Award: OCE-1923687

This project sought to quantify the relative importance for two biogeochemical processes to explain the occurrence of a nitrogen anomaly that arises in the shallow subsurface waters underlying the low nutrient and low biological productivity subtropical gyre regions of the world ocean. The upper 100-150 meters of the oceanic water column receives sufficient sunlight to permit net photosynthesis by marine phytoplankton, often resulting in a surplus of organic matter beyond the needs of the upper ocean ecosystem, that is in turn exported or transferred to depths below. This biological pump is the primary process by which the ocean sequesters carbon dioxide away from the atmosphere at great ocean depths. As exported organic material sinks it is respired by marine microbes, consuming oxygen and releasing inorganic nutrients back into the water column. For decades, marine scientists have believed the coupling between organic matter respiration, oxygen consumption, and nutrient release to follow constant proportions, or stoichiometry, encapsulated in the concept of the Redfield ratio. However, observations of oxygen and nitrate collected across the global ocean over decades have revealed that the Redfield ratio is not strictly observed in the shallow subsurface, ~100-250 meters depth, within the subtropical regions of the world ocean. The anomaly observed indicates that not enough nitrate (the main N nutrient) accumulates at these depths based on the observed consumption of oxygen and applying the Redfield ratio. This project addressed two of the proposed biogeochemical processes that might explain this ‘missing nitrogen’. This project took advantage of the rich history of biogeochemical observation as part of the Hawaiian Ocean Time-series (HOT) program, choosing to place our field study at its Station ALOHA in the center of the North Pacific Subtropical Gyre (NPSG), the ocean’s largest biome harboring the ‘missing nitrogen’ anomaly. A researcher from UT-Austin collected data with a Video Plankton Recorder, deployed over multiple cruises at Station ALOHA with the goal to quantify the abundance of known vertically migrating phytoplankton types. Some large phytoplankton including some species of diatoms are known to employ vertical migration to travel downwards in the marine water column to access nitrate where it accumulates, returning towards the surface to photosynthesize, causing the anomaly. A second researcher from the University of New Hampshire collected water samples across multiple cruises to Station ALOHA for the chemical analysis of the marine gel pool, often termed Transparent Exopolymer Particles (TEP). Marine TEP is carbohydrate-rich and nitrogen deficient. If this gel pool is larger and more dynamic in time and space than previously assumed, it could also contribute to the ‘missing nitrogen’ anomaly by significantly decreasing the amount of nitrogen required to explain biological productivity and respiration patterns in the ocean subtropics relative to the Redfield ratio. The data collected on the abundance of migrating phytoplankton at Station ALOHA and the surrounding NPSG was inconclusive. Migrating types were observed but at low abundances and the researcher team had uncertainty that the instrumentation deployed was operating as expected. Data collected on the role of TEP cycling in the NPSG was more conclusive. The researchers observed a seasonal cycle in which TEP concentrations build up in surface waters throughout the summer into fall which is then largely returned to lower levels of TEP in winter, matching the background levels observed in deeper waters. Some evidence of export events of this material was also observed with higher concentrations of TEP found at depth in late summer. Assuming this seasonal pattern of TEP concentrations reflects seasonal net production in surface waters with subsequent export and consumption at depth, the TEP cycle at Station ALOHA could possibly explain 22-67% of the ‘missing nitrogen’ anomaly there. The lower nitrogen demand by the marine phytoplankton community for the NPSG implied by the TEP cycle also explains about half of the ‘unexplained’ biological productivity at Station ALOHA. Here unexplained biological productivity refers to the fact that researchers have been unable to sum up all of the biogeochemical processes delivering nitrogen to surface waters to explain the observed rates of biological productivity on annual timescales. The fact that the TEP cycle in the NPSG doesn’t completely explain all of the ‘missing nitrogen’ and ‘unexplained’ productivity signals suggests that vertically migrating phytoplankton may still play a quantitatively significant role as well, but this process awaits further validation. Broader impacts of this work include helping to close the gap in marine biogeochemists collective understanding of how nutrient cycles and the biological pump, responsible for ocean carbon storage, operate in the ocean’s largest biomes. The training of two persons for the scientific workforce was achieved. A postdoc was trained and one M.S. student now working as an ocean field engineer. Dissemination of the project findings to a scientific audience was achieved via professional conferences and to a general public audience via the social media campaign, #SaveOur70. Last Modified: 10/26/2023 Submitted by: Robert T Letscher