Coastal wetland degradation and ecological problems in the Yellow River Delta during 2005-2021

The Yellow River Delta wetland is one of the rare estuarine wetland ecosystems in the world, and it is also the most complete wetland ecosystem in the warm temperate zone in China. To understand its ecological problems as a sensitive and fragile new wetland ecosystem, we investigated and analyzed its coastal erosion, changes in wetland types, encroachment of invasive species, changes in the areas of seagrass beds and oyster reefs, ecosystem health status, and low salinity area. The investigation of the coastal wetlands in the Yellow River Delta revealed that from 2005 to 2020, the maximum deposition distance of the Yellow River port beach was about 10 km, and the maximum erosion distance was about 5 km. The siltation rate gradually decreased. From 2016 to 2021, the total area of the coastal wetlands in the Yellow River Delta remained stable. The area change rates of Spartina alterniflora, Phragmites australis, and Tamarix chinensis were 587, 282, and 57 ha/year, respectively, and the annual rate of change in Suaeda salsa was �60 ha/year. From 2015 to 2021, S. alterniflora increased the most sharply in the southern part of the Yellow River Estuary, and this region became the largest area of S. alterniflora in the delta. In recent years, the area of the coastal seagrass beds in the Delta has decreased sharply, and the area of the degraded oyster reefs has decreased further. The high nitrogen and phosphorus content of seawater is the main factor leading to the continuous eutrophication of the delta sea area. From 2011 to 2021, the health status of the Yellow River Estuary ecosystem remained in a sub-healthy state. The monitoring results for the low salinity area revealed that the areas of the low salinity area in 2020 and May 2021 were about 1200 and 1650 km2, respectively. Based on the survey results, the ecological problems faced by the wetlands in the Yellow River Delta such as dynamic coastline changes, coastal wetland degradation, and invasion of S. alterniflora were investigated.