Implications of aquaculture-derived organic matter enrichment for sustainable mariculture density management in coastal waters

Sediments in coastal waters are major reservoirs of organic matter (OM), whose turnover plays a critical role in carbon cycling and ecosystem functioning. Suspended mariculture of fish cages generates substantial biodeposits, leading to OM enrichment in coastal sediments. Yet, the impacts of mariculture-derived OM enrichment on sediment environments are not fully understood, particularly with respect to how enrichment alters redox stratification, how the redox changes interact with microbial community dynamics, and whether such responses are dose-dependent across graded levels of organic enrichment. To address this gap, we conducted a controlled laboratory incubation experiment to quantify sediment responses to low, medium, and high OM enrichment, simulated by adding graded amounts of fish cage-derived biodeposits to sediments, representing varying mariculture densities. We found that increasing OM enrichment progressively altered sediment redox stratification by compressing oxic and suboxic zones, accelerating O₂ and NO₃⁻ depletion, and shifting OM mineralization toward sulfate reduction. Under high OM enrichment, sulfidic conditions prevailed below ~3.5 cm, resulting in high H₂S accumulation. OM enrichment significantly reshaped microbial communities, reducing diversity at low input levels but promoting recovery by favoring anaerobic fermenters and sulfate reducers at higher OM enrichment levels. Functional genes associated with carbon degradation, denitrification, and sulfate reduction increased in abundance with increasing OM enrichment. These findings highlight the importance of incorporating sediment redox buffering capacity, sediment-water interface fluxes, and benthic biogeochemical responses into mariculture carrying capacity assessments and management strategies to ensure ecological sustainability in coastal waters.