Aims The Changdao waters represent typical temperate marine and island ecosystems situated at the convergence of the Bohai and Yellow seas, functioning as a crucial east-west migration corridor for numerous economically significant fishery species. As essential secondary producers, variations in zooplankton community dynamics have the potential to significantly impact biogeochemical cycles and energy fluxes within aquatic ecosystems. This study aims to analyze the annual fluctuations in zooplankton species composition and community structure in the southern waters of Changdao. The findings aim to furnish scientific insights into the mechanisms shaping biodiversity formation and facilitate the effective conservation of local biodiversity.

Methods Zooplankton samples were collected from the southern waters of Changdao using a plankton net with a mouth area of 0.08 m² and a mesh size of 160 μm. Sampling was conducted during 10 monthly voyages spanning from March to December, 2021. Zooplankton specimens were meticulously identified to the species level or the lowest feasible taxonomic category. Subsequently, species diversity metrics, including the Shannon-Wiener index, richness, and evenness indices, were computed. Additionally, multivariate analyses were employed to discern variations in zooplankton communities across different months.

Results A total of 53 zooplankton taxa (mostly at the species level) were identified, with pelagic larvae (17 taxa) and Copepoda (16 species) comprising the predominant groups, accounting for 32.1% and 30.2% of the total taxa, respectively. Additionally, the community included 8 species of Cnidaria, 3 species of Amphipoda, 2 species each of Cladocera, Cumacea, and Tunicata, and 1 species each of Ctenophora, Decapoda, and Chaetognatha. Zooplankton species richness exhibited a temporal pattern, increasing from March, peaking in July, and gradually declining thereafter, with counts ranging from 19 to 38. Seasonally, species richness was highest during summer (June to August), followed by autumn (September to November), spring (March to May), and lowest during winter (December). The dominant zooplankton species in the southern waters of Changdao were primarily small copepods and pelagic larvae, likely influenced by the 160 μm mesh sampling nets. Notably, small copepods, such as Oithona similis, Paracalanus parvus, and Acartia hongi, were consistently prevalent across all survey months, while larger copepods, like Calanus sinicus,were relatively less frequent. Copepods exhibited the highest abundance among different taxa, followed by pelagic larvae. Multivariate analyses revealed significant differences (P < 0.01) in zooplankton community structures across different months, with exceptions observed only between March and April. Compared to neighboring regions, such as Laizhou Bay, Laoshan Bay, and Jiaozhou Bay, the southern waters of Changdao exhibit lower zooplankton species richness but higher overall species Shannon-Wiener index, ranging from 2.73 to 3.15.

Conclusion The unique environmental conditions, characterized by low temperatures, high primary productivity, and robust seawater circulation, likely underpin the observed pattern of low zooplankton species richness but high biodiversity in the southern waters of Changdao. Despite the relatively modest species count, the zooplankton community in this region, predominately comprised of small and medium-sized copepods and pelagic larvae, exhibits remarkable biodiversity. This diverse zooplankton assemblage serves as a dependable source of food support for migrating fish and other marine organisms traversing the waters of Changdao.