Trace nitrate in oxic waters

Abstract

We describe a modified (Garside, 1982, Marine Chemistry, 11, 159–167) nitrite method that permits measurements down to subnanomolar concentrations and present datafrom Atlantic and Carribean deepwater profiles for comparison with a published Pacific section.

This important intermediate in the nitrogen cycle was detected in all samples. Concentrations were consistently lowest (0.1–0.4 nM) in oligotrophic surface waters. Below 1 km, carribean and Southwest Sargasso sea nitrite concentrations were 0.4–1 nM, decreasing with increasing depth; reported Pacific [NO₂] averages are several times higher. Profiles in the upper kilometer beneath the classical primary nitrate maximum (PNM) were qualitatively similar, exhibiting a smooth supra-exponential drop with depth to vvalues of ∼1–4 nM at 1 km.

Then nitrite inventory in this “tail” of the PNM above 1 km with 1 nM ≤[NO₂]≤50 nM roughly equals that in the classical PNM. Significant differences among profiles in the 0.1–1 km regionn are observed, consistent with nitrite pool turnover of 3–7 days estimated from Redfield stoichiometry and tritium-helium ages. Thus seasonal and/or regional variations in factors altering the nitrite production-consumption balance, rather than transport, seem to be responsible for nitrite variability.

Nitrite profiles with anomalous midwater or near-bottom fine structure, including multi-point maxima and minima, were found along the Venezuelan continental margin and at ≈ 13°N. These featurers are tentatively ascribed to boundary effects, as hydrographic and circumstantial evidence suggests that these waters interacted previously with the bottom.