Presenter/Author Information

Li Gu, Metro VancouverFollow

Type of Presentation

Oral

Session Title

General oceanography

Description

The subsurface dissolved oxygen in Burrard Inlet is studied through instrument moorings and seasonal basin-wide CTD surveys. Located on the southern coast of British Columbia, Burrard Inlet is divided into the outer and inner basins, separated by a combination of contraction and underwater sill, known as First Narrows. Similarly, further to the east, Second Narrows separates the Vancouver Harbour from the eastern portion of the inlet, leading to Indian Arm, a deep and narrow fjord.

Time series from the Vancover Harbour basin mooring revealed intriguing features of tidal flushing and varied influence of vertical mixing at the First narrows through the spring-neap tidal cycle. Small floods under the neap tide bring the source Strait of Georgia water from the outer basin through the narrows with less mixing which sinks to depth. In contrast, strong mixing with highly oxygenated surface water during large floods lead to frequent replenishing of highly mixed water up to 1 mg L-1 higher in oxygen concentration and 1 kg m-3 lower in density. The dissolved oxygen at depth is shown to follow a distinct seasonal pattern with level ranging from 4.4 to 7.5 mg L-1 (47-79% saturation), comparable to that observed in the intermediate water of the Strait of Georgia.

Unlike the frequently flushed harbour basin, the deep water renewal in Indian Arm happens at most once a year. After a renewal in the winter of 2013/14, the deep Indian Arm oxygen shows a steady decline through remineralization of organic matter, dropping well below the hypoxic level by late 2015. The observed oxygen decline rate is about 3.7 mg L-1 yr-1, compared with an average decline of 3.3 mg L-1 yr-1 during the 1970s and 80s. The deep water density shows a steady decrease through diffusion, setting the condition for the next exchange event.

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Monitoring Subsurface Dissolved Oxygen in Burrard Inlet

2016SSEC

The subsurface dissolved oxygen in Burrard Inlet is studied through instrument moorings and seasonal basin-wide CTD surveys. Located on the southern coast of British Columbia, Burrard Inlet is divided into the outer and inner basins, separated by a combination of contraction and underwater sill, known as First Narrows. Similarly, further to the east, Second Narrows separates the Vancouver Harbour from the eastern portion of the inlet, leading to Indian Arm, a deep and narrow fjord.

Time series from the Vancover Harbour basin mooring revealed intriguing features of tidal flushing and varied influence of vertical mixing at the First narrows through the spring-neap tidal cycle. Small floods under the neap tide bring the source Strait of Georgia water from the outer basin through the narrows with less mixing which sinks to depth. In contrast, strong mixing with highly oxygenated surface water during large floods lead to frequent replenishing of highly mixed water up to 1 mg L-1 higher in oxygen concentration and 1 kg m-3 lower in density. The dissolved oxygen at depth is shown to follow a distinct seasonal pattern with level ranging from 4.4 to 7.5 mg L-1 (47-79% saturation), comparable to that observed in the intermediate water of the Strait of Georgia.

Unlike the frequently flushed harbour basin, the deep water renewal in Indian Arm happens at most once a year. After a renewal in the winter of 2013/14, the deep Indian Arm oxygen shows a steady decline through remineralization of organic matter, dropping well below the hypoxic level by late 2015. The observed oxygen decline rate is about 3.7 mg L-1 yr-1, compared with an average decline of 3.3 mg L-1 yr-1 during the 1970s and 80s. The deep water density shows a steady decrease through diffusion, setting the condition for the next exchange event.