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

Abstract

ContextEstuarine 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, coupled 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.ObjectivesGenerate annual mapping of salt marsh vegetation in the YRD wetland from 1986 to 2022, analyze the trends of wetland patch area and landscape pattern, and explain the hydrological drivers of landscape pattern evolution.MethodsWe combined Landsat 5‒8 and Sentinel-2 images, vegetation phenology, remote sensing indices, and Random Forest supervised classification to map the typical salt marsh vegetation of the YRD. We applied piecewise linear regression to analyze YRD wetland changes and stepwise multiple linear regression to assess the impact of hydrological factors on landscape pattern.ResultsWe identified three stages of landscape pattern evolution with 1997 and 2009 as critical junctures, including the rapid expansion stage, gradual decline stage, and bio-invasion stage. In the rapid expansion stage, the wetland area expanded by 70%, while the typical salt marsh vegetation (Phragmites australis) area was reduced by 25%. In the gradual decline stage, the wetland was reduced by 21% and the Phragmites australis area was reduced by 16%. In the bio-invasion stage, coverage of Spartina alterniflora expanded rapidly, with a 68-fold increase in area relative to 2009, expanding at an average rate of 344 hm2 per year.ConclusionsAreas of total wetland, tidal flat, and Phragmites australis were significantly influenced by cumulative sediment delivery and cumulative runoff, which together explained 61.5%, 75.7% and 63.8% of their variation, respectively. Wetland and tidal flat areas increased with cumulative sediment delivery, while cumulative runoff had a weak negative effect. For Phragmites australis, cumulative runoff had a positive effect, whereas cumulative sediment delivery had a negative effect. Water resources regulation measures should be taken to prevent the degradation of wetland ecosystems, and intervention measures can be implemented during the seedling stage to control the invasion of Spartina alterniflora.