Spatial and Temporal Origins of the La Perouse Low Oxygen Pool: A Combined Lagrangian Statistical Approach

A shortage of dissolved oxygen in seawater can adversely impact marine life and ecosystems. Low oxygen conditions at depth occur in many coastal regions, driven by both local productivity and remote changes in the source waters. A low-oxygen dense pool of water is observed every summer over the mid-shelf off southwest Vancouver Island in the Juan de Fuca Eddy region. We trace the dense pool waters back to their source using Lagrangian Particle tracking in an ocean model. The model accuracy is evaluated against a set of dense observations collected in August 2013. Only locations where the model represents water properties well are used as starting locations for the tracking. These locations are selected using a K-Means clustering algorithm. Tracking particles backwards in time showed that the low oxygen dense pool is primarily composed of water from the California Undercurrent, shallower water from further offshore of the Washington shelf, and offshore water. Waters from the northern shelf were not a primary source even though summer currents are from the north. Correspondingly, the water primarily arrived in the south Vancouver Island region in spring, months before the cruise. A kernel density estimate shows the final water properties of the dense pool are well represented by this mixture. The dense water pathways up and onto the shelf are primarily through coastal submarine canyons in early summer. Combining high spatial resolution observations, a carefully evaluated numerical model, Lagrangian tracking, and statistical techniques reveal detailed answers not obtainable through a single method.