Decoupling of wetland quantity and landscape integrity in megacities: a 36-year spatiotemporal analysis of Wuhan from 1991 to 2025

Wuhan, as a representative megacity in the Yangtze River Basin, provides an ideal case for investigating the quantity-quality decoupling paradox in urban wetland restoration. Using annual Landsat imagery from 1991 to 2025, object-oriented classification was employed with overall accuracy ranging from 85.66% to 88.32% and Kappa coefficients exceeding 0.80. Landscape pattern analysis incorporated fragmentation metrics including patch density and landscape division index, alongside connectivity indices such as contagion index and aggregation index, to examine wetland spatiotemporal dynamics over 36 years. The results revealed four major findings: (1) A critical turning point occurred around 2010. Lake wetlands, providing essential ecosystem services including flood regulation, water purification, and biodiversity habitat, declined from 756.80 km² in 1991 to 635.35 km² in 2010, then recovered to 737.00 km² by 2025, driven by policy interventions such as the Wuhan Lake Protection Ordinance. (2) Constructed wetlands represented by reservoirs and ponds expanded from 72.50 km² in 1991 to 487.50 km² by 2025, a 6.7-fold increase relative to the 1991 baseline; however, these artificial systems provide limited ecological services in biodiversity support and hydrological regulation compared to natural wetlands. (3) A quantity-quality decoupling paradox emerged: despite area recovery, landscape fragmentation persisted with patch density elevated at 16.55 in 2025 versus 13.85 in 1991, while the contagion index declined from 58.25 to 53.77 in 2015, indicating structurally compromised restoration. (4) Land surface temperature exhibits a significant negative correlation with lake area, while groundwater level shows a positive correlation, confirming the coupling mechanism between biophysical and anthropogenic drivers. This study reveals that policy-driven wetland restoration in megacities can recover area quantity but struggles to restore landscape integrity due to urbanization lock-in effects, providing critical insights for global urban wetland management by exposing the inherent limits of area-focused restoration in fragmented urban landscapes.