Fish carcass decomposition drives water quality deterioration and microbial instability in aquaculture systems

Decomposition of animal biomass represents an under-recognized source of toxicological stress in aquatic ecosystems. In Eriocheir sinensis aquaculture, large quantities of fresh-chilled fish are routinely used as feed, creating persistent organic inputs that may form localized decomposition hotspots. To evaluate the ecological consequences of carcass decay under contrasting trophic backgrounds, we conducted a 15-day microcosm experiment using eutrophic Yangcheng Lake water and oligotrophic tap water, with and without fish carcass addition. We tracked physicochemical parameters and bacterial community succession using 16S rRNA gene sequencing. Carcass decomposition rapidly depleted dissolved oxygen, lowered pH and oxidation-reduction potential, and increased total organic carbon, chemical oxygen demand, dissolved solids, conductivity, and inorganic nutrients. Microbial communities showed pronounced temporal turnover, with a shift from aerobic decomposers to anaerobic and fermentative taxa, accompanied by enrichment of opportunistic genera such as Acinetobacter, Aeromonas, and Chryseobacterium. Neutral community model analysis indicated a decline in neutral assembly and a greater role for deterministic environmental filtering in carcass-amended treatments, particularly in oligotrophic water. Predicted functional profiles further suggested enhanced metabolic activity and nutrient-responsive functions under carcass loading. Taken together, the results show that fish carcass decomposition can trigger severe water-quality deterioration and marked microbial instability in aquaculture systems, especially where baseline buffering capacity is low.