Freshwater inflows influence aerial extent, oysters, and resident macrofaunal communities of a restored oyster reef in the Yangtze River estuary.

The study aimed to gauge the influence of freshwater inflows on a restored intertidal oyster reef in the Yangtze River estuary by examining spatial and temporal changes in reef area, oyster densities, and resident macrofaunal communities from 2011 to 2021. Regression analysis indicated that freshwater inflows exceeding 56000 m/s resulted in hyposaline conditions (salinity <1) at the upstream reef. The reef's upstream boundary moved downstream during the study period. The reef area declined from 260 ha in 2011 to 170 ha in 2021, representing a net loss of 35 %. The restored reef experienced more than 30 % decrease in oyster densities and a 40 % decline in resident macrofauna densities. The pattern of loss and decrease was non-random and concentrated on the upstream section. Multivariate analysis revealed significant changes and increased relative dispersion in resident macrofaunal communities from 2011 to 2021, driven by the greater dominances of soft-habitat species, and decreased densities of mollusks and annelids in 2021 relative to 2011. The increased multivariate variations indicated that resident communities faced significant environmental stress from high freshwater inflows. This study concluded that freshwater input was the primary driver of declines in reef area, oyster densities, and macrofaunal abundances at the restored oyster reef. Predicted increases in precipitation in Yangtze River basin due to climate changes are expected to further lower salinities in the estuary, leading to the transitions of intermediate salinities to hyposaline condition that negatively impacts oyster survival, potentially disrupting oyster meta-population and impeding oyster restoration efforts.