Effects of Bellamya Stocking Density on Water Quality and Bacterial Community Responses in Aquaculture Effluent

Aquaculture effluent commonly contains suspended solids, inorganic nitrogen, reactive phosphate and algal biomass, creating a need for low-input ecological treatment approaches. This study assessed endpoint water-quality and bacterial-community responses to Bellamya stocking density in aquaculture effluent. A no-snail control (CON) and four stocking-density treatments (LD, MD, MHD and HD) were established with three independent tank replicates per treatment. Suspended solids (SS), NH4+-N, NO2−-N, NO3−-N, PO43−-P and chlorophyll a (Chl-a) were quantified, and bacterial communities were characterized by 16S rRNA gene sequencing of the V3–V4 region. At the 60-day endpoint, all Bellamya-stocked treatments had lower NH4+-N and NO3−-N concentrations than the control, and MHD and HD also had lower PO43−-P and Chl-a. SS showed a nonsignificant downward tendency, whereas NO2−-N showed a nonsignificant upward tendency. Bray–Curtis NMDS visualized treatment-associated separation, and PERMANOVA detected significant differences among treatments (R2 = 0.62, p = 0.001; stress = 0.0822). However, PERMDISP was also significant (F4,10 = 4.99, p = 0.001), indicating heterogeneous within-treatment dispersion and requiring cautious interpretation of the PERMANOVA result. Representative genera showed distinct treatment-associated abundance patterns, and 46 of 90 genus–environment associations remained significant after Benjamini–Hochberg correction. Predicted KEGG Level 3 pathways varied numerically among treatments, but none remained significant after false-discovery-rate correction. These findings indicate that Bellamya stocking may provide a low-input ecological component of aquaculture-effluent management, while the microbiome results should be interpreted as community-level associations and predicted functional trends rather than evidence of microbial causality or pathway activation.