Warming and acidification shape zooplankton community dynamics in Jiaozhou Bay: evidence from an Acid-Heat Vector...

Ocean warming and coastal acidification often co-occur in nearshore embayments, yet their joint associations with zooplankton remain difficult to characterize because responses are frequently nonlinear and spatially heterogeneous. In this study, we developed an integrated quantitative framework for characterizing joint warming-acidification exposure in Jiaozhou Bay. Based on six cruises conducted at nine stations in May (spring) and August (summer) from 2022 to 2024, we characterized zooplankton communities using abundance, biomass, Shannon-Wiener diversity (H'), and Pielou's evenness (J). We further introduced a joint-exposure indicator, the Acid-Heat Vector Intensity Index (AHI_vec), which summarizes joint temperature-pH departure based on standardized anomalies of temperature and pH. Generalized additive models (GAMs) were then applied to examine nonlinear response patterns and zone-specific associations between environmental gradients and zooplankton community attributes. Within the 2022-2024 observation window, sea surface temperature was generally higher in 2024 than in 2022, whereas surface pH was generally lower; however, these patterns should be interpreted as short-term observations rather than formal long-term trend estimates. Zooplankton also showed pronounced spring-summer and spatial variability. Notably, abundance showed pronounced temporal variation and generally increased with temperature, whereas biomass peaked at intermediate temperatures (∼20-22 °C) and declined thereafter, indicating a mismatch between abundance and biomass. Although pH-related associations were weaker than temperature-related associations, they became more evident at the lower end of the observed pH range (approximately 7.7-7.9). The AHI_vec gradient was associated with contrasting zone-specific patterns in zooplankton community attributes. Overall, recent temperature-pH variations were associated with changes in zooplankton abundance, biomass, and diversity in Jiaozhou Bay. The AHI_vec + GAM framework provides a monitoring-oriented complement to direct temperature-pH analyses for characterizing joint exposure in coastal ecosystems.