A preliminary study of the variation of phytoplankton absorption coefficients in the northern South China Sea

The temporal and spatial variabilities of phytoplankton absorption coefficients (aph(λ)) and their relationships with physical processes in the northern South China Sea were examined, based on in situ data collected from two cruise surveys during May 14 to 25, 2001 and November 2 to 21, 2002. Significant changes in the surface water in aph values and B/R ratios (aph(440)/aph(675)) were observed in May, which were caused by a phytoplankton bloom on the inner shelf stimulated by a large river plume due to heavy precipitation. This is consistent with the observed one order of magnitude elevation of chlorophyll a and a shift from a pico/nano dominated phytoplankton community to one dominated by micro-algae. Enhanced vertical mixing due to strengthened northeast monsoon in November has been observed to result in higher surface aph(675) (0.002–0.006 m −1 higher) and less pronounced subsurface maximum on the outer shelf/slope in November as compared with that in May. Measurements of aph and B/R ratios from three transects in November revealed a highest surface aph(675) immediately outside the mouth of the Zhujiang (Pearl) River Estuary, whereas lower aph(675) and higher B/R ratios were featured in the outer shelf/slope waters, demonstrating the respective influence of the Zhujiang River plume and the oligotrophic water of the South China Sea. The difference in spectral shapes of phytoplankton absorption (measured by B/R ratios and bathochromic shifts) on these three transects infers that picoprocaryotes are the major component of the phytoplankton community on the outer shelf/slope rather than on the inner shelf. A regional tuning of the phytoplankton absorption spectral model (Carder et al., 1999) was attempted, demonstrating a greater spatial variation than temporal variation in the lead parameter a0(λ). It was thus implicated that region-based parameterization of ocean color remote sensing algorithms in the northern South China Sea was mandatory.