Projected Effects of Climate-induced Changes in Phytoplankton biomass in the Southern South China Sea

Abstract. Phytoplankton lies at the base of the marine pelagic food webs influencing the planet’s net primary production and nutrient cycles. Understanding the impacts of anthropogenic climate change on the phytoplankton dynamics is crucial due to their pivotal role. Numerous studies across various latitudes have investigated the effects of climate change on the world’s oceans, focusing on future projections on plankton. However, despite being one of the largest marginal seas of the world, the impact of such future projections on marine plankton in the South China Sea has rarely been documented. Monsoon derived productive upwelling areas in the South China Sea serve as ideal sites for studying plankton dynamics. In this study, a 3D coupled physical-biogeochemical model is used to examine the response of phytoplankton biomass and selected abiotic and biotic factors to future anthropogenic climate change, focusing on two upwelling areas of the southern South China Sea: Southeast Vietnam and Northwest Sabah. The results show declined phytoplankton biomass associated with warming and nutrient depletion, particularly silicate. However, the grazing pressure by mesozooplankton is predicted to be reduced, suggesting that this studied system is under bottom-up control. This study highlights the anticipated amplification of climate change-induced impacts on the phytoplankton over to higher trophic levels, which may influence both ecosystems and socioeconomics in the region.