Temporal and Vertical Variations of Particulate and Dissolved Optical Properties in the South China Sea

Two Bio-Argo floats measured the concentration of chlorophyll-a, the backscattering coefficient, the fluorescence of humic-like dissolved organic matter, dissolved oxygen, and temperature and salinity in the northern and central basins of the South China Sea for over 2 years. Temporal evolutions of bio-optical properties were analyzed at surface, subsurface, and in the whole water column, respectively. It was found that (1) The seasonal variability of the surface chlorophyll-a was highly controlled by photoacclimation, especially in the central basin; (2) backscattering in the upper 150 m was nearly constant, exhibiting no distinct seasonality; (3) with vertical mixing, particles from the deep chlorophyll maxima were entrained into the mixed layer resulting in enhanced surface chlorophyll during the early winter. This phenomenon may mislead a study based on satellite data which is likely to interpret it as blooming rather than a redistribution of phytoplankton within the water column; (4) analysis of a winter bloom and an anticyclonic eddy reveal that physical entrainment and biological photoacclimation modulated the vertical distributions of chlorophyll-a and particles and potentially also changes of phytoplankton community composition; and (5) fluorescent dissolved organic matter was found to be highly coupled to phytoplankton dynamics in both basins, with a maximum (after removing the contribution of physical convective mixing) located at the depth of chlorophyll-a subsurface maximum.