Salish Sea Central - the Salish Sea is an Estuary

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The Salish Sea is an Estuary:

    A.  How it works; (its not obvious)

    B. What it does;    (it’s important)

The Salish Sea is an estuarine ecosystem.  Estuaries are places where fresh water from land drainage mixes with salt water from the ocean and results 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 to make large amounts of “food”. The high biological productivity of the Salish Sea produces more organic material in a given area than forests, grasslands or intensively cultivated agriculture lands. The rich biological resources of the Salish Sea are a product of this high biological productivity. That’s the first gift of the estuary.

The 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. The Salish Sea estuary functions by moving a lot of water.

How does the estuary move water? Imagine that you could empty Puget Sound from Admiralty Inlet south. Then measure how long it takes to fill it with 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 Estuary 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. 

This rapid exchange of water in the Puget Sound is also found elsewhere in the Salish Sea. The residence time of the total volume of the Salish Sea is one year or less. That means the total volume of water in the Salish Sea is replaced at least once a year. This movement of estuarine waters helps with maintaining good water quality and aids in the high biological productivity

The question now is how does the water movement occur and how does it result in  high biological productivity? The answer is the continual flow of the rivers and the pushing of the river water over the surface of the Salish Sea. To learn more about this force works look up estuary entrainment. 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 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 (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 is at least equal to 8 times the annual flow of the Columbia River. That’s big!

For many years people have noted that the surface waters of the Salish Sea move towards the ocean, but at depth the oceanic water moves into the Salish Sea towards the land.  This observation gives a clue to why the Estuary has high biological productivity.

So how does this high biological productivity happen? We know that one of the most important factors is the amount of biological nitrogen dissolved in the water. This form of nitrogen is in many places of the estuary the limiting factor in rates of photosynthesis leading to “food production”.  The deeper water flowing into the estuary has high concentrations of biological nitrogen. The Salish Sea Estuary’s nitrogen levels come from deeper Pacific ocean water and are particular high compared to the surface waters of the ocean. The high nitrogen levels in turn, helps drives the biological productivity.

The source of this high concentration of nitrogen is not from the fresh water river sources.  Some argue that the rivers provide up to 30% of the biological nitrogen in the estuary, one report put the amount much lower perhaps around 5%. 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 large amount of surface water driven ultimately by the force of the fresh water rivers brings in deep ocean water that replaces what flows out at the surface.  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.

Can we see this 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 tidal circulation where the speeds of the surface flow are less than 1 km/hr. (or1mph/hr.).

What about the tides? Estuarine circulation is a slow moving “river” that cannot be easily observed.  On the other hand it’s likely that all of us have watched the water moved by the tides.  Most people when asked how surface water moves, think about the tides and their power in places like Deception Pass, the entrance to Admiralty Inlet, the narrows in Tacoma, the currents in Rosario or Haro straits or Seymour Narrows at the north end of the Salish Sea.  The champion tidal currents in the Salish Sea are at Skookum Chuck Narrows where current speeds of more than 18 mph (20 km/hr.) can be expected on the right tides. These tidal currents are easy to understand and most of us have experienced the ebb and flood tides. These tidal currents are only indirectly related to the estuarine circulation.  Their primary impact is to mix the water in the rapids from the bottom to the surface. Where this happens the water is fairly salty.  However tides “slosh” the water out on the ebb tide and in on the flood tide and once beyond the rapids the estuarine flow reestablishes itself with higher salinity on the bottom and slightly fresher water on the surface. 

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 more information about the Salish Sea visit Salish Sea Central