1066 / 2024-09-20 10:19:11

Revealing cascade effects of typhoon process on the eutrophication, algal bloom, hypoxia, and acidification in coastal water: insights from online high frequency observations and model in Zhanjiang Bay

Typhoon,Eutrophication,Algal blooms,Hypoxia,Acidification,Cascade effects

The sequential occurrence of eutrophication, algal blooms, hypoxia and acidification following typhoons has been documented in previous studies. It is possible that a cascading effect has been caused by the typhoon. However, the mechanisms behind the typhoon-induced cascading effect remain poorly understood. This study employed high-frequency in situ observation and machine learning model to track the dynamic variations in meteorological, hydrological, physicochemical, and chlorophyll-a (Chl-a) levels throughout the entire passage of super typhoon “Yagi”through Zhanjiang Bay (ZJB) in September 2024. The results showed that the mean Chl-a, pH and dissolved oxygen (DO) levels were 6.69 μg L^{- 1}, 7.88 and 5.78 mg L^{- 1} respectively, during pre-typhoon period. It is noteworthy that an algal bloom occurred 5 days after the typhoon, when temperatures reached a suitable range and there was an ample supply of nutrients and water stability. Consequently, the concentration of Chl-a, pH and DO reached a peak of 136.35 μg L^{- 1}, 8.68, and 21.07 mg L^{- 1} respectively, during the algal bloom period. However, these values subsequently declined, resulting in a pH of 7.59 and a DO concentration of 4.56 mg L^{- 1}, which were both lower than the pre-typhoon values. Moreover, the slow decline of 6.4 % in salinity suggested a substantial freshwater input, leading to an increasing estuary nutrient flux. To examine the role of typhoons in regulating algal blooms and nutrient fluxes, a preliminary machine learning model was established to predict Chl-a and estuary nutrient fluxes. This study elucidated the mechanisms underlying the cascading effects of typhoon-induced eutrophication, algal blooms, hypoxia, and acidification in coastal waters. It provided insights into the prediction of algal bloom events and estuary nutrient flux, which are crucial for the control of eutrophication and the sustainable development of coastal waters.