Impacts of large-scale suspended mussel farm on seston depletion

The productivity of a suspended mussel farm depends on the abundance of naturally available food. This study constructs a tracer decay model that incorporates mussel filtration and integrates it with a well-established 3D ocean circulation model to examine the spatiotemporal variation of seston depletion at a large-scale suspended mussel farm on Gouqi Island, located offshore in the East China Sea. A novel Wirewalker ocean-wave-powered vertical profiling system was deployed to monitor chlorophyll-a concentration at the edge of the mussel farm. The observed profiles of chlorophyll-a concentration revealed that the current exiting the farm during ebb tide is associated with a lower concentration compared to the current entering the farm during flood tide. This suggests that the filter feeding activity of mussels within the farm is primary contributor to this reduced chlorophyll-a concentration. This finding is consistent with the tracer model results, which indicates a higher tracer concentration entering the farm and a lower tracer concentration exiting it. The model results suggest that the blocking effect of the farm can significantly exacerbate seston depletion, particularly during neap tide as opposed to spring tide. The interplay of tidal characteristics and topography has resulted in two areas that are particularly vulnerable to seston depletion: one centrally situated within the mussel farm, and another near the islands. However, it is possible to alleviate seston depletion by aligning the waterway within the farm with the major axis of the tidal ellipse. Consequently, it is crucial to consider tidal conditions surrounding the farm for optimal farm layout design, increased production, and enhanced stock quality in shellfish farms facing seston depletion.