Abstract:

Aims: The frequency of harmful algal blooms (HABs) in the Bohai Sea and the North Yellow Sea has been increasing continuously in the context of coastal eutrophication and climate change. Dinoflagellate cysts are considered a potential seed bank and play a key role in the occurrence of HABs. This study aimed to investigate the diversity, distribution patterns, and environmental drivers of harmful dinoflagellate cysts in the Bohai Sea and North Yellow Sea, and to assess the potential risk of HABs in these regions. 

Methods: A DNA metabarcoding approach was applied, targeting the small subunit ribosomal (18S V4) and internal transcribed spacer (ITS1) to investigate the diversity and distribution of harmful dinoflagellate cyst assemblages in surface sediments from the Bohai Sea and North Yellow Sea in May 2023. Canonical correspondence analysis (CCA) was used to assess the correlations between cyst assemblages and environmental variables. 

Results: The environmental DNA (eDNA) information revealed 38 potentally toxic and harmful dinoflagellate cyst species, including 5 species not previously known to produce, and 8 newly recorded species. Among them, Azadinium spinosum, Gymnodinium nolleri and Alexandrium fragae were detected for the first time in sediments samples from the coastal waters of Chinese. Alpha diversity analysis revealed a clear decreasing trend in species richness from the Bohai Sea, through the Bohai Strait, to the central North Yellow Sea, and this trend was positively correlated with the water temperature gradient. CCA result indicated that the abundance and distribution of dominant harmful dinoflagellate cysts in the two seas were influenced to varying degrees by environmental factors (temperature, salinity, water depth, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and N/P), with temperature being the main explanatory variable. Temperature, in combination with the N/P ratio and DIN, affected and regulated the abundance and distribution of most coastal-estuarine species, resulting in higher detection rates and/or abundances in the warm and eutrophic southwestern Bohai Sea. Low temperature, high salinity, and low nutrient levels were key factors contributing to the dominance of A. catenella cysts in the North Yellow Sea, particularly around the central cold-water mass. 

Conclusion: This study provides crucial information on the diversity and distribution patterns of HAB dinoflagellate cysts in the whole Bohai and North Yellow Seas in spring. The southwestern Bohai Sea remains a high-risk area for HABs due to both established toxic and harmful species and emerging potential risks, such as HAB-forming Lepidodinium chlorophorum, whereas the North Yellow Sea faces elevated risks from the widespread high relative abundance of A. catenella. These findings demonstrate that molecular techniques are a powerful tool for monitoring HAB cysts, underscoring the need for their use in early warning of HABs and for protecting marine aquaculture from blooms.

Key words: harmful dinoflagellate cysts, DNA metabarcoding, diversity and distribution, environmental factors