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