Saanich Inlet Oceanography and Ecology

By: David Timothy

In this report I discuss the unique conditions that lead to very high rates of primary production in Saanich Inlet.  By primary production I mean the photosynthetic rate of the single-celled phytoplankton living at the surface of the ocean where both sunlight and nutrients are present.  Phytoplankton photosynthesis generates the main flow of organic matter into the marine ecosystem, providing most of the energy that fuels growth at higher trophic levels.

Saanich Inlet is the most productive fjord yet to be measured, according to a table published by Grundle, Timothy and Varela (Variations of phytoplankton productivity and biomass over an annual cycle in Saanich Inlet, a British Columbia fjord; Continental Shelf Research, 2009, vol. 29, pp. 2257-2269).  Although the list is limited to 26 fjords in Scandinavia and here in the NE Pacific, it’s safe to say that, at a photosynthetic rate of about 40 moles of carbon per square meter per year (~40 mol C / m2 / yr), Saanich Inlet is an exceptionally productive ecosystem.  To put this rate into perspective, it’s 30% higher than the next highest fjords on the list (Puget Sound and Indian Arm, both at 30 mol C / m2 / yr), about double estimates for the Strait of Georgia (ranging from 10 to 30 mol C / m2 / yr) and more than triple the average rate of primary production for the global ocean (~12 mol C / m2 / yr).
This high rate of primary production, which allowed for the diversity and abundance known historically to the local First Nations and which today is returning to the fjord, is due to the fact that Saanich Inlet (1) receives a relatively high amount of sunshine, (2) has a calm surface layer and (3) has an unusual but effective nutrient supply to the main body of the inlet.
The phytoplankton here are bathed in sunshine, mostly because Saanich Inlet is in the rain shadow of the mountains on the west coast of Vancouver Island and of the Olympic Peninsula.  Less rain = more sun!  Also, at 48.5oN, Saanich Inlet is a relatively tropical fjord.  That is to say that fjords, which are flooded mountain valleys carved out by Pleistocene glaciers, are high-latitude features not found much closer to the Equator than about 44o in either the northern or southern hemisphere.
Phytoplankton grow well in calm, undisturbed surface waters, provided sunshine and nutrients (discussed below) are available.  In Saanich Inlet, the winds and currents that generate surface turbulence are both low when compared to other fjords.
Wind mixing
The N-S orientation of Saanich Inlet is ~perpendicular to prevailing winds along the BC coast, so large-scale weather patterns coming off the Pacific Ocean tend not to be funneled within the fjord.  Also, diurnal winds that are a prominent summer feature throughout coastal BC are nearly absent in Saanich Inlet.  The lack of a continental landmass behind Saanich Inlet, and thus the land-sea temperature gradients that drive diurnal winds in other fjords, has much to do with this atmospheric stability.
Tidal mixing
The funnel shape of Saanich Inlet (wide mouth narrowing towards the head) makes for very weak tidal currents within the fjord.  Many other fjords have expansive inner basins that are filled and emptied by strong tidal currents pushing their way through narrow mouths that lead to the sea.  For example, currents through restrictive Skookumchuck Narrows at the mouth of Sechelt Inlet can reach 16 knots, forming huge standing waves with each tidal cycle.  Tidal currents are also very high through Haro Strait separating the Gulf Islands and San Juan Islands (about 80% of tidal exchange for the Strait of Georgia passes this way, with the remainder passing through Johnstone Strait to the north), but these currents do not reach Saanich Inlet.
Explaining the nutrient supply of Saanich Inlet, controlled largely by surface circulation, is tricky.  I’ll start by describing positive and negative estuarine circulation, since Saanich Inlets demonstrates both.
Positive Estuarine Circulation
The vast majority of estuaries, including almost all fjords, are positive estuaries, where surface waters get saltier towards the ocean.  For the typical fjord, a river enters at the head, setting up a surface current flowing “downhill” towards the ocean.  This surface flow (up to ~20 meters deep) entrains the seawater below it, so that the volume of water leaving the mouth of the fjord is many times larger than the river input at the head.  The entrained seawater exiting the fjord is replaced by a subsurface inflow of seawater from outside of the fjord.
Negative Estuarine Circulation
Negative or reverse estuaries occur where surface salinity gradients and circulation are opposite those of the positive estuary.  They tend to occur in hot and dry climates where evaporation causes surface waters inside coastal bays to be more salty than in the nearby ocean.  The Mediterranean Sea, with saltier surface waters than those of the Atlantic Ocean, is an example.  Surface waters flow into the Mediterranean through the Strait of Gibraltar, while entrainment drives a subsurface outflow, in reverse direction of the positive estuary’s subsurface flow.
Positive and Negative Circulation in Saanich Inlet
How does this apply to Saanich Inlet?  The biggest sources of fresh water to the surface of Saanich Inlet are the nearby Cowichan River (in winter) and the Fraser River (especially during the spring-summer freshet).  These are both seaward of Saanich Inlet, with the Goldstream River and Shawnigan Creek delivering smaller flows directly into the fjord.  Surface salinity thus tends to be lowest towards the mouth of Saanich Inlet, making it, in general, a negative estuary.  However, as I explain next, the spring-neap tidal cycle impacts surface salinity outside of Saanich Inlet in a way that causes the circulation to periodically flip into positive estuarine mode (Gargett, Stucchi and Whitney, 2003; Physical processes associated with high primary production in Saanich Inlet, British Columbia; Estuarine, Coastal and Shelf Science, vol. 56, pp. 1141-1156).
The intense tidal currents passing through Haro Strait thoroughly mix surface freshwater with the saltier waters below.  However, during neap tides when mixing is weakest, the surface layer is slightly fresher than at any other time in the fortnightly tidal cycle.  The relatively fresh surface plume then tends to flow “downhill” into Saanich Inlet, driving a weak negative estuarine circulation.  But with the eventual shift to stronger spring tides (new and full moons), more salt is mixed into surface waters around the entrances to Saanich Inlet.  Now surface waters are fresher inside Saanich Inlet than out, causing the surface layer to flow downhill out of the fjord.  Positive estuary!
In summary, surface waters tend to flow into Saanich Inlet during neap tides (negative estuarine flow), and out of Saanich Inlet during spring tides (positive estuarine flow).  Thus there is a fortnightly cycle of surface circulation, set up by the freshwater balance and driven by the tides.
Surface circulation and nutrients
This description of surface circulation in Saanich Inlet was made by Gargett et al., in part to explain surface drifter data collected over a fortnightly tidal cycle (5-20 July 1995).  During the neap tide, drifter motion followed an eddy located between Patricia Bay and Mill Bay in the main body of the inlet, while farther up inlet drifters moved towards the head of the fjord (weak negative estuarine flow).  The surprise occurred with the shift to spring tides, when drifters from throughout the inlet were rapidly carried out of the fjord into Satellite Channel, indicating positive estuarine flow.
The exiting layer would be replaced by waters from directly below (from depths of roughly ~20 m but not from depths of 100 m or more where waters are often anoxic), which are higher in nutrients than surface waters.  New nutrients fuel phytoplankton blooms, which indeed have been observed in Saanich Inlet as summertime mini-blooms occurring on a fortnightly basis (Takahashi, Seibert and Thomas, 1977; Occasional blooms of phytoplankton during summer in Saanich Inlet, BC, Canada; Deep-Sea Research, vol. 24, pp. 775-780).  These blooms consume surface nutrients, while simultaneously fresher waters from the Fraser and Cowichan Rivers leak into Saanich Inlet (the weak negative estuarine flow), conditioning the surface layer for another surface exchange at the next spring tide.
Nutrient summary
This exchange mechanism provides a pulse of nutrients to the surface of Saanich Inlet, which then becomes a quiescent and sunny place for phytoplankton to grow.  The exchange occurs every two weeks - a Goldilocks time period for phytoplankton growth (phytoplankton tend to divide once per day).  Much shorter and there would not be enough time for a bloom to develop.  Much longer and a bloom would use up the nutrients and go into a period of nutrient depletion with low primary production.

