The Salish Sea Estuary
Written by Bert Webber
The Salish Sea: an estuarine ecosystem
Since around 1930 with the work of ecologist Eugene Odum the concept of the ecosystem has been used to describe areas of nature that show similar characteristics and about which we have interest. The entire Salish Sea is such an ecosystem. As well, the fresh water flowing into this ecosystem interacts with salt water and the entire Salish Sea is considered an estuarine ecosystem.
The fundamentals of the Estuary
1. Source of Energy: Movement of the water in the Salish Sea Estuary is driven by the hydraulic energy of the rivers. This energy powers the circulation of water in the Estuary.
2. Fresh and Salt Water Mix: Fresh water mixes with the salt and surface waters move seaward increasing in salinity in part due to entrainment (friction between fresh and salt water) and in part by mixing in areas with turbulent tides.
3. Direction of Flow. Circulation is characterized by flow of surface waters towards the ocean and a compensatory flow of Pacific Ocean water that enters the Salish Sea at depths.
4. Residence Time: Estuarine circulation rapidly replaces the water in the Salish Sea Basins. In most of Puget Sound the total volume of water is replaced 6 times a year. In other parts the exchange is slower (ie. The deep basins of the Strait of Georgia may be replaced only once a year). The impact of residence time on water quality is noteworthy.
5. Estuarine Circulation and Biological Productivity: The deep ocean water is rich in biological nitrogen and as this deep water nitrogen works its way to the shallower waters, light energy powers photosynthesis resulting in high levels of primary productivity (food production). Much of this productivity provides the basis of the pelagic food web. In its simplest form phytoplankton is grazed by zooplankton, which feeds (so called) baitfish that in turn provides a food source for larger fish, birds and mammals. Intertidal and subtidal communities (kelps and sea grasses) also benefit from the primary productivity.
More on how the estuary works:
Estuaries are places where fresh water from land drainage mixes with salt water from the ocean resulting in water with a measurable, although sometimes small amount of fresh water. What the Salish Sea does, like other estuaries in North America—San Francisco Bay The Everglades, Chesapeake Bay, The St Lawrence River and Bristol Bay to name a few, is make large amounts of “food” that supports rich food webs. The biological productivity of the Salish Sea is high (it produces more organic material in a given area than forests, grasslands or intensively cultivated agriculture lands, and is up to 20 times more productive than the open ocean). The rich natural resources of the Salish Sea are a product of this high biological productivity. That’s the main gift of the estuary.
A second benefit of the Estuary is the relatively good water quality found in most places of the Salish Sea. Both water quality and high biological productivity are a result of how the estuary works. In short the estuary functions by moving a lot of water.
Sources of energy: the Rivers. Of the many rivers and creeks flowing into the Salish Sea one stands out. The Fraser River flowing from southern British Columbia is the largest single source of fresh water. I have seen estimates that up to 80% of the Salish Sea fresh water is from the Fraser River. However the contribution of fresh water from the Fraser River to the Salish Sea estuary is more complicated. If the question is what is the contribution of the Fraser River to the total fresh water flowing into the Salish Sea an analysis of the “Salish Sea MEOPAR” database finds that the amount is much less-- somewhat less that 50%. If you ask how much of the fresh water exiting the surface waters of the Strait of Juan de Fuca the value is around 70%. If the question is how much of the Fraser River water enters Puget Sound, Parker MacReady of the University of Washington has reported that a third of the fresh water in Puget Sound comes from the Fraser River. That Fraser River water enters Puget Sound is testimony that the Salish Sea is a single integrated estuarine ecosystem. Many of the Salish Sea Rivers have glaciers or snow packs as their source. Sources that is vulnerable to impending climate change. Annual fresh water flow amounts for the Salish Sea should be monitored.
Fresh and salt water mix. As mentioned above two factors are at play mixing the fresh and salt water: Entrainment and tidal mixing. First entrainment. Because fresh water is somewhat lighter than salt water, the fresh water, when it meets the salt water, “floats”. The river flow pushes the fresh water over the salt. This causes a friction that “pulls” the salt water towards the surface and tends to mix the fresh with the salt. In an estuary like Chesapeake Bay (having a gradual smooth flat bottom) entrainment is of particular importance for mixing and salinity gradually increases as water moves seaward.
In the Salish Sea, entrainment is one important mixing force, but there is a second mixing force--- tides. On an ebb tide the water moves seaward for about 6 hours and which point the tide turns to a flood tide and moves water towards the land for another 6 hours. If tidal flow is not constrained by narrow passages or an irregular shallow bottom, and you watched carefully over a tidal cycle you would noticed that at the end of a tidal cycle the water is a little further seaward that at the beginning of the cycle. The difference is the seaward flow of the surface waters of estuary.
However many places in the Salish Sea are constrained either by narrow passages or by irregular bathometry. Tacoma Narrows, Rossario and Haro straits are examples. As the tidal flow becomes turbulent the fresh and salt-water mix often erasing any salinity gradient. Once the water reaches a wide deep area on its seaward travels the salinity gradient, although decreased, is still measurable.
