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Saanich Inlet

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Introduction

Saanich Inlet is one of the better-studied marine basins in the world. The combination of easy access and unusual features has attracted researchers since the 1930s. The most interesting feature of Saanich Inlet is the combination of high plankton populations and deep water that is naturally depleted in oxygen. Once a year, in the fall, oxygen is restored. The result is a fascinating study area for biologists, chemists, and sedimentologists.

Composite image of Saanich Inlet showing depths

Composite image of Saanich Inlet showing depths

The composite image to the left is colour-coded to represent different depths in the inlet.

If you have Google Earth installed on your system, you can visit the Saanich Location virtually (Google Earth KML file).

The backbone cable and Node were deployed in early February 2006. On February 20, 2006 the first suite of instruments, including the VIP, Hydrophone, and Camera were deployed. See our Notice to Mariners page for a table showing the current location of the node and instruments.


The Physical Setting

Cross-section of Saanich Inlet

Cross-section of Saanich Inlet


Saanich Inlet is a glacially carved fjord. The Inlet basin is about 230m deep and is separated from tidally well-mixed waters in the Strait of Georgia by a shallow (70m) sill that restricts water inflow. Currents are usually low, although wind squalls are common in winter. Tidally-driven mixing occurs across the fjord mouth during times of spring tides. There is little freshwater input directly into Saanich Inlet; most comes from the Cowichan River to the northwest of the Inlet (see satellite inset).


The Biological Setting

Diatom Species from Saanich

Diatom Species from Saanich


Primary production from phytoplankton in the Inlet can be very high. Stable stratification of the water in the Inlet is established in the spring as the water warms. This density layering reduces vertical mixing and maintains phytoplankton in the light. Nutrients are periodically renewed by upwelling and advection across the sill. Diatoms and dinoflagellates change the water colour in large ‘blooms.' A succession of species of these single-celled algae occurs through the spring and again in fall. The zooplankton populations are also very high in Saanich Inlet. Euphausiids, arrow worms, copepods, pteropods and amphipods feed on phytoplankton and upon each other. These zooplankton rise to the surface at night to feed but, as soon as the sun rises, they migrate as deep as possible to avoid predation by fish that are visual hunters. At a depth of about 100m, the zooplankton form very dense masses that can be seen on acoustic sounders. Fish are also abundant: herring, hake, and dogfish as well as salmon that spawn in Goldstream River.


The Chemical Setting

Dense bacterial mat in Saanich Inlet

Dense bacterial mat in Saanich Inlet


Dying plankton fall to the deep basin where bacteria degrade them. The breakdown of organic matter by respiring microbes quickly depletes dissolved oxygen in the deep waters. Other compounds build up in these anoxic conditions: hydrogen sulphide, methane, and ammonia, among others. Researchers are often interested in the behaviour of trace elements in such conditions.

In the fall each year, dense, oxygen-rich water intrudes over the sill and fills the Inlet from the bottom up, creating a sandwich of anoxic water that is pushed upward. On the seafloor, chemosynthetic bacteria begin to grow in dense mats using hydrogen sulphide from the sediment and oxygen from the water to provide energy to fix carbon dioxide into organic carbon.


The Sediments of Saanich

X-ray of two sediment cores from Saanich Inlet

X-ray of two sediment cores from Saanich Inlet

The low oxygen prevents animals from living in the seafloor of the central basin. An undisturbed sedimentary record accumulates where macrofauna are absent. The nature of the sediment reflects input changes throughout the year. Distinct layers, or varves, form by alternations in plankton deposition in spring and silt from the Cowichan River in the winter. This layering preserves a history of Saanich Inlet that extends back to glacial times 13,000 years ago. Sediment cores reveal changes in plankton, fish, vegetation, and oxygen levels in the Inlet. In addition, slides, glacial deposits, and volcanic ash all record major events of the past.

Saanich Inlet is rich in information of past and present conditions. Building on such a fine record, we can examine how the Inlet is now responding to local pressures and global changes.

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