An upper mesopelagic zone carbon budget for the subarctic North Pacific

Abstract. Mesopelagic zone (MZ) carbon budgets comparing supply with demand can be difficult to constrain due to the temporal and spatial offsets between key sources and sinks, and due to nuances of the measurement techniques used, their associated uncertainties, and potential sampling biases. To address some of these challenges, the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign increased the number and variety of simultaneous measurements to monitor temporal variability in the MZ carbon budget using both a Lagrangian frame of reference and long-term autonomous collection. In this study, we collate a comprehensive combination of new and previously published organic carbon supply and demand measurements collected from the surface (5 m) to the upper MZ, defined here as depths from 100 m to 500 m. Research cruise-based measurements were collected near the subarctic North Pacific’s Ocean Station Papa (OSP) during the August 2018 EXPORTS field campaign. The supply of particulate organic carbon (POC) into the MZ averaged 3.0 mmol C m^-2 d^-1, with roughly equal contributions from passively sinking particles and deposits from active diel vertical migration of zooplankton. MZ carbon demand, in the form of respiration, averaged 5.7 mmol C m^-2 d^-1, with most of this demand from free-living bacterioplankton and minor contributions from zooplankton and particle-attached bacterioplankton. The ship-based estimate of water column demand exceeded ship-based supply. Moreover, the MZ carbon demand may have been even higher based on trends in dissolved oxygen concentration from a glider and a biogeochemical float operating from August to November 2018. This imbalance could be resolved by particle dynamics influencing timescales of organic carbon utilization prior to the field campaign. Net community production (NCP) rates measured during the preceding spring and early summer of 2018 based on long-term mooring estimates of dissolved inorganic carbon concentrations. Seasonal trends in upper MZ backscattering measurements in the vicinity of OSP, in addition to long-term decreases in dissolved organic carbon, suggest that the excess in organic C demand in the upper MZ could be accounted for by the release, disaggregation, and subsequent slow degradation of particles from NCP earlier in the year. The OSP MZ carbon budget presented here demonstrates that studies attempting to constrain the fate of exported POC require the integration of samples over short-time (days to weeks; ships) and long-time (months; remote observations) scales. Finally, based on this carbon mass balance approach, we highlight that studies attempting to validate carbon dioxide removal through particle export should consider comparing multiple sample collections and monitoring over longer time scales.