Dissolved organic matter (DOC, CDOM, FDOM) in the western Tropical South Pacific: Depth- and subregion-resolved variability, and hydrothermal influence

The Western Tropical South Pacific has recently been identified as a global hotspot for microbial dinitrogen fixation and shallow hydrothermal activity, yet the dynamics of dissolved organic matter (DOM) in this ecosystem remains understudied. During the TONGA cruise (2019), we investigated the distribution of dissolved organic carbon (DOC), chromophoric DOM (CDOM) and fluorescent DOM (FDOM) from Melanesian waters to the South Pacific Gyre, including the Lau Basin/Tonga-Kermadec Volcanic Arc. DOC concentration, CDOM absorption [aCDOM(254)], the CDOM spectral slope (S275-295) and tyrosine-like fluorescence decreased from surface to deep waters across subregions. In contrast, apparent oxygen utilization (AOU), nutrients, aCDOM(350), specific UV absorbance (SUVA254), humic-like fluorescence, humification (HIX) and combustion (COX) indices increased with depth. These distributions reveal 1) the production of labile, low molecular weight DOM by phytoplankton, and photobleaching in the photic layer, and 2) the production of higher molecular weight, bio-refractory DOM from the remineralization of sinking particulate organic carbon and DOC in deeper waters. Also, the tryptophan-like fluorescence peaks at depth could be associated with the presence of sinking Trichodesmium spp. Regional variations in DOM characteristics were less pronounced than water-mass-related differences but revealed subtle trends along the west-east gradient, with overall higher DOC, CDOM and FDOM levels in the Melanesian and Lau Basin/Arc subregions compared to the South Pacific Gyre. At 200-m depth near the Arc, the release of hydrothermal fluids altered the DOM composition close to the vent, with significant increases in aCDOM(254) and tyrosine-like material, and significant decreases in HIX and COX indices. We further show an indirect, large-scale impact of shallow hydrothermal vents on the DOM stock in the 0–50-m surface layer, driven by the iron fertilization-induced stimulation of planktonic activity in the photic zone. The increased DOM stocks were observed mostly in the Lau Basin/Arc subregion but extending to Melanesian waters and the western edge of South Pacific Gyre. Collectively, these processes shape the optical properties and biogeochemical behavior of DOM, highlighting the importance of hydrothermal systems in the oceanic carbon cycle.