Spatial Patterns and Regulation of Nanomolar‐Level Nutrients in the Surface Oligotrophic Oceans

Abstract Ambient nutrient concentrations and their ratios regulate phytoplankton physiology and biomass in the ocean. In oligotrophic surface waters, nitrate and ammonium (NH 4 ) concentrations are often below conventional analytical detection limits (50–100 nM). However, nanomolar‐level nutrients can strongly influence phytoplankton growth in oligotrophic regions. Using highly sensitive colorimetry with detection limits of 3 nM, we quantified surface nitrate plus nitrite (NO X ), NH 4 , and phosphate (PO 4 ) concentrations across the low‐latitude Pacific and Indian Oceans. Concentrations from over 47,648 measurements were analyzed. NO X , NH 4 , and PO 4 ranged from ≤3 nM to several hundred nM, and after calculations of mean NO X , NH 4 , and PO 4 into 0.5° latitude‐longitude intervals, ∼60% of intervals for NO X and NH 4 showed <5 nM, and ∼80% of them showed <10 nM. Spatial PO 4 patterns were significantly associated with phosphacline depths, highlighting the importance of vertical PO 4 supply. The spatial NO X pattern was weakly associated with nitracline depth, and several to tens of kilometers processes, including upwelling and rainfall, contribute to its elevations. The combined NO X + NH 4 to PO 4 ratio was consistently below Redfield ratios. Subtle increases in NO X and/or NH 4 concentrations corresponded to slight increases in chlorophyll a levels, suggesting that both recycled and newly supplied nitrogenous nutrients contribute to phytoplankton biomass in oligotrophic low‐latitude oceans. Overall, our findings emphasize that highly sensitive nanomolar‐scale nutrient analyses are important for understanding the environmental controls on phytoplankton in low‐latitude oligotrophic oceans.