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

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.

Effects of rainfall on oil droplet size and the dispersion of spilled oil with application to Douglas Channel, British Columbia, Canada.

Raindrops falling on the sea surface produce turbulence. The present study examined the influence of rain-induced turbulence on oil droplet size and dispersion of oil spills in Douglas Channel in British Columbia, Canada using hourly atmospheric data in 2011-2013. We examined three types of oils: a light oil (Cold Lake Diluent - CLD), and two heavy oils (Cold Lake Blend - CLB and Access Western Blend - AWB). We found that the turbulent energy dissipation rate produced by rainfalls is comparable to what is produced by wind-induced wave breaking in our study area. With the use of chemical dispersants, our results indicate that a heavy rainfall (rain rate>20mmh-1) can produce the maximum droplet size of 300µm for light oil and 1000µm for heavy oils, and it can disperse the light oil with fraction of 22-45% and the heavy oils of 8-13%, respectively. Heavy rainfalls could be a factor for the fate of oil spills in Douglas Channel, especially for a spill of light oil and the use of chemical dispersants.

Budget analysis of Escherichia coli at a Southern Lake Michigan Beach.

Escherichia coli (EC) concentrations at two beaches impacted by river plume dynamics in southern Lake Michigan were analyzed using three-dimensional hydrodynamic and transport models. The relative importance of various physical and biological processes influencing the fate and transport of EC were examined via budget analysis and a first-order sensitivity analysis of model parameters. The along-shore advective flux of EC (CFU/m(2).s) was found to be higher compared to its cross-shore counterpart; however, the sum of diffusive and advective components was of a comparable magnitude in both directions showing the importance of cross-shore exchange in EC transport. Examination of individual terms in the EC mass balance equation showed that vertical turbulent mixing in the water column dominated the overall EC transport for the summer conditions simulated. Dilution due to advection and diffusion accounted for a large portion of the total EC budget in the nearshore, and the net EC loss rate within the water column (CFU/m(3).s) was an order of magnitude smaller compared to the horizontal and vertical transport rates. This result has important implications for modeling EC at recreational beaches; however, the assessment of the magnitude of EC loss rate is complicated due to the strong coupling between vertical exchange and depth-dependent EC loss processes such as sunlight inactivation and settling. Sensitivity analysis indicated that solar inactivation has the greatest impact on EC loss rates. Although these results are site-specific, they clearly bring out the relative importance of various processes involved.