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UBC Theses and Dissertations

Characterizing the lowest oxygen waters on the southern continental shelf off Vancouver Island Sahu, Saurav


A shortage of dissolved oxygen in seawater can adversely impact marine life and ecosystems. Coastal waters deeper than 100 m do not gain oxygen directly from the surface, and thus transient, seasonal, or permanent oxygen deficit conditions can occur in such deeper coastal waters. A low oxygen, dense pool of water is formed every summer over the mid-shelf off southwest Vancouver Island in the Juan de Fuca Eddy region, known for its high primary productivity. In this thesis, the waters of the dense pool are traced back to their source using Lagrangian Particle tracking and a regional numerical ocean model, and upwelling hotspots of deep water leading to the pool are discovered. The model accuracy in simulating the local circulation is evaluated based on a statistical skill score, root mean square error and bias upon comparing the model variables to the observations. The numerical model simulates the local circulation well, except it under-predicts the variation along isopycnals. Due to this lack of variation in the model, only the particles which agree well between the model results and the observations are selected using a K-Means clustering algorithm. Tracking these particles backwards in time showed that the dense pool under the Juan de Fuca Eddy is primarily composed of water from the California Undercurrent, water from Washington State shelf and offshore water. Signatures from mixing the source waters in proportion closely approximate the final signatures of water inside the dense pool. The investigation of upwelling hotspots revealed that 1) the dense pool water primarily upwells through Spur Canyon and the convoluted Juan de Fuca Canyon bathymetry near Swiftsure Bank, and 2) The southern side of Nitinat Canyon acts as a dominant upwelling site for water ending on the south-outer shelf of South Vancouver Island. This study helps in climate change predictions, as the pathways identified for different source water origins help to determine the change in source composition over time and understand the potential low dissolved oxygen implications over the Vancouver Island shelf, in Juan de Fuca Strait and in the Strait of Georgia.

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