptake ( 18 ε / 15 ε = 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 δ 18 O NO 3 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.

Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes

“…This change may be related to a decrease of PN content in sinking particles due to a preferential remineralization of PN relative to POC with depth, and resulting in nitrification during the bloom period. Several authors have confirmed the significance of nitrification in the Kerguelen Plateau area, with nitrification equivalent to up to 47% of the nitrate uptake Dehairs et al, 2015;Fripiat et al, 2015). Other studies, investigating suspended particles in the subsurface ocean or particles from deep sediment traps, have generally found a more rapid remineralization of N than organic C (Boyd et al, 1999;Boyd and Trull, 2007;Schneider et al, 2003), consistent with the more labile nature of N (Gordon, 1971).…”

“…Similarly, the NPP increased from 578 to 1064 mgC m À 2 d À 1 from E-1 to E-5 stations respectively. A highly productive station (E-4W) located closer to the Kerguelen Plateau margin was also sampled but excluded from the time series, since being nonrepresentative to trace the seasonal evolution in the less productive PF meander Dehairs et al, 2015;Planchon et al, 2015). -a station north of the PF (F-L) was sampled at mid-cruise and displayed moderate dFe concentrations in surface waters (0.26 nmol L À 1 ; Quéroué et al, 2015), the highest chlorophyll-a (3 mg L À 1 ; Lasbleiz et al, 2014) and NPP (3380 mgC m À 2 d À 1 ; Cavagna et al, 2015) levels.…”

Nutrient cycling in the Atlantic basin: The evolution of nitrate isotope signatures in water masses

“…The δ 18 O NO3 of NADW at 40°S (2.0‰) is too high to produce the low δ 18 O NO3 reported in the Southern Ocean AABW (1.6‰); therefore, these low signatures may be produced by remineralization processes. Recent work has identified low δ 18 O NO3 in the Kerguelen Plateau area of the Southern Ocean, which has been attributed to nitrification [ Dehairs et al , ]. This may suggest that nitrification processes may be prevalent in some regions of the Southern Ocean, causing the decreases in δ 18 O NO3 to lower values in Southern Ocean‐sourced deepwater masses [ Rafter et al , ; Dehairs et al , ].…”

Significant mixed layer nitrification in a natural iron‐fertilized bloom of the Southern Ocean

“…KEOPS 1 was carried out in January–February 2005 and KEOPS 2 in October–November 2011. The data include vertical profiles of nitrate δ 15 N and δ 18 O [ Trull et al ., ; Dehairs et al ., ], mixed layer particulate nitrogen (PN) δ 15 N [ Trull et al ., , ], NO 3 − and NH 4 + uptake rates in the 100 and 0.1–0.01% PAR (photosynthetically active radiation) depth ranges [ Cavagna et al ., ; P. Raimbault, unpublished results], and 234 Th‐based particulate N (PN) export rates [ Savoye et al ., ; Planchon et al ., ]. The discussion is focused on the southeast Kerguelen Plateau because this area was studied during both the spring and late summer expeditions, allowing us to constrain the seasonal evolution of the N‐biogeochemical cycle (from October to February).…”

“…Despite high primary productivity, the seasonal nitrate (NO 3 − ) drawdown was only slightly elevated (6 µmol L −1 ) in comparison to surrounding HNLC areas (3–4 µmol L −1 ) [ Pondaven et al ., ; Nelson et al ., ]. To tackle that apparent paradox, we investigated the upper ocean seasonal nitrogen (N) cycle over the southeast Kerguelen Plateau using model and data for nitrate and ammonium concentrations and uptake rates, particulate nitrogen accumulation and export, and nitrate δ 15 N and δ 18 O [ Savoye et al ., ; Trull et al ., , ; Cavagna et al ., ; Dehairs et al ., ; Planchon et al ., ].…”