Short-Term Variability of Biological Production and CO2 System Around Dongsha Atoll of the Northern South China Sea: Impact of Topography-Flow Interaction

The short-term variabilities in temperature, salinity, nitrate, chlorophyll a (Chl a), and carbonate chemistry data (i.e., pH, partial pressure of CO₂ (pCO₂), total alkalinity, and dissolved inorganic carbon) were concurrently investigated in the shallow (∼50 m) and deep (∼300 m) water areas around Dongsha Atoll of the northern South China Sea (NSCS). The results show that surface temperature and pCO₂ were lower but that upward nitrate flux and Chl a were higher in the upper euphotic zone at the shallow-water area than those at the deep-water area. We suggest that the observed contrasting biogeochemical properties between the two areas could be attributed to the impact of topography-flow interaction. As tidal currents (or any other horizontal currents) interacted with the shallow topography, they may induce vertical isotherm displacements (e.g., internal tides/internal waves) that may enhance turbulent mixing, and thus can transport more nutrient-replete subsurface water into the euphotic zone and stimulate phytoplankton production. The stimulated biological production and the cooling effect induced by topography-flow interaction may collectively drive surface pCO₂ down in the shallow water area. Though the present short-term hydrological and CO₂ data, which to our knowledge were concurrently investigated for the first time in the NSCS, reveal that the topography-flow interaction could be a favorable mechanism for atmospheric CO₂ uptake around Dongsha Atoll, more long-term observations are still needed to confirm that the similar processes can repetitively take place in topography-flow interaction prevalent areas.