Oxygen in the deep Strait of Georgia, 1951-2009: The roles of mixing, deep-water renewal, and remineralization of organic carbon

The concentration of oxygen in the deep Strait of Georgia has declined at a rate of 0.45-1.3 mu mol L-1 yr(-1) since 1971 and now seasonally approaches thresholds of biological tolerance. The decline has resulted principally from the increasing hypoxia of upwelled Pacific Ocean water. Local anthropogenic loadings have little potential to reduce bottom water oxygen concentrations. Hypoxic, upwelled water mixes vigorously with surface water in the tidal passages of Haro Strait before entering the deep Strait of Georgia during a series of deep-water renewal events in late spring and late summer. Remineralization of at least 22 mu mol L-1 yr(-1), together with diffusive mixing, reduces the bottom water oxygen concentration to a winter minimum of 90-110 mu mol L-1. Linear extrapolation of the long-term trend indicates that parts of the Strait could become episodically hypoxic (< 62.5 mu mol L-1; < 1.4 mL L-1) as early as 2042. However, water mass modeling shows that the mixing with surface water in Haro Strait limits the potential of the shelf water to reduce the oxygen concentration in the deep Strait; even should the shelf water become completely anoxic, the concentration of oxygen in bottom waters would level off just above 90 mu mol L-1 after 3 yr. Increasing surface water temperature will reduce the solubility of oxygen, but this effect is projected to cause a further decline of only 2.2 mu mol L-1 over the next 25 yr. Despite its restricted circulation, the deep Strait of Georgia is less threatened by hypoxia than are many other coastal areas, because of the combination of light-limited primary production and intense tidal mixing in the approaches, which replenishes the deep-water oxygen annually.