ESTIMATING RATES OF ANAEROBIC NITRITE OXIDATION IN THE EASTERN TROPICAL NORTH PACIFIC OXYGEN MINIMUM ZONE THROUGH INCUBATION EXPERIMENTS

Abstract

Nitrogen is an important nutrient for life in the ocean, and up to thirty percent total marine nitrogen loss occurs in ocean Oxygen Minimum Zones (OMZs), where the nutrient is converted to biologically unavailable gaseous forms. The budget of biologically available marine nitrogen, determined by reactions in the ocean nitrogen cycle, is not well-quantified, especially since nitrite oxidation – a step in the nitrogen cycle – surprisingly occurs in OMZs despite the lack of dissolved oxygen. To estimate nitrite oxidation in OMZs and consider the effects on the marine nitrogen budget, this study utilized tracer experiments with five incubation timepoints and the denitrifier method on seawater samples from the Eastern Tropical North Pacific (ETNP) OMZ. Seawater was sampled from three different locations in the ETNP OMZ: coastal, open ocean (central), and peripheral. Each location was sampled at ten depths, and the tracer experiment replicated three times for each depth, resulting in 450 data points. The calculated nitrite oxidation rates were compared to various proxies from shipboard measurements, such as dissolved oxygen, to assess possible environmental and chemical relationships. Nitrite oxidation was detected at all sampled locations in anoxic waters; the coastal environment had the greatest nitrite oxidation maximum of 48.5 nM/day, and the peripheral environment had the lowest maximum of 12.8 nM/day. These magnitudes of nitrite oxidation are comparable to those of the dominant biological processes (anammox and denitrification) that occur in OMZs; this stresses the importance of the reaction and the need for incorporation in models that utilize the nitrogen cycle.