Marine Oxygen Deficient Zones serve as hotspots for the loss of fixed nitrogen for the world’s oceans, and fixed nitrogen limits primary productivity in large expanses of the ocean. This fixed nitrogen loss occurs primarily through denitrification, where the stepwise reduction of nitrate to nitrite and ultimately to dinitrogen gas is coupled to organic matter oxidation. Nitrite, the first intermediate in denitrification, can also be re-oxidized back to nitrate in a reaction by chemoautotrophic microbes. Nitrite’s partitioning between reduction and oxidation determines if marine fixed nitrogen is lost or recycled. Nitrite oxidation in anoxic waters has been previously studied through stable and tracer isotope experiments, but the difficulty of these measurements has limited their geographical distribution and therefore requires extrapolation to understand their impact on the nitrogen cycling. Using basin-scale data, we analyze the progression of nutrients within the three water masses that feed the Eastern Tropical North Pacific Oxygen Deficient Zone. Significant deviations from the expected stoichiometry for denitrification demonstrate that 79% of the nitrite produced in the upper region of the Oxygen Deficient Zone is re-oxidized, whereas only 54% of the nitrite produced in the lower region of the Oxygen Deficient Zone is re-oxidized. These large estimates for nitrite re-oxidation reveal significant fixed nitrogen recycling across the Eastern Tropical North Pacific.
