Variability of Dissolved Oxygen in the Strait of Georgia: Seasonal, Intraseasonal, and Interannual Changes Based on the SalishSeaCast Model

Dissolved oxygen (DO) variability in the Salish Sea reflects interactions among circulation, deep-water renewal, and biological activity across multiple timescales. Using a coupled physical-biogeochemical model, we investigate the drivers of DO dynamics in its largest basin, the Strait of Georgia (SoG), between 2012-2020. Surface DO production peaks in spring due to enhanced photosynthesis and declines through summer to reach a minimum in fall. Sediment oxygen demand also peaks in spring, consuming up to 50% of surface production, primarily due to the deposition of organic-matter. Vertical DO variations result from the balance between oxygen consumption at depth and physical transport processes. Seasonal intrusions of dense, oxygen-rich water from Juan de Fuca and Haro Straits ventilate the SoG, elevating deep DO from winter minima (120 mmolO2m-3). Advection dominates this renewal, while mixing and organic-matter remineralization shape the vertical DO profile. As inflows weaken, sustained oxygen consumption returns the deep basin to depletion. Interannually, temperature and salinity anomalies mainly affect renewal strength and residence times, whereas particulate organic nitrogen (PON) anomalies control deep oxygen consumption. Strong renewal events (e.g., May 2015) mitigate oxygen depletion by importing oxygen-rich waters, while weak renewal periods (e.g., summer 2017) prolong residence times and increase hypoxia risk. Elevated PON during weak renewal amplifies deep DO depletion, showing a lagged effect of spring production on subsurface oxygen. Overall, the interaction between physical drivers and biogeochemical processes is critical to understanding and predicting DO dynamics in the SoG and informing management of its ecosystem health.