Abstract: 

Harmful algal blooms (HABs) are a prevalent yet environmentally detrimental phenomenon in the seawater surrounding Hong Kong. Previous studies have identified several harmful algae species, including those capable of causing red tide or shellfish toxins. However, few studies have highlighted the spatial-temporal patterns of harmful algae in Hong Kong regional seawater. This study employs an amplicon sequencing approach to detect both harmful and non-harmful algae species during a one-year monthly sampling surveys in Hong Kong waters. The results showed that harmful diatoms dominate during the wet season, while harmful dinoflagellates dominate during the dry season. In the wet season, inner bay area exhibits the highest relative abundance of harmful algae, while the outer sea area shows a higher relative abundance of harmful algae during the dry season. The community pattern of harmful algae exhibits spatio-temporal variations, with seawater temperature and nutrient factors primarily influencing these patterns. The relationship between the relative abundance of algae species with environmental factors is investigated with generalized additive mixed model (GAMM) approach. Harmful diatoms exhibit higher relative abundances in higher seawater temperatures, while harmful dinoflagellates exhibit higher relative abundances in lower seawater temperatures. However, this pattern is not observed in non-harmful diatoms and dinoflagellates. In comparison to non-harmful diatoms and dinoflagellates, harmful species exhibit a weaker relationship with nutrient factors such as nitrate and phosphate. Both harmful and non-harmful diatoms prefer high N/P ratio, while harmful dinoflagellates show a peak of preference when total inorganic nitrogen to phosphate ratio is 48.5. The interaction between temperature with nitrate:ammonium ratio and N/P ratio suggests that non-harmful diatoms/dinoflagellates, rather than harmful species, show pattern closer to previous studies. This study can support future research on harmful algae spatiotemporal dynamics in Hong Kong waters and facilitate the environmental conservation in local waters.