Photosynthesis-Irradiance Response In The Eddy Dipole In The Western South China Sea

Photosynthesis-irradiance (P-E) parameters, such as the maximum photosynthetic rate (PmB) and maximum light utilization coefficient (alpha), are key parameters of primary production (PP) models. It is necessary to understand the response of P-E parameters to mesoscale eddies to accurately model PP in eddies. The P-E parameters and phytoplankton communities were examined at the surface and the deep chlorophyll maximum (DCM) of the eddy dipole in the western South China Sea. The surface PmB differed significantly between the edge and center of both eddies. The PmB at the center was significantly higher than that at the edge of the cyclonic eddy (CE), while the opposite trend was observed in the anticyclonic eddy (AE). Phytoplankton community composition modulated variability in P-E parameters in the eddy dipole. Over 75% of the variance in surface P-E parameters could be explained by the pico-phytoplankton communities and environmental factors (nutrients and temperature). In contrast, the ratio of Synechococcus abundance to chlorophyll a concentration was primarily responsible for the variability in P-E parameters at the DCM. These relationships allowed us to model PmB and alpha at all stations and depths sampled. The integrated primary production (IPP) determined from these modeled P-E parameters was coincident with the IPP determined from on-deck incubations. The modeled PmB -alpha-based IPP exhibited a higher IPP at the CE center than that at the AE center. Our findings can improve the parameterization of the physical-biogeochemical coupling models of eddies.