Anthropogenic and climate-driven shift in gelatinous zooplankton from jellyfish and thaliaceans to larvaceans over three decades in a subtropical bay.

Gelatinous zooplankton play a crucial role in marine ecosystems by regulating food web dynamics, mediating carbon flow, and affecting fisheries productivity and coastal resources. However, the mechanisms by which combined anthropogenic and climatic stresses drive their long-term dynamics remain poorly understood. Here, we document a shift in gelatinous zooplankton communities on both interannual and seasonal scales in Daya Bay, a subtropical, eutrophic coastal bay in the northern South China Sea, over the past three decades (1994-2023). We identified 2007 as a statistical break point marking an abrupt change in community composition, with winter being the most responsive season to this shift. Following this break point, we observed: (1) a pronounced decline in both species richness and diversity; (2) An abrupt decline in the relative abundance of gelatinous zooplankton in 2008, followed by a gradual increase that did not fully recover to pre-break point levels; (3) a shift in the community composition from jellyfish and thaliaceans to larvaceans dominance; and (4) an increasing contribution of Oikopleura dioica to total gelatinous abundance, particularly in winter. We interpret this community reorganization as resulting from the synergistic effects of chronic anthropogenic stress and acute climatic perturbation. Long-term anthropogenic stressors progressively destabilized the gelatinous zooplankton communities, while the prolonged cold anomaly of winter 2008 (January-February) likely acted as an acute event that accelerated this compositional restructuring, particularly by reinforcing the wintertime dominance of small-bodied taxa such as O. dioica. These findings highlight the sensitivity of gelatinous zooplankton to both climatic and human-driven environmental changes and provide critical insights for monitoring and predicting the functioning of subtropical coastal ecosystems at the gelatinous taxa level.