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

Abstract

AbstractHigh‐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 aCDOM(254) with the maximum chlorophyll (Chl a) layer infers phytoplankton‐sourced dissolved organic matter (DOM). The aCDOM(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°N–7.5°N) than that from the subtropical‐gyre branch (12°N–17°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.