FY 2025 An upper-mesopelagic-zone carbon budget for the subarctic North Pacific Stephens, B.M., M. Roca-Martí, A.E. Maas, V.J. Amaral, S. Clevenger, S. Traylor, C.R. Benitez-Nelson, P.W. Boyd, K.O. Buesseler, C.A. Carlson, N. Cassar, M. Estapa, A.J. Fassbender, Y. Huang, P.J. Lam, O. Marchal, S. Menden-Deuer, N. Paul, A.E. Santoro, D.A. Siegel, and D.P. Nicholson Biogeosciences, 22(13), 3301–3328, doi: 10.5194/bg-22-3301-2025, View open access article at Copernicus (external link) (2025) 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, their associated uncertainties, and potential sampling biases. To address 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 observations. 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 to 500 m. 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 organic carbon into the upper MZ averaged 3.0 mmol C m–2 d–1, with roughly equal contributions from passively sinking particles and from active diel vertical migration of zooplankton. Upper-MZ carbon demand, in the form of respiration, averaged 5.7 mmol C m–2 d–1, with the greatest fraction of demand from free-living bacterioplankton and minor contributions from zooplankton and particle-attached bacterioplankton. Estimates of ship-based upper-MZ demand exceeded ship-based supply. Moreover, the upper-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 the production and export of organic carbon prior to our measurement period. 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, were higher than those measured during the EXPORTS field campaign. 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 carbon 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 upper-MZ carbon budget presented here demonstrates that studies attempting to constrain the fate of exported particulate organic carbon require the integration of samples over short (days to weeks; ships) and long (months; remote observations) timescales. Finally, based on this carbon mass balance approach, we highlight that studies attempting to test strategies of carbon dioxide removal through particle export should consider comparing multiple sampling platforms and monitoring over short and long timescales. Feature Publications | Outstanding Scientific Publications Contact Sandra Bigley | Help
