Inferring the potential for nitrogen toxicity on seagrass in the vicinity of an aquaculture site using mathematical models

Abstract

Finfish aquaculture is a source of dissolved nutrients, which can impact water quality in the wider environment. Therefore, the potential effects of dissolved nutrient loading must be considered if management is to transition towards an Ecosystem Approach to Aquaculture. In this study, the dissolved nitrogen dispersion pattern from a rainbow trout farm in Port Mouton (Nova Scotia, Canada) was simulated and evaluated in the context of potential toxicity for a foundation seagrass species. A range of scenarios defined under a precautionary approach were simulated using a fully spatial hydrodynamic model. These worst-case scenarios predicted a maximum nitrogen concentration at any moment of the day of 7.5 μM, which is below the expected toxicity threshold for seagrass. Further scenarios demonstrated that the increased dispersion caused by the wind could drop these values by 45-50% in the vicinity of the farm, suggesting the relevant role of wind forcing in nitrogen dispersion. This outcome suggests that the decline of seagrass reported in some parts of Port Mouton bay are unlikely to have been triggered by dissolved nutrients discharged from the farm. This case-study demonstrates the value of ecosystem modelling to make science-based and transparent decisions to implement an ecosystem approach to aquaculture.

Keywords: Aquaculture; Finfish; Nitrogen; Nutrient loading; Seagrass; Toxicity.