Notable Variations in Chlorophyll a in Response to Successive Typhoons in and near the Pearl River Estuary

Two successive Typhoons, Cempaka (2107) and Lupit (2109), passed through the Pearl River estuary (PRE) in 2021 late July and early August, when an extremely low discharge was recorded. On the basis of field observations, a physical–ecological coupled model, and a large discharge-sensitive experiment, chlorophyll a (Chla) responses to these typhoons and driving mechanisms in and near the PRE were investigated. The results show that Chla decreased considerably during the passage of typhoons and subsequently recovered in Lingdingyang and the upper nearshore region. The upper Chla bloomed notably in the shelf region following successive typhoons. Conversely, the bottom/subsurface Chla maximum (BCM/SCM) decreased following Cempaka and disappeared after Lupit. Dynamically, variations in Chla were primarily dominated by different physical processes in different areas during the typhoon periods. They were driven by horizontal advection in Lingdingyang and vertical and horizontal advection in the nearshore region. Vertical diffusion and vertical advection (horizontal advection) were responsible for the Chla changes after Cempaka (Lupit) in the shelf region. In addition, enhanced net primary productivity contributed to Chla increases or recoveries of 2 to 3 (4 to 5) d after Cempaka (Lupit), except for the upper shelf region. Compared with the large discharge experiment, variations in Chla were larger (weaker) in Lingdingyang and in the BCM/SCM layer (at the upper layer of the nearshore and shelf regions) under low discharge. Owing to ongoing ocean warming and increasing extreme climate events, the frequency of successive typhoons and extreme discharge will increase; therefore, more attention should be paid to the impacts of these events on estuarine ecosystems in the future.