Dissolved Organic Matter Dynamics in the Epipelagic Northwest Pacific Low-Latitude Western Boundary Current System Insights From Optical Analyses

High-resolution horizontal and vertical distribution of dissolved organic carbon (DOC), chromophoric, and fluorescent dissolved organic matter (CDOM and FDOM) were investigated in the western boundary current system of the tropical Northwest Pacific (<200 m) in autumn 2017. A strong correlation between DOC and stratification index indicated that the vertical DOC profile was primarily regulated by physical processes. The association of high a(CDOM)(254) with the maximum chlorophyll (Chl a) layer infers phytoplankton-sourced dissolved organic matter (DOM). The a(CDOM)(325) and humic-like FDOM (FDOMH) showed an accumulation in the deeper layer and positive correlations with apparent oxygen utilization and Chl a concentration at the maximum chlorophyll layer, suggesting that these components are related to microbial degradation of biogenic materials. Elevated Chl a at the frontal area between the North Equatorial Current (NEC) and cold Mindanao Eddy enhanced DOM production. Input waters from the NEC showed higher DOC, but lower FDOMH, than inflow waters from the New Guinea Coastal Current/Undercurrent (NGC(U)C). A mass balance model estimated a 6-times higher lateral DOC flux from the NEC tropical-gyre branch (12 degrees N-7.5 degrees N) than that from the subtropical-gyre branch (12 degrees N-17 degrees N). Based on comparison with long-term (1994-2015) average DOC fluxes for the same season, eddy and upstream processes contributed 38%, 46% and 40% of lateral DOC fluxes for the NEC tropical-gyre branch, NGC(U)C and export North Equatorial Counter Current, respectively. These results demonstrated that the quasi-permanent Mindanao and Halmahera eddies greatly enhance lateral export of DOM with altered properties throughout this large conjunction area. Plain Language Summary The tropical Northwest Pacific Ocean is a crossroads linking large-scale water transport among the North, South and central Pacific, and tropical Indian Ocean. Hydrological processes within this area are very dynamic due to mixing of multiple water currents and the occurrence of mesoscale cold and warm eddies. In this study, we assessed how these complex hydrological processes regulate the quality and quantity of dissolved organic matter (DOM) based on high-resolution horizontal and vertical investigations of DOM optical properties. The mixed layer depth largely regulated the vertical patterns for dissolved organic carbon and DOM optical properties. Notably, elevated chlorophyll-a concentrations associated with mixing dynamics at the frontal area increased DOM production. A mass balance model using field current velocity data revealed that dynamic eddy processes greatly enhanced lateral export of DOM. This work highlights the importance of lateral and vertical hydrodynamic processes in controlling carbon biogeochemistry of the surface ocean.

Automated summary

High-resolution distribution of dissolved organic carbon and optical properties of dissolved organic matter in the tropical Northwest Pacific Ocean were studied in autumn 2017. Physical processes play a key role in regulating the vertical distribution of dissolved organic carbon. The presence of specific optical properties in deeper layers suggests a microbial degradation of biogenic materials.