Effects of upwelling and runoff on water mass mixing and nutrient supply induced by typhoons: Insight from dual water isotopes tracing

Abstract

AbstractThe issue of whether the increase in nutrients is supplied by increasing runoff or by enhancing upwelling in coastal marine ecosystems after a typhoon is considered controversial. This is because it is relatively difficult to quantify water mixing using traditional methods. Dual water isotopes in the northwestern South China Sea during Typhoon Wipha (July–August 2019) were investigated to address this issue. Weak mixing among different water masses was observed before the typhoon because of strong thermal stratification. After the typhoon, temperature and salinity dropped in the water column, particularly in the upper layer (<20 m), due to strengthened mixing. Mixing in the upper layer predominantly originated from coastal diluted water (36%) in the nearshore, whereas it changed to upwelling (51%) offshore. Upwelling was stronger on the right of the typhoon path (50%) than on the left (36%). Nutrients predominantly originated from the coastal diluted water input (50%) in the upper nearshore layer, while upwelling occurred (53%) offshore. The nutrient supply from upwelling on the right of the typhoon path (43%) was also more considerable than that on the left (25%). This study suggests that typhoon‐induced nutrient supply to the coastal upper water layers from the upwelling water (38%) was more substantial than that from the runoff input (30%), providing new insight into the biogeochemical and ecological responses of coastal marine systems to typhoons.