Abstract:

Under the dual influence of human activities and natural factors, the coastal zone patterns are prone to rapid changes which can directly or indirectly affect the structure, function, and sustainable development of the coastal ecosystem. Using the coastal zone of Nanhui Dongtan in Shanghai as a typical research area, we used remote sensing interpretation, sea chart digitization, and field investigation to analyze changes in spatial patterns and changes in coastal zones over the last 20 years (from 2000 to 2020). In addition, the effects of coastal engineering (including reclamation engineering and siltation promotion engineering) and S. alterniflora invasion on coastal pattern dynamics were analyzed. The results showed that: ① Since 2000, under the influence of coastal engineering and biological invasion, the land use types of Nanhui Dongtan coastal zone changed from a simple pattern dominated by coastal wetlands to a complex pattern which included multiple land use types (i.e. coastal wetlands, inland wetlands, constructed wetlands, farmland, and construction land). ② Coastal reclamation engineering decreased 11894.7 hm² of coastal wetlands in the Nanhui Dongtan coastal zone from 2000 to 2005. The reclaimed coastal wetlands were transferred into land use types such as rice fields, ponds, and farmland due to human activities; the reclamation engineering promoted deposition of sediment in the estuary and tidal mudflat (above 0 m) and the intertidal salt marsh developed with increased rates of 320.5 hm²/a and 110.9 hm²/a, respectively; meanwhile, the siltation rate decreased to 286.8 hm²/a and 15.7 hm²/a, respectively, after 2015. After 10 years (2005—2015) of natural recovery, the area of coastal wetlands did not reach the levels seen before reclamation in Year 2000. ③ Two types of siltation promotion engineering—hard siltation promotion engineering and biological siltation promotion engineering—have both significantly promoted the rapid development of coastal wetlands in Nanhui Dongtan. Hard silting promotion engineering with propagation rates of 516.9 hm²/a in tidal mudflats (above 0 m) and 915.7 hm²/a in intertidal salt marshes, respectively, was 5.4 times and 13.9 times higher than rates observed in non-siltation areas; hence, the effects were more significant than biological siltation promotion engineering which only resulted in pattern changes in a limited area between the seawall and the wave dissipation dike by planting S. alterniflora. ④ After introducing S. alterniflora in Nanhui Dongtan, it became the most dominant plant in the Nanhui Dongtan salt marsh, accounting for 56% of the total area; this significantly changed the ecological structure and function of coastal wetlands in 2020. Coastal engineering and biological invasion have a great impact on coastal zone patterns. Although the coastal ecosystem showed a certain resilience to coastal human activities, changes in the wetland type, area, and function were difficult or impossible to recover at a great cost in the future. How to integrate the ecological functions of coastal wetlands and inland wetlands through the implementation of coastal zone restoration, ecological protection, and other measures to achieve the sustainable and healthy development of coastal zones is an important problem for future land and sea planning.

Key words: coastal zone, pattern change, coastal engineering, biological invasion, Nanhui Dongtan