Predictive depositional modelling (DEPOMOD) of the interactive effect of current flow and resuspension on ecological impacts beneath salmon farms

ABSTRACT: Sediment resuspension is an important factor in controlling the impact of any localised point source impacts such as salmon farms; at high-flow (dispersive) sites, resuspension can significantly reduce potential effects. Depositional modelling (DEPOMOD) is widely used to predict localised seabed impacts and includes an optional flow-related resuspension module. This study examined the observed impacts at 5 farms with contrasting flow regimes to evaluate the role of modelled resuspension dynamics in determining impacts. When resuspension was included in the model, net particle export (i.e. no significant net downward flux of organic material) was predicted at the most dispersive sites. However, significant seabed effects were observed, suggesting that although the model outputs were theoretically plausible, they were inconsistent with the observational data. When the model was run without resuspension, the results were consistent with the field survey data. This retrospective validation allows a more realistic estimation of the depositional flux required, suggesting that approximately twice the flux was needed to induce an effect level at the dispersive sites equivalent to that at the non-dispersive sites. Moderate enrichment was associated with a flux of ~0.4 and ~1 kg m−2 yr−1, whilst highly enriched conditions occurred in response to 6 and 13 kg m−2 yr−1, for low and dispersive sites, respectively. This study shows that the association between current flow, sediment resuspension and ecological impacts is more complex than presently encapsulated within DEPOMOD. Consequently, where depositional models are employed at dispersive sites, validation data should be obtained to ensure that the impacts are accurately predicted.