Nitrification rates, ammonium and nitrate distribution in upper layers of the water column and in sediments of the Indian sector of the Southern Ocean

Abstract

During the ANTARES 1/F-JGOFS cruise (April–May 1993) concentrations of inorganic nitrogen, nitrification rates (ammonium and nitrite oxidation) and 14C carbonate incorporation by nitrifying bacteria were measured at 0, 50 and 100 m in the water column, and at the water-sediment interface between the polar front (52°S) and the subantarctic and subtropical fronts (42°S) in the Indian Ocean Sector of the Antarctic Ocean. The 0–100 m water layer showed a global increase of NH 4 + (from 0.4 to 0.6 μM) and a global decrease of NO 2 − + NO 3 − (from 25 to 5 μM) along the south-north transect. Anomalies were detected for both ammonium and nitrite + nitrate concentrations in the SubTropical Front (STF) and SubAntarctic Front (SAF). Pelagic nitrifying activities (N oxidation) did not demonstrate any latitudinal gradient in the water column. In terms of integrated rates between 0 and 100 m, the nitrification increased by a factor 1.8–2 in the STF-SAF area. The N oxidation processes mostly depended on the substrate availability. For both ammonium and nitrite oxidizers the C fixation was well correlated ( r = 0.68, p = 0.001 and r = 0.98, p = 0.0001, respectively) with N oxidation, in the range of 15 N (ammonium oxidizers) and 20 N (nitrite oxidizers) oxidized for 1 C incorporated. For nitrite oxidizers, C incorporation was principally influenced by temperature ( Q 10 = 1,73). From the calculation of nitrogen fluxes, the nitrifiers would be able to compete with the primary producers for the regenerated ammonium, while the flux of nitrate produced by nitrification could sustain 10 to 100% of the primary producer requirements for this nitrogen source. In the sediment pore waters, the inorganic nitrogen compounds displayed two different latitudinal distributions. The ammonium concentrations were lower (around 4 μM) south of the 48°S than in the northern part (values reaching 10 μM in the first centimetre). The nitrite + nitrate concentrations were close to 40–45 μM at each end of the transect, and increased to 50–55 μM near 45°S. The benthic nitrification, measured from 0 to 5 cm depth in the sediment cores, showed high rates in the upper 3 cm. The rates of ammonium oxidation demonstrated a good correlation ( r = 0.64, p = 0.0001) with the ammonium content of the interstitial water. The nitrite oxidation rates were well correlated ( r = 0.92, p = 0.0001) with the ammonium oxidation rates. The integrated rates (0–5 cm) of ammonium oxidation were of about 35 nmol NO 2 − produced cm −2 day −1 in the south, and increased by a factor 1.3–1.9 after 44°S. Both ammonium and nitrite oxidation rates showed a maximum near 44°S, corresponding to the SAF-STF frontal zone.