Spatio-temporal evolution monitoring and analysis of tidal flats in Beibu Gulf from 1987 to 2021 using multi-source remote sensing

Tidal flats are an important part of coastal wetland systems and play an irreplaceable role in maintaining the health of global coastal ecosystems, resisting natural coastal disasters, and sequestering blue carbon. For the coastal zone around Beibu Gulf, where tidal flats are densely distributed, the spatiotemporal characteristics of tidal flats evolution and its key driving factors remain unclear. This article took the Beibu Gulf as the study area, and constructed an integrated tidal flats extraction algorithm of “maximum spectral index composite (MSIC) + Otsu algorithm (OTSU) + DEM correction” based on the cloud computing platform of Google Earth Engine to realize a large-scope and high-accuracy automatic extraction of tidal flats. The results discovered that: (1) the tidal flats extraction method proposed in this study has high accuracy, with the overall accuracy of the confusion matrix reaching 93.9%, and the Kappa coefficient reaching 0.82. The user accuracy and mapping accuracy of the tidal flats are both greater than 85%. (2) In 35 years, the total tidal flats area of the Beibu Gulf has decreased by 683.9 km2, a reduction of 29%, and the average annual rate of change was -19.5 km2/a. (3) The fragmentation state of the tidal flats in Beibu Gulf intensified with time, and the natural driving forces of the tidal flats were mainly the rise in sea level, decrease in the amount of sediment in the river entering the sea, and expansion of mangrove trees. Human-driven forces were mainly coastal economic development and offshore aquaculture.