Award: OCE-1657958

Currently there are observational data as well as many global model studies that suggest a warming climate is decreasing oxygen concentrations in the oceans. An expansion of very low oxygen regions would likely result in an expansion of the major water column denitrifying zones as well where biologically available nitrogen is converted to biologically unavailable nitrogen gas, potentially decreasing marine fertility. Horak et al.1 (2016) compared 4 oceanographic cruises between the years 1973 and 2012 to see if the oxygen minimum zone (OMZ) and the denitrification regime in the eastern tropical North Pacific had expanded during that time. After adjustment to a standard oceanographic database, GLODAP, the Horak et al. data suggested both the OMZ and the denitrification had expanded with most of the expansion coming after year 2000. The goal of this study was to reoccupy the same eastern tropical North Pacific stations to see it the apparent expansion was a limited perturbation to the system or a more permanent regime shift. To that end we conducted three more sets of measurements in the study area in 2016, 2018 and 2019. Again, we adjusted measurements to the same standard database, calculated N* and oxygen content and added them to the previous published data. The station locations and typical oxygen and N* values are shown in the attached figure 1. Integrations of our new measurements added to historical data are shown in the attached figure 2, where the integrated oxygen in the OMZ (blue points) and the integrated denitrification proxy, N* (red points), are plotted versus time. The data clearly show a general decrease in oxygen and an expansion of the denitrification regime (N*) over the last 50 years with the majority of the denitrification expansion occurring since the year 2000. The eastern tropical North Pacific is one of the global oceans 3 major oxygen minimum zones; the other two are the eastern tropical South Pacific and the Arabian Sea. Together they constitute roughly 50% of the denitrification in the marine environment, with the remainder occurring in marine sediments. The attached figure shows that the increase in denitrification loss of biologically available nitrogen is on the order of 25%. This would suggest that marine productivity in the downstream waters might decrease by 25%. In order to maintain marine fertility, the loss of biologically available nitrogen must be balanced by the addition of available nitrogen from biological nitrogen fixation. At present we do not know if balancing nitrogen fixation is occurring. More study of the marine nitrogen cycle is required to understand the effects of expanding OMZs on the marine nitrogen cycle and marine fertility. 1Horak, R., Ward, B., & Devol, A. (2016). doi:10.1002/2016GL068871. . Last Modified: 09/02/2021 Submitted by: Allan H Devol