Dynamic changes in the sea-land convergence zone: Long-term coastal wetland dynamics, landscape pattern evolution, and human-activity impacts in Zhejiang

Coastal wetlands serve as vital ecological buffer zones between marine and terrestrial systems, performing crucial roles in the protection of coastal erosion, wave and tsunami protection, and maintaining ecological functions. However, these important wetlands are increasingly being destroyed and incessantly threatened by human activities. The literature shows existing studies are primarily focused on specific periods or single wetland types, lacking long-term and multi-type systematic analyses. Therefore, this study utilizes Landsat time-series imagery and multiple auxiliary data sources, employing GEE and the Random Forest (RF) algorithm to construct a 35-year time-series wetland classification dataset. It analyzes the dynamic changes of coastal wetlands in Zhejiang Province from 1990 to 2024. Based on the analyses, the primary results are:1) The overall classification evaluation of mapping precision in coastal land types in Zhejiang Province reached 78.6 %, with a Kappa coefficient of 0.75. Although the wetland area peaked in 2005 at 10,700 km 2 , the overall picture from 1990 to 2024 shows that the wetland area had clearly declined, with the total area in 2024 about 20 % smaller than in 1990. Specifically, tidal flats and inland marshes showed significant shrinkage. In contrast, the area of building land increased by over 1900 km 2 , primarily through the continued conversion of tidal flats, inland wetlands, and croplands.2) Inland marshes experienced the most severe degradation. Landscape spatial heterogeneity decreased, while overall landscape connectivity improved. The distribution of landscape units became more fragmented, the area distribution among different landscape types became more balanced, and spatial heterogeneity increased. 3) The overall area of wetlands converted to Building Land and cropland exceeded 2400 km 2 , with transformation hotspots concentrated along the southern coast of Hangzhou Bay, northern Zhoushan, and the Wenzhou Bay shoreline. Additionally, the wetland area exhibited a strong negative correlation with human activity factors. Direct land-use expansion exerts a much stronger and more immediate pressure on wetland loss than aggregated economic or demographic growth indicators. These findings highlight the urgent need for integrated land management and wetland conservation policies in rapidly urbanizing coastal areas, aiming to strike a balance between ecological protection and socioeconomic development. • Generated annual 30-m coastal wetland maps of Zhejiang from 1990 to 2024. • Quantified wetland change, with ∼20 % loss and conversions to built-up or cropland. • Identified land-use expansion as the dominant driver of coastal wetland loss.