Oxygen in Deep Waters of the Strait

From  ”State of physical, biological, and selected fishery resources of Pacific Canadian marine ecosystems in 2012

Sophia Johannessen, Diane Masson and Robie Macdonald of the Institute of Ocean Sciences, Fisheries and Oceans Canada, have been examining changes in deep water oxygen levels in the Strait of Georgia between 1951 and the present.

Marine hypoxia, or lack of dissolved oxygen in seawater, is a growing problem worldwide and one that can have dramatic impacts on marine life and ecosystems. It is now recognized as one of the likely consequences of global warming, since warmer water does not hold as much oxygen as cooler water. There is evidence that oxygen levels are declining in the deep waters of the Strait of Georgia.

The concentration of oxygen in the deep Strait of Georgia is determined by several events, namely, tidal mixing, deep-water renewal, diffusion and respiration. Hypoxic, upwelled ocean water flows eastward along the bottom of Juan de Fuca Strait and into Haro Strait, where it is partially reoxygenated by contact with the surface water during vigorous tidal mixing. During the late spring and late summer repeated deep-water renewal events replace the pre-season bottom water in the Strait of Georgia with mixed water from Haro Strait (see the figure below). Diffusive mixing rapidly reduces the concentration of oxygen in the new bottom water, while remineralization of organic matter continues to consume oxygen throughout the year, some of which is replaced by mixing from above.

Johannesson 1
Figure 1. Physical circulation of the Strait of Georgia and approaches. The figure represents a transect from the mouth of Juan de Fuca Strait (at left, labelled upwelling) through Haro Strait (in left-centre, labelled tidal mixing), where the strong mixing occurs, and then into the southern and northern basins of the Strait of Georgia (Figure from Johannessen and Macdonald, 2009. Climate Research 40: 1-21.)

Dissolved oxygen is declining in subsurface seawater around the world. Similarly, in the Strait of Georgia, the concentration of oxygen in the deep water has declined at a rate of about 0.02 ml L-1 yr-1 since 1971 (Johannessen et al., in press) and now seasonally crosses the precautionary threshold of 3 ml L-1 which applies to many species of marine animals. With a winter minimum of 2.0 to 2.5 ml L-1, the oxygen concentration is approaching biological thresholds for tolerance of hypoxia. The authors believe that this decline in oxygen is primarily caused by the increasing hypoxia of Pacific Ocean water along the continental margin of southern British Columbia.

Another major concern is that of ocean acidification. Each year, about one third of the carbon dioxide in fossil fuel emissions dissolves in ocean surface waters, forming carbonic acid and increasing ocean acidity. Over the next century or so, it is predicted that acidification will intensified in surface waters where much of the marine life that humans depend upon live. Ocean acidification threatens marine food webs, ecosystem productivity, commercial fisheries and global food security.

The range of pH in the Strait of Georgia is about 7.8-8.1 at the surface and 7.3-7.6 at depth (Johannessen et al., unpublished data). Unfortunately, scientists lack the time series to assess pH change in the Strait of Georgia. Acidification has been documented in the hypoxic, subsurface water off the west coast of North America, which is the source water for deep-water renewal; thus, the authors believe that pH in the deep Strait of Georgia is also likely declining.