Typhoon Effects on the Vertical Chlorophyll Distribution on the Northern Shelf of the South China Sea

Typhoon Merbok passed through the northern South China Sea and landed near the Pearl River Estuary in June 2017. Field observations showed that the chlorophyll-a (Chla) concentration in the bottom/subsurface chlorophyll maximum (BCM/SCM) layer decreased in both nearshore and offshore regions after Merbok. In addition, the BCM/SCM layer became shallower in the nearshore region and deeper in the offshore region after Merbok. A coupled physical-biogeochemical model was applied to investigate the driving mechanism for this Chla change. During the typhoon, the spatial movement of plume water was responsible for the surface Chla change in the nearshore region, and vertical mixing increased Chla in the upper layer and decreased it in the SCM layer in the offshore region. Shortly after the typhoon, surface Chla increased and bottom Chla decreased in the nearshore region, which was associated with strengthened bottom flow and vertical mixing. When the typhoon-induced mixing decreased, upwelling tended to shoal the BCM layer in the nearshore region, and phytoplankton production increased Chla in the BCM layer. In the offshore region, vertical mixing was the dominant mechanism for Chla redistribution during the typhoon, which decreased Chla in the SCM layer and resulted in a deeper SCM layer. A few days after the typhoon, phytoplankton production also contributed to the Chla change particularly in the subsurface layer, which was stronger in the nearshore region than that in the offshore region. These findings may contribute to the understanding of phytoplankton dynamics in response to typhoons in coastal areas. Plain Language Summary Typhoons occur every year in the South China Sea (SCS). The northern shelf of the SCS is also affected by the Pearl River plume and coastal upwelling. Understanding how these features interact and affect marine ecosystem dynamics is particularly important. We conducted two cruises in June 2017 before and after Typhoon Merbok. Based on these in situ measurements, we observed a clear change in the bottom/subsurface chlorophyll maximum after Merbok. By using a coupled physical-biogeochemical model, we examined the underlying dynamics of this change. We showed that the immediate change in chlorophyll was caused mainly by physical processes, and the subsequent gradual change in chlorophyll was due to both biological and physical processes, particularly in the subsurface layer. Different mechanisms at different stages in surface and subsurface layers, as well as in nearshore and offshore regions, were identified, suggesting the complex dynamics of marine ecosystems in response to typhoons in coastal areas.