Overview of harmful algal blooms (red tides) in Hong Kong during 1975–2021

Hong Kong has a long historical record of harmful algal blooms (HABs). In the 1980s–1990s, HABs were mainly pollution-related and most of the events happened in estuaries and enclosed embayment such as Tolo Harbour and Port Shelter. The major cause of HABs in 1980s–1990s was closely related to nutrients enrichment; included but not limited to changes in the concentration and ratio of soluble N and P in seawater. The major causative organisms of HABs in Hong Kong by then were meso-plankton such as selected species of diatoms and dinoflagellates. Sometimes, zooplankton was also a cause of red tides (the common name of HABs). There has been gradual change after the 2000s. It is attributed to higher and higher influences from the region, namely the Zhujiang (Pearl) River delta. There are increasing influences from Chinese mainland due to extraordinary social and economic growths during the past 20 years. In the past 10 years, HABs in Hong Kong was mainly subject to regional impacts in the Zhujiang River delta. Both the duration period and covered areas have been enlarged which overwhelmed the localized influences of stream pollution and self-contamination of aquaculture zones. More flagellates of uncommon happening become dominant species in HAB now. Nevertheless, the seasonal impact of Noctiluca scintillans in late winter to mid spring remained.