Exploring the long-term dynamics of detailed wetland types and their driving forces in coastal metropolitan areas from 1990 to 2020

The Guangxi Beibu Gulf Economic Zone (GBGEZ) and Guangdong-Hong Kong-Macao Greater Bay Area (GBA) have rich wetlands. However, long-term maps with detailed wetland types in these two regions are scarce, and studies on the driving forces of wetland changes are insufficient. To this end, our study produced long-term wetland maps to explore wetland dynamics and their driving forces. From 1990 to 2020, the mangrove, tidal flat, coastal shallow water and reservoir had similar change trends in two metropolitan areas. For the GBGEZ between 1990 and 2020, serious wetland degradation, slight wetland degradation, obvious wetland restoration and slight wetland restoration had areas of 569.23 km2, 426.89 km2, 274.83 km2 and 375.78 km2, respectively; for the GBA, the four change types had areas of 986.80 km2, 1854.63 km2, 547.34 km2 and 2183.57 km2, respectively. Overall, wetland degradation in the GBA were significantly larger than that in the GBGEZ. Samplebased analysis indicated that wetland changes in the GBGEZ and GBA mainly attributed to direct drivers, with the GBA suffering more from the influence of these direct drivers. Specifically, over 90 % of slight wetland degradations and 89 % of slight wetland restorations were affected by direct drivers (e.g. aquaculture). Serious wetland degradations were most affected by direct drivers (e.g. land reclamation) from 1990 to 2010 and by indirect drivers from 2010 to 2020 (e.g. water shrinkage). For obvious wetland restorations, most of them in GBGEZ were affected by indirect factors (e.g. water expansion) while in GBA were affected by direct factors (e.g. new reservoir). Overall, our study could offer valuable information to support wetland protection and urban sustainable development.