Stage-specific nutrient pulses and transient microbial-network intensification in deep-water croaker cage aquaculture

Deep-water cage mariculture is expanding rapidly worldwide, yet its stage-specific ecological effects remain poorly understood. We assessed physicochemical conditions and bacterial communities surrounding the net-pen farm for large yellow croaker in Dinghai Bay, a pioneering deep-water cage mariculture zone in the East China Sea. Seawater was sampled from farming and reference sites at early, mid-, late, and harvest-culture stages. Most water-quality variables rose over time, but only nitrogen and phosphorus-related parameters showed modest, statistically significant increases in the late stage (p < 0.05). Spatial contrasts between farm and reference sites were minimal throughout the cycle, indicating generally mild water-column impacts. Amplicon sequencing revealed stable alpha-diversity between sites, whereas beta-diversity demonstrated subtle but consistent shifts linked to mariculture activity. The number of unique amplicon sequence variants and bacterial network complexity increased notably from mid-to late stages, suggesting intensified microbial interactions under farming conditions. The structural equation modeling further revealed that the complexity of microbial networks was primarily driven directly by mariculture activities. Both environmental and microbial assemblages trended toward baseline conditions within weeks after harvest, implying rapid recovery. Overall, deep-water cage mariculture exerted limited and largely reversible effects on the adjacent water column, although cumulative nutrient enrichment and microbial reorganization in the late stage warrant continued monitoring and spatial planning. These findings provide empirical evidence for the sustainable development of offshore aquaculture and highlight the value of full-cycle assessments in management frameworks.