Strait of Georgia

On this page:

• Introduction
• The physical setting
• Geological Features and Anthropogenic Impacts
• The biological setting
• The chemical setting
• Cable Route and Study Areas
• CODAR

Also see:

• Fraser Delta

Introduction

The Strait of Georgia is a deep, inland basin located between Vancouver Island to the west and the Lower Mainland of British Columbia to the east, including the City of Vancouver. The Strait of Georgia branch of the VENUS network resides in the southern portion of the Strait. This deep basin is nearly 200 km long and 40 km wide, and has maximum water depths of more than 400 m in the central/northern region. To the south, the Strait borders onto the Gulf Islands and San Juan Islands, connecting through Haro and Juan de Fuca straits to the Pacific Ocean. At the far northern end, it connects through Johnstone Strait around the north end of Vancouver Island to the Pacific Ocean.

Vancouver, Canada’s third largest city and busiest sea port, sits on the east shore of the Strait of Georgia. As such, these local waters are heavily used for commercial transportation, public ferries, commercial fisheries, recreational boating, cruising, and fishing. Data generated on the VENUS network is supporting research in the Strait on both natural processes and anthropogenic impacts.

The Physical Setting

Fraser River

The marine environment in the Strait of Georgia is influenced by several dominant physical characteristics. The southern part of the Strait is strongly influenced by the fresh water discharged from the Fraser River, with peak river runoff of nearly 10,000 cubic metres a second during the spring snowmelt/freshet. This lighter, nutrient-rich, often silt-laden water enters the Strait along the Delta banks, and spreads out over the southern regions, often covering tens of square kilometres. The river discharge is often visible both to satellites and to the naked eye as a brownish plume extending from Tsawwassen in the south, all the way to Galiano Island in the west, and Burrard Inlet in the north (see satellite inset). This discharge dilutes the seawater in the surface layer, causing a strongly stratified interface between a shallow (<10m) brackish surface layer and the deeper (>10m) more saline seawater. Sediments from the river settle primarily where the salty water meets the fresh in the near-shore delta, but sediment settling extends over an area comparable to that occupied by the “visible” plume.

Circulation and Tides

Mean currents in the northern basin of the Strait maintain a slow persistent anticlockwise circulation driven predominantly by winds from the northwest. Mean currents in the southern part of the Strait, including the areas occupied by the VENUS network, are influenced by both the wind and the buoyancy flux of fresh water from the Fraser River, with mean currents circulating in a clockwise direction. Superimposed on these slower mean currents (~0.1 to 0.2 m/s) are mixed, mainly semidiurnal (twice a day) tides, which are stronger (~1 m/s) in the south, decreasing in strength toward the north. Flood tides, entering primarily from the south (Boundary Passage), drive currents northward, while ebb tides, exiting toward the south, are southward. The animated image to the right was generated using the tidal model of the Institute of Ocean Sciences. Maps of currents for the entire region of southern Vancouver Island can be viewed at the IOS site.

Geological Features and Anthropogenic Impacts

The Strait of Georgia is bounded by islands to the south and west, and the Lower Mainland and the City of Vancouver to the east. More than 70% of the population of British Columbia lives along these shores. As such it is considered one of Canada’s most important marine environments. River drainage from Vancouver Island is limited, because the east coast of the Island is on the lee side of the Island mountains. However, the west side of the mainland Coastal Mountains are notorious for precipitation, both rain and snow, all of which drains into the Strait over the course of a year. A significant fraction of that fresh water flows into the Strait via the Fraser River, just south of the City of Vancouver. The VENUS network is located in the centre of the Fraser River delta.

The Fraser River delta is a key region of study for the VENUS network. In addition to the fresh water buoyancy input into the Strait, Fraser River water is also laden with suspended sediments, dissolved nutrients, and other chemicals. As the fresh water mixes with the salty marine water, many of the particulates and sediments are precipitated out and settle to the bottom. Whereas typical sedimentation rates in the coastal ocean are centimetres per decade, the delta region can experience rapid deposition of centimetres per month. Due to the strong tidal currents and seismicity of the region, the accumulated sediments along the delta slope often become unstable and result in underwater landslides, slumps, and gravity currents (see image to the left). A key suite of experiments on VENUS will be to study this dynamic delta region for slope stability and slope failure.

The Port of Vancouver is large by any standards. In North America, it ranks #1 in total foreign exports. In Canada it ranks #1 in total cargo handled and #1 in total container throughput. The deep-sea cargo vessels transit to and from the Pacific Ocean almost exclusively via Juan de Fuca Strait, passing the Fraser River delta region and transiting over the VENUS network. Passenger cruise ships also use the northern passage through Discovery Passage on the way to the northern BC and Alaskan coasts. Recreational boating and fishing is popular throughout the year, but is particularly heavy during the summer months. Commercial fisheries for Pacific salmon, herring, and shrimp also occur at specific intervals during the year throughout the Strait. In short, the potential for anthropogenic impacts on this marine environment are significant.

