Deepening of the thermocline increases primary production in winter vs summer in the northern South China Sea

Abstract

Primary production (PP) is higher in winter than in summer in the northern South China Sea (NSCS), which is different from temperate seas. Few field studies have been conducted to examine the regulatory factors of the seasonal difference in PP. In this study, we conducted two cruises to investigate PP in large areas in the NSCS in summer and winter. In summer, integrated primary production (IPP) was about 349 mg C m−2 d−1, lower than the winter value of around 449 mg C m−2 d−1. IPP exhibited no significant seasonal difference on the shelf between summer and winter, while beyond the shelf, IPP was significantly (p<0.05) higher in winter compared to summer. Deepening of the thermocline increased the availability of nutrients in the upper euphotic layer and was responsible for the increased IPP in winter. Furthermore, the deepened thermocline induced changes in the photosynthetic efficiency and light adaptation of the phytoplankton community, which might be another important reason for the IPP increase in winter. Integrated new production (INP) and POC flux exported from the euphotic layer were also higher in winter than in summer. The increase in POC flux was more pronounced compared to INP because in addition to being affected like INP by the increase in nutrient supply and IPP in winter, the decrease of sea surface temperature would also lead to an increase in POC flux.