Nutrient provinces in the sea: Concentration ratios, reaction rate ratios, and ideal covariation

Abstract

Global distributions of the ratios of the concentrations of nitrate + nitrite (= [N]) and phosphate (= [P]) are evaluated from Geochemical Ocean Sections Study (GEOSECS) and Transient Tracers in the Ocean (TTO) data sets. If large oceanic regions (or provinces) can be identified on the basis of constant [N]: [P] ratios, then the distribution equation for a reactive variable shows that the ratio of the net reaction rates involving N and P in each one is equal to its concentration ratio. Organisms within the interiors of the provinces would then be in balance with the ratios in which the nutrients are present, producing a non‐fractionated or “ideal” nutrient covariation. Such provinces can be observed throughout the ocean. Notable features are as follows: (1) Between the euphotic zone and 500 m in the west central North Atlantic is a large region in which N‐P regeneration produces very high [N]:[P] ratios: ∼50 mol mol−1. Potential causes are 18° Water formation, coccolithophorid growth, nitrogen fixation, or atmospheric fixed‐nitrogen deposition. (2) Most oligotrophic surface waters seem to have [N]:[P] between 0 and 3 mol mol−1, implying that the net removal ratio of N and P in those waters is 0–3 mol mol−1. (3) Below 600 m, the ocean contains large provinces with N‐P regeneration ratios of 12–18 mol mol−1. The dominant ratio is slightly sub‐Redfield at 14.5–15 mol mol−1, with the entire Indian Ocean below 3000 m being ideally covariant at 14.7 mol mol−1. The northeastern Pacific has provinces with very low regeneration ratios (<14 mol mol−1). Vertical boundaries between deep provinces in the western Pacific and eastern Atlantic suggest that particles from immediately above control regeneration ratios, whereas the more horizontal boundaries between western Atlantic provinces appear to reflect a greater importance of horizontally transported particles in water masses like Antarctic Intermediate Water. N‐P reaction rate ratios along deep isopycnal surfaces are quite variable.