Award: OCE-1154741

Oxygen minimum zones of ?the ocean (OMZs; Fig. 1), are unique in having naturally very low oxygen levels. These functionally anoxic conditions promote anaerobic microbial pathways that uniquely impact a wide-?array of biogeochemical processes including those ?affecting trace greenhouse gases (e.g. N2O), trace metals ?and in particular, the microbial production of N2 gas from NO3- (nitrate). Whereas NO3- can be used by phytoplankton for their growth, N2 gas generally cannot. Therefore OMZs are important for regulating oceanic productivity as well as the global ocean carbon and nitrogen? cycles. OMZ?s host 30 to 50% of the total global conversion of NO3- to N2 gas (N-loss)! In addition, the presence of low oxygen restricts the movement of populations of fish, squid, and other oxygen requiring organisms. This in turn affects the use of living marine resources particularly by nations bordering OMZs. A particular challenge is evidence for OMZ expansion in the recent past that may be linked to global warming. If continued into the future, there will be unforeseen global impacts in additional to the more obvious regional ones. Moreover, the behavior of OMZs may be fundamentally misunderstood as a consequence of lack of observations on appropriate temporal and spatial scales. We have recently discovered that circulation features imbedded within OMZ, known generally as mesoscale eddies, can be hotspots for N-loss. Since eddies occur frequently in the Peru ODZ, their presence may substantially increase overall ODZ N-loss as well as its spatial/temporal heterogeneity. Because of there size (~ 100 km diameter) and transient nature, mesoscale eddies are typically overlooked by typical ship-based studies. From their performance in other regions of the ocean, autonomous platforms have great potential for both continuously monitoring OMZ?s for possible responses to global warming as well as targeted studies of important transient phenomenon such as mesoscale eddies. Two key chemical signatures of an OMZ that autonomous platforms need to monitor for biogeochemical studies are very low oxygen concentrations and supersaturated N2 concentrations due to conversion of NO3- to N2 that characterize OMZs. Our primary goal for this project was to develop a sensor suite for such platforms appropriate to OMZs, install it on two floats with a 0 to 500 m depth range (where oxygen is lowest in the OMZ) and verify its performance against standard methods as part of a deployment in the OMZ south of Baja, Mexico. This project produced essential technology for future study of OMZ biogeochemistry. We developed autonomous platforms and instrumentation (Fig. 2) for measuring biogeochemical processes within OMZs. Two Lagrangian Floats, a mature, flexible, and reliable instrumentation platform, were adapted for OMZ operations by hardening them against anoxic corrosion, developing and installing new sensors for measuring O2 and N2 gas as well as other relevant parameters, and developing a new mission for OMZ operations. The floats were deployed for several weeks during May-June 2014 in the Mexican OMZ and their performance assessed by comparison with ship-based and shore-based measurements (Fig. 3). Results showed that observations from the floats were just as precise as existing ship- and shore-based technology (Fig 3 and 4), thereby achieving our primary goals and making available essential tools for study of OMZ?s To assess the long-term, autonomous performance needed for future monitoring of OMZ?s we deployed similar technology during the recent GO-SHIP P18 cruise (Nov. 2016) in the OMZ off Mexico. Six months later the float continues to send high quality data via satellite (Fig. 5). It as anticipated that we reasonable efforts at power consumption, we will be receiving data for another two years! Autonomous ocean observation is increasingly critical to studies of important processes at important temporal and spatial scales. We have developed and validated through this project a float system optimized for studies of OMZ's particularly for measurement of biogenic N2 produced by the N-loss processes unique to these systems and the very low levels of O2 they require. Our team now plans to use this technology for studies of OMZ's focusing on the role of mesoscale of eddies as N-loss hotspots as well as long-term monitoring for climate-change impacts. Last Modified: 05/19/2017 Submitted by: Mark A Altabet