A time series analysis of transparent exopolymer particle distributions and C:N stoichiometry in the subtropical North Pacific: a key process in net community production and preformed nitrate anomalies?

Abstract. Within the oligotrophic subtropical oceans, summertime DIC drawdown despite nutrient limitation in surface waters and subsurface oxygen consumption in the absence of Redfieldian stoichiometric nitrate release are two phenomena still awaiting a full mechanistic characterization. The distribution, intensity and seasonality of these phenomena are identified with preformed nitrate as a tracer, where negative preformed nitrate (NPN) anomalies below the euphotic zone correspond to oxygen consumption without Redfieldian NO₃^- release, and positive performed nitrate (PPN) anomalies found within the upper 100 m occur where O₂ is produced without stoichiometric NO₃^- drawdown.  Many processes that may contribute to these anomalies including N₂ fixation, non-Redfieldian DOM cycling, vertically migrating phytoplankton, heterotrophic NO₃^- uptake and vertical NO₃^- injection events have been measured or modelled, yet generally cannot fully account for the magnitudes of preformed nitrate anomalies and excess DIC drawdown observed in many oligotrophic subtropical waters. One other candidate process that may contribute to both phenomena is the formation of carbon-rich transparent exopolymer particles (TEP) and Coomassie-stainable particles (CSP) from dissolved organic precursors in surface waters and their subsequent remineralization below the subsurface chlorophyll maximum (SCM). However, few data exist to quantify exopolymer production and vertical distributions in oligotrophic oceans over an annual cycle, which is necessary to understand their potential role in the evolution of seasonal preformed nitrate anomalies and DIC drawdown.

To investigate the significance of exopolymer formation and export to North Pacific subtropical gyre biogeochemistry, we undertook a multi-year time-series (Jan 2020 – Sep 2022) analysis of TEP, CSP and total dissolved polysaccharides concentrations at Station ALOHA (22° 45’,158° W), and along a transect from 22° 45’ to 31° N to measure latitudinal variability in June 2021.  Exopolymer C:N stoichiometry at Station ALOHA varied between 16.4 – 34.3, with values being more carbon-rich in summer; ratios were higher (32–38) toward the gyre centre at 31° N. TEP concentrations were consistently elevated in surface waters through Spring–Autumn (4–8 µM C after carbon conversion) at Station ALOHA with lower concentrations (~1.5–3 µM C)  and more uniform vertical distribution during winter, indicating that TEP accumulated in surface waters may vertically sink and be exported with winter mixing. The accumulation of TEP in surface waters through Spring–Autumn and its subsequent export may account for 6.5–20 % of net community production (NCP), helping reduce the estimated imbalance of N supply and N demand at this site to <10 %. The upper ocean TEP cycle may explain 22–67 % of the observed PPN/NPN anomalies, helping to close the C, N, and O₂ budgets at station ALOHA, while leaving room for significant contributions from other processes such as vertically migrating phytoplankton and heterotrophic nitrate uptake to be further validated. These results suggest that exopolymer production and cycling may be more important to open ocean carbon biogeochemistry than previously expected, with considerable seasonality and spatial variability influenced by physical processes and phytoplankton activity.