Award: OCE-1537943

Tracking the temporal and spatial variability of dissolved organic matter, its diagenetic state, and bioavailability during various bloom states in the western North Atlantic Spring phytoplankton blooms, like those in the western North Atlantic Ocean, are significant sources of new organic matter that fuel food webs in the oceanic water column. These spring bloom events represent important sinks for atmospheric CO2; thus, influencing the balance of carbon between the ocean and atmosphere. The organic matter produced by phytoplankton blooms is subject to various food web processes that control the partitioning of organic carbon into the particulate and dissolved phases. Some of the organic matter that accumulates as dissolved organic carbon (DOC) is consumed and fuels bacterial carbon demand for bacterial biomass production and respiration. Marine bacteria are the primary biological entities the control DOC distribution in the ocean and when and where DOC is recycled back to carbon dioxide. The DOC that resists or escapes bacterioplankton utilization can accumulate and persist for months to years. Some of this persistent DOM can be mixed out of the sunlit surface to the dark deep ocean during winter storms. Once delivered to deep ocean depths, the carbon can be stored in the deep sea for periods of years to millennia and thus, represents an important carbon export and potential carbon sequestration pathway in the ocean's biological carbon pump. The work conducted through this award sought to clarify the magnitude and divers of surface DOC accumulation over the course of the annual phytoplankton bloom in the Western North Atlantic, the contribution of DOC to annual vertical carbon export via deep winter mixing, and the fate of surface accumulated DOC following physical mixing (export). This project collaborated with the NASA interdisciplinary field program entitled North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), designed to resolve the dynamics and drivers of the annual phytoplankton bloom and their subsequent impacts on the atmosphere. As a part of the science program, my group collected microbial and geochemical data from four 26-day repeat latitudinal transects in the western North Atlantic, spanning different seasons over four years between 2016 - 2018. This project supported the successful completion of Nicholas Baetge's Ph.D. thesis and twelve peer-reviewed journal articles to date. Primary scientific outcomes are highlighted below. The manuscript entitled "Net Community Production, Dissolved Organic Carbon Accumulation, and Vertical Export in the Western North Atlantic" by Baetge et al. 2020 (Frontiers in Microbiology) combined autonomous float and ship-collected data to constrain the fraction net community production (NCP) that was partitioned into seasonally accumulated DOC. This analysis showed that the mixing of DOC from the sunlit surface ocean to the deeper dark sea significantly contributed to the biological carbon pump in the Western North Atlantic. The results also revealed seasonality in the partitioning of NCP, with more DOC accumulating in the later summer/autumn periods. The manuscript entitled "The Seasonal Flux and Fate of Dissolved Organic Carbon through the Food Web in the Western North Atlantic" by Baetge et al. 2021 (Frontiers in Marine Science) describes microbial growth experiments conducted aboard three of the research expeditions. These experiments measure how much organic carbon that accumulates seasonally as DOC is accessed daily by the bacterial community to support their growth. These experiments' empirically derived estimates of bacterioplankton growth efficiency allowed us to estimate the total amount of DOC required to flow through bacterioplankton to support their growth. A significant finding from this study was that while the bacterial carbon demand became more strongly correlated to primary production as seasons transitioned from high to low productivity, a larger fraction of the DOC production accumulated in the surface ocean and persisted; thus, DOC was available to be exported under the right physical conditions. The manuscript entitled "Bacterioplankton Response to Physical Stratification following Deep Convection" by Baetge et al. (Elementa, in review) captured a rare field opportunity to observe how microbes respond to organic matter during and shortly following physical exported from the sunlit ocean into the dark mesopelagic ocean. In May of 2016, we tracked the temporal evolution of phytoplankton and bacterioplankton processes as the mixing depth shoaled from ~250 m upon arrival on station to < 25 m over three days. Using molecular microbiological approaches, this manuscript describes the rapid changes in bacterioplankton carbon and carbon demand throughout the water column, with relatively smaller and more gradual changes in community composition in response to physical stratification following deep convection. Last Modified: 11/30/2021 Submitted by: Craig A Carlson