Seasonal fluctuations of nitrite concentrations in the deep oligotrophic ocean

Abstract

Concentrations of nitrite in the Sargasso Sea near Bermuda were measured monthly for 3 years using a chemiluminescent analysis capable of precise determination of concentrations as low as 1 nM. Ammonium and dissolved primary amine concentrations were also determined occasionally. The mean nitrite concentration over the entire period declined exponentially with depth between 300 and 1000 m ( r 2 > 0.99) and then declined slightly from 1.9 nM to 1.3 nM at 2600 m. From 150 to 250 m, the data had a bimodal distribution, with more than one-third of the observations comprising higher concentrations (mean ∼ 147 nM) during February-April that are characteristic of the “classic” primary nitrite maximum (PNM) and almost two-thirds comprising low concentration values (mean ∼ 17 nM) that would not have been detected by colorimetric analysis. A rapid oscillation between the two modes was observed. Combining the exponential [N0 2 −]-depth relationship with Redfield ratio assumptions and apparent oxygen utilization (AOU) rates for the region (Jenkins, 1982), it was calculated that at steady state, nitrite turnover rates over the 150–1000 m depth interval range from 3 to 7 days. The depth integrated nitrite inventory reaches a maximum in the spring and is correlated with peaks in primary productivity and sediment flux. Ammonium concentrations were similar to, or higher than, nitrite concentrations and also increased dramatically during winter mixing with values of 50–100 nM in the 100–300 m depth interval before decreasing to 5–20 nM values at greater depths. During the remainder of the year, concentrations were relatively constant with depth compared to the nitrite concentration profile.