Abstract
An accurate representation of the particle organic matter (POM) footprint is necessary in order to effectively predict impacts upon benthic communities and the risk of excessive organic enrichment beneath aquaculture sea-cages. Consequently, bottom-related processes such as particle resuspension must be adequately parametrized and evaluated in the available numerical models. We implemented two approaches to model POM resuspension in a Lagrangian particle tracking model and compared their influence on footprint extension and gradients of depositional flux against a no-resuspension scenario. We performed simulations in both exposed and protected aquaculture locations, and at different stages of the Atlantic Salmon (Salmo salar) production cycle in Norway. Our results indicate that the use of sediment-dependent thresholds for resuspension has the potential to regulate the high levels of erosion produced when selecting a low critical value in constant-threshold approaches, particularly in dynamic environments with mixed sediment types.
Highlights
Open cage finfish aquaculture is associated with a broad spectrum of environmental impacts, ranging from disease transference and ge netic interactions between reared and wild organisms to direct and indirect impact of fish farm wastes on the surrounding benthic eco systems (Forrest et al, 2007; Taranger et al, 2014; Grefsrud et al, 2018)
Our results indicate that the use of sediment-dependent thresholds for resuspension has the potential to regulate the high levels of erosion produced when selecting a low critical value in constant-threshold ap proaches, in dynamic environments with mixed sediment types
By comparing no-resuspension, constant-value and substrate-dependent resuspension scenarios, we evaluate whether the inclusion of the new substrate-dependent approach for particle re suspension provides a better prediction of the extent of the particle organic matter (POM) foot prints beneath fish farms
Summary
Open cage finfish aquaculture is associated with a broad spectrum of environmental impacts, ranging from disease transference and ge netic interactions between reared and wild organisms to direct and indirect impact of fish farm wastes on the surrounding benthic eco systems (Forrest et al, 2007; Taranger et al, 2014; Grefsrud et al, 2018). All studied farms are located over a mixture of sediments with textures equal or coarser than sand, i.e. very few areas near the farm present cohesive beds Under these conditions, a mean shear stress in the order of 0.1–0.3 Pa would be required to guarantee material transport, with sites like Farm F1 in Frøya Archipelago and both farms in Altafjorden falling short with respect to this condition. We see that a rapid-assessment method based on the calculated bed shear stress, is a practical and simple way to identify the potential for resuspension events at a fish farming location, and serves as an indicator of the complexity of the model required to simulate the spread of the particles and the extent of the POM footprint
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