Annual Cycle of Hypoxia at Saanich Inlet
In the fall, oxygen in the Saanich Inlet bottom water is restored through deep-water renewal events. For a deep-water renewal event to occur, high density water needs to make it up over the shallow sill and sink down to the bottom of Saanich Inlet. The high density water needed for these events comes from the salty, deep-ocean water upwelled off the coast that comes up through the Straits of Juan de Fuca. This salty water builds up on the sill during the fall until it reaches a critical mass and boluses of the water move across the sill and into the deep basin of the inlet. These dense, oxygenated waters displace the hypoxic/anoxic waters upward through the water column. The exact timing, intensity and duration of depletion and renewal phases vary between years, but seasonally they predictably occur each year.
The VENUS array in Saanich Inlet captures this annual oxygen cycle. As the array is located at approximately 100m depth, hypoxia/anoxia at the Saanich Inlet node results from the deep, low oxygen water being displaced upwards from the deeper portions of the inlet.
High Dive into the Data
How were the data collected?
- Cabled Observatory: ONC VENUS Observatory
- Location: Saanich Inlet, Patricia Bay
- Lat/Lon: 48.6513N, 123.4863W
- Oceanographic Tool: VENUS Instrument Platform
The ONC VENUS Observatory is the world’s first complex, cabled seafloor observatory. Data began streaming live from instruments deployed on the seafloor to computers around the world in February 2006. This cabled array includes and instrument platform deployed at approximately 100m depth in Saanich Inlet. On the instrument platform is a suite of sensors including a CTD (conductivity, temperature and depth sensor) and an oxygen sensor. These sensors collected data continuously at 1-minute intervals, 24 hours a day, 7 days a week.
Saanich Inlet is a 274 metre deep glacially carved fjord separated from the Salish Sea by a shallow sill that restricts water inflow, making it naturally low in oxygen. Credit: Ocean Networks Canada