Macrozoobenthic Spatial Organization Along Water-Depth and Upstream–Downstream Gradients in a Sand-Mining-Related Reach of Poyang Lake

Operational areas and continuous habitat gradients should be distinguished in sand-mining-related channels. We sampled 17 sites in November 2024 under low-water conditions and 18 sites in July 2025 under high-water conditions along the Poyang Lake–Yangtze River waterway near Duchang. Upstream reference, core mining, adjacent disturbed, and downstream potential-impact areas were compared using permutational multivariate analysis of variance and multivariate dispersion analysis; distance-based redundancy analysis, variation partitioning, and Threshold Indicator Taxa Analysis evaluated depth and spatial associations. Fourteen taxa were recorded in the low-water survey and 16 in the high-water survey. Community composition at the 17 shared stations showed no significant turnover between surveys (paired PERMANOVA, p = 0.068). The full low-water dataset showed an area-associated contrast, but this depended strongly on three zero-record core-mining sites. In high water, water depth had the strongest univariate environmental association, and environmental and spatial variables jointly explained 42.9% of variation. Branchiura sowerbyi showed a stable deeper-site response. These patterns support assessment using predefined areas together with continuous depth and upstream–downstream gradients without causal inference.