Can we see this surface flow of estuarine circulation? Probably not. In spite of the large amounts moving, the speed is slow. Much of the Strait of Juan de Fuca has minimal surface and deep water mixing and the speeds of the surface flow are less than 1 km/hr. (or1mph/hr.).
Two layer flow. What goes out must come in. Now we understand why the surface waters flow outward. For many years people have noted that although surface waters of the Salish Sea move towards the ocean, at depth the oceanic water moves into the Salish Sea towards the land. How do we explain this? What is the force pushing the surface water seaward?
First, Estuarine scholars have determined that in estuaries of many types, the annual estuarine flow is between 10 and 20 times the total river flow. Since we know the average annual flow of all the Salish Sea Rivers, we can estimate the total surface estuarine flow of the Salish Sea. The average rate of flow of Puget Sound Rivers is 1174 m3/sec. That of the Fraser River is 3475m3/sec. These values allow us to calculate the total annual estuarine flow. If we assume that estuarine flow is 15 times the total fresh water sources (a conservative estimate), the amounts are immense. An estimate indicates that the amount of the outward estuarine surface flow of the Salish Sea through the Strait of Juan de Fuca each year is at least equal to 8 times the annual flow of the Columbia River. That’s big!
The surface outflow is matched by a compensatory inflow of ocean water at depth that replaces the water leaving the Salish Sea. This deep water is “pulled” in to all parts of the Salish Sea. It is cool and explains why the surface waters of the Salish Sea do not warm up much in the summer. The average temperature most of the year hovers around 50 degrees farenheight. The water is also high in biological nitrogen. More about that below.
Residence time. Let’s consider the boundaries of the Puget Sound, (ending northward at Admiralty Inlet). Imagine that you could empty the main basin of Puget Sound and then measure how long it takes to fill it with just the rivers that flow into it. If you relied on these rivers to refill the basin, it would take about 5 years to fill it to sea level. In reality though, the estuarine circulation adds enough water to Puget Sound to fill it every two months. We call this the residence time. Five years from river flow, a couple of months from estuarine flow of water. That further demonstrates the immense amount of water that is moved by estuarine circulation. As mentioned above other parts of the Salish Sea have different residence times. The resident time of southern Puget Sound is no more than one month; the longest is that of the deep water of the Strait of Georgia that is as much as a year. We can say with confidence that all areas of the Salish Sea have a resident time of one year or less. Resident time has a measured impact on water quality. Remember the saying, “The solution to pollution is dilution.”
Estuarine Circulations and Biological Productivity. The question now is how does the estuarine circulation result in high biological productivity? The single most important limiting factor in levels of biological productivity in the Salish Sea (as well as in the World Ocean) is the availability of biological nitrogen. In fresh water it is the availability of biological phosphorus.
The Salish Sea Estuary’s biological nitrogen levels come from deeper Pacific Ocean water and are particular high compared to the surface waters of the Ocean. These high nitrogen levels in turn, are instrumental in the levels of the biological productivity.
The source of this high concentration of nitrogen is not from the fresh water river sources. Some values are that the rivers provide up to 30% of the biological nitrogen in the estuary. However, in surface waters in the Strait of Juan de Fuca, only 2% of the biological nitrogen is from rivers, the rest is from estuarine circulation. If the majority of the nitrogen comes from Pacific Ocean deep water, how does the nitrogen get into the Estuary? The answer is in how water circulates in the Salish Sea estuary. The deep ocean water that replaces what flows out at the surface compensates the large amount of surface water driven ultimately by the force of the fresh water rivers. This inflow, with high nitrogen levels, works, its way into all parts of the Salish Sea providing the relatively high concentrations of biological nitrogen that helps fuel the productive ecosystem.
The water flowing out of the Salish Sea is a significant contribution to the productivity of Coastal Pacific Ocean. Surface areas off the south west coast of Vancouver Island and the north west coast of Washington get as much nitrogen from estuarine circulation as they get from coastal upwelling.
One additional feature of interest is that the outgoing estuarine flow creates a circular eddy at the mouth of the Strait of Juan de Fuca. This circular flow of water, with high levels of biological nitrogen supports blooms of diatom growth that produces the toxic domoic acid that can contribute to shellfish poisoning. This is one additional impact of the Salish Sea estuarine circulation.
The take away from this story is that the Salish Sea estuary moves very large amounts of water, helping water quality and that the large concentrations of biological nitrogen brought by the deeper water contributes to the large biological productivity that is the basis of the marine resources we use and cherish.
For additional information on the estuarine properties of the Strait of Juan de Fuca see: Davis, K.A. et al. 2014. Estuarine enhanced upwelling of marine nutrients fuels coastal productivity in the U.S. Pacific Northwest. In the Journal “Advancing Earth and Space Science.”