Response of Residence Time to Coastline Change in Xiamen Bay, China

Xiamen Bay (XMB), a representative semi-enclosed bay, demonstrates hydrodynamic conditions and water exchange characteristics that are significantly influenced by alterations in the coastline. The three-dimensional hydrodynamic model and remote sensing interpretation techniques were utilized to examine coastline changes and evaluated the spatio-temporal variations in water residence time in XMB from 1955 to 2021. The results indicate that the coastline of the XMB has been considerably modified by extensive reclamation activities. The total reclaimed area reached up to 188.08 km2 during the period of 1955–2021, resulting in a 17.8% reduction in the total bay area. The average residence time increased from 13.28 days in 1955 to 16.94 days in 2003 and then decreased to 16.12 days because of ecological restoration initiatives. Spatially, water residence time increased from the outer sea towards the inner bay, with the high value observed in the northwest part of XMB while the low value was observed in the southeastern region. Among the various sub-regions, Tong’an Bay experienced the most significant change in residence time, followed by the West Sea. Conversely, the Dadeng Waters and Jiulong River Estuary showed relatively minor increases in residence time. The primary factors influencing variations in water residence time are large-scale reclamation projects and ecological restoration measures. These findings provide a significant scientific foundation and technical support for the integrated management of the coastal zone and ecological restoration construction in XMB.