Mixing control on the photosynthesis-irradiance relationship and an estimate of primary production in the winter of the East China Sea

Abstract

The response of marine phytoplankton primary productivity to available light (photosynthesis-irradiance relationship, P-B-E, curve) is the principle physiology of plant that is particularly important in the estimation of primary production and in physical-biogeochemical modeling. In this study, we conducted on-board incubation experiments to examine photosynthesis-irradiance relationships in samples collected from the subtropical shelf of the East China Sea during a winter cruise. Depth and spatial variation of the maximum photosynthesis rate normalized to chlorophyll a (P-m(B)) and the initial slope (alpha) of the P-B-E curve parameters were insignificant in the East China Sea. The vertical homogeneity was due to the strong winter mixing. However, spatial homogeneity could be possibly caused by different factors. With relatively low solar radiation in the winter, strong vertical mixing deteriorated light dose and may lead to homogeneously low photosynthetic parameters. In addition, differential inhibitions by temperature and nutrient availability may potentially cause the spatial homogeneity. The mean values of P-m(B) and alpha were 2.82 +/- 0.50 gC (gChl)(-1) h(-1) and 0.010 +/- 0.002 gC (gChl)(-1) h(-1) (mu mol photons m(-2) s(-1))(-1), respectively. Using the photosynthetic-irradiance model (P-B-E curve model), the estimated value of shelf-averaged euphotic zone integrated primary production was 186 +/- 117 mgC m(-2) d(-1). With an overestimate of satellite-derived primary production, field observations will be required.