Modelling sediment assimilative capacity and organic carbon degradation efficiency at marine fish farms

ABSTRACT: The eutrophication of sediments underlying marine fish farms is one of the major environmental concerns of the industry and regulatory agencies. In this study, a mechanistic model of sediment geochemistry was developed to: (1) determine maximum organic loads that can be degraded without leading to eutrophicated conditions, (2) predict the transition to suboxic conditions as a result of organic enrichment, and (3) predict sulfide levels in surface sediments, a key regulatory variable in Canada. A new definition is proposed for sediment assimilative capacity (AC) of marine fish farms, as the gross deposition rate of organic wastes that maximizes total organic carbon (Corg) degradation rates while preventing sediment sulfide accumulations in surface sediments (upper 2 cm) above regulatory limits (AC-H2S). Model results were consistent with empirical observations and highlight the influence that organic loading history, hydrodynamics, and microbial activity have on assimilative capacity. AC-H2S varied between 0.6 to 22.1 g Corg m-2 d-1 in poorly flushed environments, with no upper limit defined in environments exposed to mean tidal currents >9.5 cm s-1 (dispersion scenarios), where most fish farm organic wastes are dispersed to the far-field or resuspended after deposition below fish cages. The combination of diagenetic modelling and geochemical indicators may contribute significantly to the development of more effective tools for site selection and environmental management of marine fish farms. At this stage, the model should be considered as a proof of concept, developed with the purpose of verifying the utility of assimilative capacity for real-world application. Further validation is still required.