Spatial segregation of calanoid and cyclopoid copepods in the coastal waters of the Yellow Sea

Understanding the mechanisms that drive ecological separation among species is essential for explaining biodiversity patterns, especially in marine systems characterized by spatial heterogeneity. This study explores the distributional dynamics of two dominant copepod groups, calanoids and cyclopoids, in the coastal waters of the Yellow Sea, using high-resolution in situ data collected using the PlanktonScope underwater imaging system. We analyzed their horizontal and vertical distribution patterns, the degree of spatial overlap, and the environmental factors influencing these patterns. Our results reveal a distinct horizontal separation between the two groups. Calanoid copepods constituted over 80% of the total copepod abundance within 30 km of the coastline, making them the dominant taxonomic group in nearshore waters. In contrast, cyclopoid copepods exhibited an inverse distribution pattern, becoming the predominant group in offshore waters beyond 30 km. This spatial boundary, aligned with coastal current patterns and temperature gradients, further points to temperature as a key environmental driver. Calanoid abundance increased in warmer waters (>18°C), whereas cyclopoid density declined linearly as temperatures rose from 14°C to 20°C. Chlorophyll a exhibited a negative correlation with individual body size, although the effect was weak. This pattern was primarily driven by the higher abundance of smaller individuals in the nearshore zone, where chlorophyll a concentrations were elevated. By integrating advanced plankton imaging with statistical modeling, this study demonstrates how physical and biological gradients interact to structure copepod communities in coastal ecosystems. These findings improve our understanding of species coexistence and ecological differentiation in dynamic marine environments.