BGC-05 Coastal biogeochemical processes in a climatically sensitive ocean
Drivers of phytoplankton variability in and Near the Pearl River Estuary, South China Sea during Typhoon Hato (2017): A numerical study
Yang Feng* , State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology & Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou, China
Jingwen Huang, University of Chinese Academy of Sciences, Beijing, China
Yan Du, State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology & Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou, China
Karthik Balaguru, Pacific Northwest National Laboratory, Richland, WA, USA
Xianghui Guo, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China

The Pearl River estuary-coastal system in the Northern South China Sea is frequently affected by tropical cyclones (TCs) every year. Previous in-situ and remote sensing studies have found typhoons can enhance phytoplankton biomass and induce blooms in this region. However, the mechanistic links between phytoplankton blooms and typhoons have not been well elucidated due to the interplay of multiple processes along the land-ocean-atmosphere interfaces. Unravelling these interactions will have to rely on an integrated modelling system. Here, we constructed a realistic, 3-dimensional, land-ocean-atmosphere modelling system with the marine ecosystem and sediment components for the China Great Bay Area. By using the integrated modelling system, we quantitatively investigated phytoplankton response to hydrological conditions variations under Typhoon Hato (2017), a strong typhoon case. We analyzed the biological and hydrological characteristics of Lingding Bay, nearshore and offshore, which represent an ecological gradient from eutrophic to oligotrophic conditions. Passive tracer experiments showed that with high river discharge induced by heavy rainfall, the residence time of Lingding Bay is as short as 15-day, less than half of that under the climatological discharge. The increase in freshwater pulse washed out the phytoplankton biomass within Lingding Bay, therefore, is positively correlated with the sea surface salinity (SSS) (0.9), contrasting the negative correlations for the nearshore (-0.7) and offshore (-0.8) regions. The source and sink terms analysis for the offshore region’s post-typhoon response showed that the increase of phytoplankton biomass in the first week was because of the uplift of nutrient-rich subsurface water, while in the second week was because of the seaward propagated nearshore high phytoplankton biomass water. While riverine nutrients support phytoplankton growth in the third week, a large part of phytoplankton biomass was lost to zooplankton grazing, showing the system shifted from the bottom-up control to the top-down control.