Variation of phytoplankton absorption coefficients in the northern South China Sea during spring and autumn

J. Wu (1) , H. Hong (1) , S. Shang (1) , M. Dai (1) , Z. Lee (2)

1 State Key Laboratory of Marine Environmental Science
2 NRL - Naval Research Laboratory

We examined the temporal and spatial variabilities of phytoplankton absorption coefficients (?_ph?)) and their relationships with physical processes in the northern South China Sea from two cruise surveys during spring (May 2001) and late autumn (November 2002). A large river plume induced by heavy precipitation in May stimulated a phytoplankton bloom on the inner shelf, causing significant changes in the surface water in ?_ph values and B/R ratios (?_ph(440)/?_ph(675)). This was consistent with the observed one order of magnitude elevation of chlorophyll ? and a shift from a pico/nano dominated phytoplankton community to one dominated by micro-algae. At the seasonal level, enhanced vertical mixing due to strengthened northeast monsoon in November has been observed to result in higher surface ?_ph(675) (0.002?0.006 m^?1 higher) and less pronounced subsurface maximum on the outer shelf/slope in November as compared that in May. Measurements of ?_ph and B/R ratios from three transects in November revealed a highest surface ?_ph(675) immediately outside the mouth of the Pearl River Estuary, whereas lower ?_ph(675) and higher B/R ratios were featured in the outer shelf/slope waters, demonstrating the respective influence of the Pearl River plume and the oligotrophic nature of South China Sea water. 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. In addition, a regional tuning of the phytoplankton absorption spectral model (Carder et al., 1999) demonstrated a greater spatial variation than seasonal variation in the lead parameter a0(?). These results suggest that phytoplankton absorption properties in a coastal region such as the northern South China Sea are complex and region-based parameterization is mandatory in order for remote sensing algorithms.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

J. Wu, H. Hong, S. Shang, M. Dai, Z. Lee. Variation of phytoplankton absorption coefficients in the northern South China Sea during spring and autumn. Biogeosciences Discussions, 2007, 4 (3), pp.1555-1584. ⟨hal-00297894⟩