Against the backdrop of national policies promoting coordinated regional development and ecological civilization construction, the contradiction between ecological security and environmental carrying capacity in the Yangtze River Delta urban agglomeration has become increasingly prominent. However, the interaction mechanisms between these two systems remain insufficiently explored. This study constructs a comprehensive evaluation indicator system for ecological security and environmental carrying capacity in the Yangtze River Delta. A double exponential function is employed to examine the intensity of interaction pressure and reveal their nonlinear relationship. The coupling coordination model is applied to assess coordinated development trends, while a vector autoregression (VAR) model is used to identify the dynamic response relationships among system variables. The results indicate that the overall levels of both systems have improved, with core areas maintaining a leading position and southeastern, northeastern, and western regions remaining in a catching-up stage, accompanied by low-level convergence. Regional coordination exhibits a positive temporal evolution from imbalance to coordination, while its spatial pattern evolves from core dominance toward multi-regional convergence. Significant regional heterogeneity is observed in shock responses, with peripheral cities facing stronger industrial restructuring pressures showing greater ecological volatility. Overall, the dynamic interaction between ecological security and environmental carrying capacity demonstrates a stage-specific transition from mutual constraint to mutual promotion. This study provides empirical support for ecological restoration and regional sustainable development policymaking.

The layout of urban blue, green and grey spaces plays a crucial role in flood risk, but the potential trade-offs and synergistic effects of these spaces on urban flood risk have not been fully studied. This research uses the spatial Durbin model to analyze the direct impact of changes in urban blue, green and grey spaces on risk changes and the spatial spillover effects, revealing how the balance and coordination states and their changes affect urban flood risk. The results show the following: (1) An increase in the balance effect is often accompanied by an increase in risk, but there are differences in the intensity and direction of response at different stages, and the risk changes show significant spatial dependence and spillover effects. (3) The analysis of the spatial Durbin model indicates that the increase in blue and green spaces mainly alleviates risks through local retention and storage. It is notable that in the later stage, the neighborhood spillover effect of green spaces has exceeded its direct local effect, highlighting the collaborative value of green network connectivity at the regional level; while the impact of grey spaces is the most complex—although the construction of sponge cities and urban rainwater management have led to a negative direct local effect (2020–2024), its positive neighborhood spillover effect still persists. Clearly understanding how “balance and coordination” affects risk at different development stages can help urban managers formulate differentiated strategies for the proportion of blue, green and grey spaces based on the development stage of the city, thereby avoiding the use of “one-size-fits-all” planning standards.