Biogeochemistry and dynamics of particulate organic matter in a shallow-water hydrothermal field (Kueishantao Islet, NE Taiwan)

The particulate organic matter (POM) of shallow-water hydrothermal fields has been studied in the context of food web reconstruction, but the processes governing its biogeochemistry and dynamics are poorly explored. Here, we investigate the POM in the Kueishantao hydrothermal field using chemical and hydrodynamic approaches. The depletion of total suspended matter, lower C/N ratios, and higher carbon isotopic values of particulate organic carbon (δ13CPOC) in the vertical plumes relative to values derived from Si-based models were attributed to the hydraulic sorting of the vented particles, which tend to have more N and 13C in the fine fraction. The particulate organic carbon (POC), unlike the total suspended matter, was enriched in the vertical plumes and explained by physicochemical processes rather than biological addition. The POC-enriched plume-top water was found to be a better endmember than the vent fluids to explain particle mixing in the lateral plume. Physical mixing played a steering role in shaping the particle chemistry of the lateral plumes, but markedly 13C-enriched POC was still observable in several near-vent, low-to-intermediate-Si plume waters, implying locally enhanced primary production of at least 0.1–0.4 mg C/m3/h. The presence of eddies, confirmed by flow field measurements, should have contributed to the detection of biogeochemical anomalies via extending the retention time of plume water to 1–2 h. The dominating mixing process resulted in decoupling between the δ13CPOC signatures and carbonate chemistry in this shallow-water hydrothermal plume.