Exploring the spatial and temporal dynamics of zooplankton assemblages in the Seomjin estuary and Gwangyang Bay, South Korea: A comparative analysis of Sequence Assignments Species (SAS) and Morphological Species Identification (MSI)

Estuarine and coastal ecosystems are dynamic environments with strong environmental gradients that influence zooplankton community structure and distribution. Comprehending these patterns is essential for ecosystem management and biodiversity evaluation; however, traditional morphological identification consistently underestimates species diversity, as demonstrated by our comparative analysis. This research examined the structure of zooplankton communities and the distribution of copepod species along an estuarine-marine gradient in the Seomjin estuary and Gwangyang Bay, employing both Sequence Assignments Species (SAS) and Morphological Species Identification (MSI) methodologies. We collected environmental variables and zooplankton samples from six stations in May and September 2023. The Copepoda group constituted 55.1 % of the zooplankton community, with Ciliophora at 31.6 %, Meroplankton at 8.3 %, and Others at 5.0 %. SAS analysis revealed the presence of 31 copepod species distributed among three orders, exhibiting similarity values between 97.03 % and 99.29 %. We observed strong spatial and temporal variations in community structure and diversity indices. Copepod abundance significantly increased in May, especially at bay stations, whereas September showed more uniform distribution. Environmental analysis revealed that salinity and water temperature were primary drivers of species distribution, resulting in distinct species composition between estuarine and bay environments. Species diversity (H′) was consistently greater in SAS (0.69–2.92) than in MSI (0.47–1.74), with both methods indicating elevated diversity in May. NMDS analysis revealed distinct spatial separation between estuarine and bay communities, indicating the impact of environmental variables on community structure. The findings improve our understanding of zooplankton ecology in transitional waters. Both methods offer valuable insights; however, SAS consistently exhibited greater accuracy, evidenced by higher detection rates, more apparent community differences, and stronger environmental correlation, indicates its better performance for biodiversity assessment in complex estuarine ecosystems.