Threshold indicator trait analysis uncovers functional shifts in macrobenthos across stress gradients: Insights from two adjacent yet contrasting bays

Bay ecosystems are highly sensitive to anthropogenic stress, requiring precise ecological monitoring and management. This study investigated Xiangshan Bay (semi-enclosed) and Sanmen Bay (open bay) using macrobenthic data from 2018 to 2022. A combination of ecological indices (AMBI, M-AMBI, bAMBI, M-bAMBI, and CWM) and Threshold Indicator Taxa Analysis (TITAN) was applied to evaluate ecological quality and identify trait-based thresholds. Results showed that Sanmen Bay exhibited higher ecological quality and clearer functional trait thresholds than Xiangshan Bay. In Sanmen Bay, AMBI (0.50–2.13) and bAMBI (0.42–1.60) indicated undisturbed to slightly disturbed conditions, while M-AMBI (0.45–0.72) and M-bAMBI (0.42–0.52) corresponded to moderate to good status. Trait thresholds (AMBI = 0.74–1.85; bAMBI = 0.60–1.45) revealed gradual replacement of sensitive taxa by tolerant ones along increasing stress gradients. In contrast, Xiangshan Bay showed higher disturbance levels (AMBI = 1.30–2.38; M-AMBI = 0.36–0.46) and evident functional convergence, reflecting an “adaptive locking” effect where long-term eutrophication constrained community resilience. The open hydrodynamics of Sanmen Bay maintained higher functional diversity, whereas the semi-enclosed Xiangshan Bay experienced stronger human-induced stress. These results suggest that restocking species with negative trait responses in highly stressed areas could enhance ecosystem resilience. Overall, the trait-based TITAN framework reveals critical thresholds of macrobenthic functional responses to environmental stress, offering new insights for the refined assessment and adaptive management of coastal bay ecosystems. • Comparative study of two adjacent bays with distinct environmental conditions. • Identified trait-based ecological thresholds in Xiangshan and Sanmen Bays. • Sanmen Bay exhibited a better ecological status and clearer functional trait thresholds. • Xiangshan Bay showed functional convergence and adaptive locking under stress. • Trait-based TITAN provides early warnings for resilience loss and guides restoration.