The Biological Setting

Although the surface waters of the Strait of Georgia are strongly influenced by local winds and discharge from the Fraser River, the ecosystems found within the Strait are highly dependant on exchanges with the open (Pacific) ocean. As with many marine environments, at the bottom of the food web are the microscopic plants known as phytoplankton. The nutrients on which their growth depends come primarily from tidal mixing in the channels of the Gulf Islands and the estuarine exchange of deep, cold, up-welled water flowing into Juan de Fuca Canyon along the west coast of Vancouver Island. This nutrient-rich water finds its way into the Strait of Georgia through the Gulf Islands to the south. Tidal mixing brings it to the surface, where the phytoplankton can grow. The food web then includes zooplankton, smaller fish including herring, larger nekton such as salmon, and ultimately large marine mammals such as sea lions and orcas.

For additional information on recent field observations of the physical and biological cycles in the Strait of Georgia, visit the STATOGEM web site.

The Chemical Setting

As a semi-enclosed basin, separated from the open ocean by narrow and restricting channels, the Strait of Georgia represents a rather unique and fertile marine ecosystem. The primary source for natural nutrients is governed by the estuarine exchange flows through Juan de Fuca Strait, while the Fraser River provides a limited and seasonal source of nutrients to the shallow surface layer. Because of erosion, commercial agriculture in the BC interior, and industrial activity along the banks of the river, the Fraser also supplies the Strait with a broad mix of natural and anthropogenic elements and chemical compounds. Dissolved gases are naturally high near the air–sea interface, but can vary at depth when dense seawater enters the Strait from the south, where vigorous tidal mixing exposes the colder salty water to the atmosphere.

River discharge from mountain snow melt peaks from May through September, the surface layer warms, enhancing stratification, and primary production in the upper euphotic layer is at a peak. The nutrients of this layer are gradually drawn down and eventually become depleted, and further phytoplankton growth is believed to be nitrogen- (nitrate) limited. Nutrient concentrations in the partially mixed deep inflow from Juan de Fuca Strait, however, are always relatively high, varying from approximately 25 micromoles (µM) in winter to more than 30 µM in summer. However, due to the enhanced stratification, this nutrient-rich water may or may not reach the euphotic zone. In the winter, primary production in the Strait is light-limited and a cold, deep, wind-driven mixed layer allows the plankton to sink out of the shallow euphotic zone. Wind events during the summer also enhance mixing, but have the alternate affect of providing a renewed influx of nutrients from the deeper regions, across the density stratification, into the surface layer.

Cable Route and Study Areas

The cable route extends from a land-fall on the Iona Causeway along the central east shore of the Strait of Georgia, heading southward, passing around the busy Fraser River mouth, to two nodes off the southern delta. These nodes accommodate the connection of dozens of instruments and sensors for monitoring the circulation, water properties, and species within the Strait.

Central Node (300m)

The node located in the south-central part of the Strait supports a variety of science interested in water properties, how and when they vary, and the structure, interactions, and seasonal variations of the resident species. Initial instrumentation includes a VENUS Instrument Platform (VIP) supporting a CTD with a dissolved gas sensors, and an ADCP for measuring the vertical structure of the currents. This node will also host a VENUS hydrophone system for measuring ambient sound, including vocalizations of resident and migratory marine mammals.

East Node (170m)

The second node is located along the eastern slope of the Strait and through an extension cable services the Delta Dynamics Laboratory (DDL) near Sand Heads. Initial instrumentation for this node site includes a VIP with a CTD and an ADCP. This node also hosts a VENUS hydrophone system for tracking and studying marine mammals. Via an extension cable, power and communications are supplied to the DDL located along the very near-shore slope of the Fraser River delta. Studies here are focussing on the sediment loading of the water column by the Fraser River discharge and sediment resuspension during wind events, the dynamics and evolution of the water–sediment interface, and the structural stability of the underlying substrate.

CODAR

Westshore Terminals

With one CODAR station in operation at the Westshore Coal Terminal in Tsawwassen, VENUS is able to measure the radial velocities–that is, towards and away from the station’s antenna–of ocean currents in the Strait of Georgia.

Iona Breakwater

A second CODAR station will be installed in early 2012 at the Iona Breakwater, near Vancouver Airport. The area covered by this second CODAR station will partially overlap that covered by the first station. In the area of overlap, the radial velocities from the two stations will be combined to create hourly maps of north-south, east-west ocean currents.