Key environmental factors for the initiation and termination of the spring dinoflagellate red tides in the southern Zhejiang coastal waters

The southern Zhejiang coastal area is severely affected by harmful algal blooms (HABs), with spring being the peak season for such events. To investigate the key environmental factors driving the initiation and termination of spring dinoflagellate red tides in this region, this study utilized in situ measurements, ERA5 reanalysis data, and satellite remote sensing data to analyze the initial dinoflagellate blooms recorded in 2017 and from 2020 to 2023, aiming to develop a pre-warning method suitable for the spring dinoflagellate red tides in the southern Zhejiang coastal area. The results indicate that in spring, water temperature and salinity in the study area increase, with overall water quality ranging from Class III to below Class IV. Water temperature and turbidity were identified as the key factors triggering dinoflagellate red tide outbreaks in this region. Specifically, the first red tide event of the year occurred within three days after the daily-averaged water temperature at Nanji Station exceeded 17.0 ℃. A sharp temperature rise of more than 2.0 ℃ was more conducive to red tide outbreaks. A notable decrease in turbidity was commonly observed before the red tide outbreaks. The red tide occurred within six days after the turbidity was lower than 10.0 NTU. The intrusion of the Taiwan Warm Current (TWC) into nearshore waters shifted the turbidity front toward the southern Zhejiang coast, thereby influencing bloom occurrence. Additionally, the weakening of the northeast monsoon enhanced the stability of water column, reducing turbidity and creating favorable conditions for blooms. In particular, if the TWC showed greater intensity and earlier intrusion than normal, the red tide outbreaks might occur earlier in the year. The termination of red tide was primarily governed by nutrient availability and salinity. A significant depletion of nutrients improved water quality to Class II or higher, creating unfavorable conditions for algal growth and resulting in the termination of red tides. Sustained salinity levels above 30.0, exceeding the optimal range for dinoflagellate growth, also hindered bloom maintenance. The results of this study suggest that early warning efforts for dinoflagellate blooms in this region should prioritize monitoring water temperature and turbidity, as well as closely tracking salinity variations. For bloom termination, salinity should be considered a key indicator.