Spatiotemporal dynamics of gelatinous and other metazooplankton in the northeastern East China Sea in 2023, highlighting the dietary composition of <i>Salpa fusiformis</i>

ABSTRACT The northeastern East China Sea (ECS) is an ecologically important marine ecosystem influenced by multiple water masses. We investigated spatiotemporal variations in metazooplankton communities, including gelatinous zooplankton, during May, July and October 2023. Both species abundance and richness were lowest in July and comparable between May and October. Spatially, along the 33°N latitude, overall individual metazoan abundances consistently decreased, whereas species richness increased, towards the east across all seasons. Although copepods dominated communities, gelatinous zooplankton—particularly salps (Thaliacea) and jellyfish (Cnidaria)—were notably abundant in eastern regions, suggesting possible introductions via Kuroshio Current–associated warm waters. A significant negative correlation between copepod and salp abundances suggests that salps contribute to reductions in copepod populations, possibly through overlapping prey utilization. A dietary analysis of Salpa fusiformis, detecting 80 distinct DNA sequences within their digestive tract, suggests that salps serve as ecologically important grazers within the region’s pelagic food web. This study demonstrates that the spatiotemporal distribution patterns of metazoans in the northeastern ECS are largely influenced by distinct water masses. The population increase of salps in this region could be partially attributed to current-driven transport and diverse prey availability, highlighting the complex ecological interactions shaping this marine ecosystem.