To summarize our current understanding, there are at least four independent conditions that have come together to drive high primary production in Saanich Inlet.
- Sunny conditions (rain shadow; “low” latitude)
- Quiescent surface waters (low wind and tidal energy)
- Nutrient replenishment that does not disturb the quiescence (negative estuary but with a fortnightly shift to positive estuarine flow)
- Goldilocks timescale for phytoplankton, of the fortnightly nutrient replenishment

Remove one or more of these conditions and production in Saanich Inlet would possibly decrease by ~30%-50%, the levels observed in nearby waters.  Indeed, the fortnightly nutrient supply is susceptible to the proposed LNG plant at Bamberton.

One design proposal by Steelhead is to use an open-loop seawater cooling system to cool the power plant required to convert methane gas to LNG.  A proposal for a similar plant at Squamish designed to produce 3.6 million tonnes of LNG per year describes 50,000 tonnes (m3) of cooling water per hour, returned to the ocean 10oC warmer than intake and possibly chlorinated (anti-fouling agent).  Scaling up to the 6 million tonnes LNG per year proposed for the Bamberton site results in 83,000 tonnes of seawater per hour for cooling.  To put this volume into perspective, it amounts to roughly 12x more water than is added directly to Saanich Inlet by Shawnigan Creek and Goldstream River combined and, over the fortnightly exchange described above, it translates to a 28 km2 surface cap (almost ½ of Saanich Inlet’s surface of 65 km2) if the cap’s thickness is set at 1 m.  (Heating by 10oC will cause the effluent to be buoyant, assuming salinity of the effluent is not more than 2.6 PSU greater than the salinity of the ambient seawater.)  Without more detail it’s hard to know how this discharge would mix with ambient waters throughout Saanich Inlet, or how that mixture would respond to the fortnightly density variations outside the fjord.  Nevertheless the possibility must be considered, that this discharge would disrupt the density-driven flips from negative to positive estuarine circulation that are so critical to the high primary production in Saanich Inlet.  Mixing caused by the tanker traffic, which will further alter surface salinity gradients, must be added to this consideration.  (LNG tankers have a draft of up to 12 m and would come and go about twice per week.)

Finally, chlorine is added to the coolant because it is an effective anti-fouling agent, and thus could kill phytoplankton and other marine organisms in contact with the effluent plume, which we now see could cover a very large area of Saanich Inlet