In subtropical eutrophic coastal waters around Hong Kong, phytoplankton or unicellular microalgae can grow rapidly to abnormally high concentrations under favourable environmental conditions. Harmful algal blooms (HAB) are often observed and bring harmful effects on marine ecology and human environments. Traditional red tide monitoring and fisheries management methods rely on resource-intensive and time-consuming field sampling and laboratory analysis of chlorophyll-a concentration (Chl-a) - an indication of algal biomass - as well as manual cell counting and species identification.

Algal bloom species and concentrations have been continuously monitored in a marine fish culture zone using high-frequency algal imaging technologies by a submerged Imaging FlowCytobot (IFCB) for the first t...[ Read more ]

In subtropical eutrophic coastal waters around Hong Kong, phytoplankton or unicellular microalgae can grow rapidly to abnormally high concentrations under favourable environmental conditions. Harmful algal blooms (HAB) are often observed and bring harmful effects on marine ecology and human environments. Traditional red tide monitoring and fisheries management methods rely on resource-intensive and time-consuming field sampling and laboratory analysis of chlorophyll-a concentration (Chl-a) - an indication of algal biomass - as well as manual cell counting and species identification.

Algal bloom species and concentrations have been continuously monitored in a marine fish culture zone using high-frequency algal imaging technologies by a submerged Imaging FlowCytobot (IFCB) for the first time. The core of this system is the development of automatic algal species classifier which hinges on the manual annotation of the IFCB images. A total of 350,000 images have been annotated into 45 destination classes (19 diatom species, 15 dinoflagellate species and 9 other species), which represent the most common red tide causative and potential harmful algal species in Hong Kong.

The algal bloom dynamics has been studied using IFCB and also a continuous real-time water quality monitoring system. Based on the study of the data from March 2019 to August 2021, the relationship between phytoplankton and DO is revealed: (i) a typical diurnal change pattern of algal abundance is observed with peaks around 4pm in the afternoon. Seasonally, greater algal abundance is observed in spring and summer months; (ii) algal species from “diatoms” and “others” genera dominant throughout the sampling period. “Diatoms” are less abundant in winter seasons while “others” are less abundant in spring and fall; (iii) the Shannon-Weaver bio-diversity index H during this period is mostly in the range of 1 - 2.5; (iv) during an algal bloom, the DO dynamics varies according to the different dominant algal species. If the bloom is dominated by Heterosigma akashiwo, a swimmable species,a marked diurnal variation in bottom DO can be observed in addition to significant vertical DO differential.

The IFCB phytoplantkon data are in agreement with the weekly phytoplantkon data provided by the Agriculture, Fisheries and Conservation Department (AFCD) over the IFCB sampling period. Further, based on the analysis on the historical data of phytoplankton and water quality from 1999 to 2020, the following trends of are revealed: (i) a slight increase in depth averaged water temperature and a decrease in nutrient concentrations were accompanied by an increase in the total number of algae cells as well as in the Diatom/Dinofalellate ratio in Yim Tin Tsai (YTT) and Lo Tik Wan (LTW); (ii) the risk of algal blooms increased in the last 20 years in YTT; (iii) the algal bio-diversity in both sites experienced a significant decrease from the early 2000s to the middle 2010s.

The carbon to chlorophyll-a ratio (CCHL) is an important parameter in water quality models for prediction of dissolved oxygen. Based on cell bio-volume obtained from digitized algal images and a well-accepted algal bio-volume to carbon conversion relations, the carbon content can be estimated. In combination with directly measured chlorophyll-a concentration, the CCHL can be determined in-situ continuously for the first time. The results are in reasonable agreement with theory and also proved that the CCHL can vary significantly over an algal bloom cycle - in the range of 10 to several hundred. The predictions of DO dynamics using IFCB estimated carbon content data are also validated by theory.

Overall, this study provides the first quantitative understanding of phytoplankton dynamics and water quality changes during an algal bloom.