Collaborative Research: High resolution glider observations enable reassessment of export production in the oligotrophic Sargasso Sea

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Observations and measurements over several decades at locations like the Bermuda Atlantic Time-series Study (BATS) site have been important to fundamental understanding of the links between the physics, chemistry, and biology of the ocean. At the BATS site however, despite the long history of observations, a mystery remains: When and how is enough nitrogen delivered to surface waters to support the biological production that is observed? The investigators will use oxygen, nitrate, and micro-turbulence sensors on autonomous underwater gliders to sample continuously for twenty months at the BATS site in an effort to resolve this mystery. Gliders make it possible to measure processes at scales that can't be captured by monthly or biweekly sampling from a ship. Regular sampling by the BATS program will still be essential to the project, however, by providing opportunities for calibration and validation of the glider sensors as well as ship support for glider deployments and recoveries. The project will also leverage existing private funding from the BIOS-SCOPE project, a multi-institutional microbial marine biogeochemistry initiative at BATS, that will provide partial support for glider operations. The investigators will recruit and advise at least two NSF Research Experience for Undergraduates (REU) students each year, and will incorporate project themes and data into educational programs at their respective institutions. This project will apply new technologies to address several factors believed to perpetuate significant underestimates of nitrate delivery and particulate organic carbon (POC) exports in the Sargasso Sea. High-resolution glider-based observations will be used to estimate annual net community production (ANCP) through oxygen mass balance, and new production (NP) through nitrate mass balance, over a period spanning an annual cycle and two consecutive oligotrophic periods. Microstructure measurements will provide much-needed profiles of vertical diffusivity to accurately constrain fluxes of oxygen and nitrate and their variability over the annual cycle. Oxygen/argon ratio measurements will be used as independent estimates of biologically mediated oxygen changes and as constraints on glider-measured rates of oxygen concentration change. Assessments of phytoplankton characteristics by flow cytometry will be used to calibrate the gliders' optical sensors to in situ biological characteristics, so that variability in phytoplankton community structure and biomass can be discerned at the gliders' high temporal resolution and related to simultaneously measured NCP, NP, and physical water column structure, both above and below the euphotic zone. POC exports will be inferred from backscatter inventories in the deeper layers. The sum of observations will enable assessments of relationships among three independently measured proxies for the strength of the biological carbon pump and how the system responds to environmental forcing over the annual cycle, with particular focus on the stratified oligotrophic months.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.