Spatio-temporal changes and hydrological forces of wetland landscape pattern in the Yellow River Delta during 1986-2022

Abstract

 Estuarine wetlands provide valuable ecosystem services, but 20-78% of coastal wetlands are facing the risk of loss by the end of the century. The Yellow River Delta (YRD) wetland, one of the most productive delta areas in the world, has undergone dramatic changes under the influence of a precipitous drop of sediment delivery and runoff, coupling with the invasion of Spartina alterniflora. Monitoring the spatio-temporal patterns, thresholds, and drivers of change in wetland landscapes is critical for sustainable management of delta wetlands. Combining 1728 Landsat 5‒8 and Sentinel-2 images, vegetation phenology, and remote sensing indices of vegetation and water body, a Random Forest supervised classification was used in the Google Earth Engine platform to map the annual typical salt marsh vegetation from 1986 to 2022. We identified three stages of landscape pattern evolution with 1997 and 2009 as turning points, including the rapid expansion stage, gradual decline stage, and bioinvasion stage. In the first phase, the wetland area was expanded by 70.45%, while the typical salt marsh vegetation, Phragmites australis area was reduced by 25%. In the second phase, the wetland was reduced by 21.33% and the Phragmites australis area was reduced by 15.96%. The third stage, Spartina alterniflora demonstrated an unstoppable trend of rapid expansion, with an area increase of 68 times relative to 2009, expanding at an average rate of 3.44 km2 per year. Areas of wetland, tidal flat, and Phragmites australis are significantly influenced by cumulative sediment and cumulative runoff, which total explain 61.5%, 75.7% and 63.8% of their variations, respectively. Wetland and tidal flat areas increased with the cumulative sediment, while cumulative runoff had a weak negative effect. As for Phragmites australis, cumulative runoff had a positive effect, whereas cumulative sediment had a negative effect. These findings provide important theoretical support for the water resources management of delta wetlands.