LOCAL AND REGIONAL DRIVERS OF OXYGEN VARIABILITY IN COASTAL EMBAYMENTS ON THE SOUTHWEST SCOTIAN SHELF: IMPLICATIONS FOR NOVA SCOTIA ATLANTIC SALMON (SALMO SALAR) FARMING
Globally, the coastal ocean is increasingly threatened by climate change-related stressors, such as ocean warming and deoxygenation. As a result, anthropogenic activities such as sea cage aquaculture, which often occurs in these ecosystems, are left vulnerable. Additionally, coastal oxygen deficits are intensified due to eutrophication as there is increased nutrient input from coastal development, which then affect cultured fish directly through stress, vulnerability to diseases and ultimately, increased mortality. Therefore, it is important to effectively monitor aquaculture farms and the adjacent coastal zone to ensure optimal fish welfare and marine management. This thesis explores the local and regional drivers of dissolved oxygen and temperature dynamics in coastal embayments within southwest Nova Scotia. Novel real-time oxygen and temperature sensors were used to understand local dynamics. First, a dense array of 63 sensors was deployed through an aquaculture farm to explore the drivers of oxygen variability. Tidal driven currents were determined to have the most significant impact on oxygen, with the influence varying depending on the position of the cage within the farm (Chapter 2). Second, the sensors were used to determine how an oxygen supplementation system affects oxygen distribution within 3 sea cages. Overall, oxygenation likely resulted in the upwelling of cool waters to the surface, increasing oxygen solubility and lowering fish metabolism, subsequently increasing oxygen concentrations (Chapter 3). To explore regional along-shore and cross-shore oxygen distribution and variability, a Slocum glider was deployed in a zigzagging pattern along the inner southwest Scotian Shelf. Wind direction had a significant affect, with strong cross-shore winds advecting water masses of differing properties to the coast, and persistent southwesterly winds causing upwelling of high DO from the subsurface. This data was then compared to a sensor within a bay to examine the offshore-inshore interaction, which has the potential to impact aquaculture farms located in these coastal embayments. A strong upwelling event 10 km from the coast was captured 30 h later within the bay, depicting possible interaction between the shelf and the bay (Chapter 4). Lastly, numerical experiments using an oxygen model were conducted for the Southwest Scotian Shelf. The effect of four potential climate change-related stressors on oxygen dynamics were examined: increased temperatures, weakened Gulf Stream-Labrador Current, increased winds, and weakened winds. To simulate the interactive effect of these stressors, a final scenario combined increased temperature, weakened currents, and weakened winds (Chapter 5). Individually, weakened Gulf Stream-Labrador Current resulted in the most significant decrease in oxygen on the inner Scotian Shelf, while increased winds (due to stronger storm events) resulted in the highest decreases in oxygen on the mid-outer Scotian Shelf. When stressors were combined, there was an overall decline in DO across the domain, whereby weakened currents caused the most significant decrease on the inner shelf. The exception to this was in the LaHave and Emerald Basins, as weakened winds caused an increase in oxygen. Improved knowledge of the drivers of oxygen dynamics within aquaculture farms and the adjacent shelf waters is important for the management of the region as well as for the future of aquaculture production and other coastal economies.