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Puget Sound: The Estuary

By South Sound Estuary Associatation

If you were unaware that the entire Puget Sound is an estuary, you aren't alone. Simply stated, estuaries are places where fresh and salt water mix. They are distinct habitats that differ from the open ocean but are connected to it.

Typically, estuaries are formed in two ways: by geological processes, or by the circulation of water. The Puget Sound in surface area the second-largest estuary in the US after the Chesapeake Bay is a fjord-type estuary, formed by glaciers that covered the area during the last ice age over 10,000 years ago. When the glaciers retreated, an inland sea was left behind, with over 2,500 miles of shoreline.

Blending rivers and oceans
One distinctive aspect of an estuary is its salinity. For example, the average salinity of the Puget Sound is 2.9%, compared with the Pacific Ocean's average salinity of 3.4%. While half a percentage point difference may not sound like much, in terms of habitat it is quite significant, an increase of over 16% total.

The Sound's actual percentage, however, varies from place to place, a result of the interaction between the fresh water discharges from rivers and streams flowing into the inland sea with the salt water from the Pacific. As the lighter fresh water from the uplands mixes with the heavier salty ocean water, the water near the surface now a diluted mixture of Pacific Ocean water and river water pushes seaward towards the western mouth of the Strait of Juan de Fuca.

The volume of water exchanged between the Pacific Ocean and the Sound by this estuarine circulation is huge. Each year a volume equal to the entire volume of the Sound exits to the Pacific Ocean in surface waters, and fills in at depth with Pacific Ocean water.

In the south Sound defined as that part of Puget Sound south of the Tacoma Narrows a complete tidal flush ranges from four days to as long as several weeks. This slower circulation pattern and lesser wave action compared to the central and northern Sound which in places see a complete tidal flush in 24 hours are distinguishing characteristics of the south Sound. Also, the tidal flow through the Narrows so completely mixes the surface and deep waters that there is little or no difference in salinity between the bottom and the surface in the south Sound.

A productive but threatened ecosystem
Estuaries are some of the most productive and biologically diverse ecosystems on earth, and the Puget Sound is no different. The richness of life forms in our inland sea is a direct result of the estuarine circulation caused by the interaction of rivers with ocean waters.

The rocky and muddy substrates of shallow waters close to shore support rich biological communities. The algae of the intertidal and subtidal areas produce as much plant material as the most productive agriculture crops. Diatoms living in the surface layers of mud flats; sea grasses found in large meadows and just offshore of many beaches; and marshes near the high tide line of sandy and muddy flats - all these contribute large amounts of energy-rich carbon molecules that in turn nourish a diverse group of animals.

At the top of this diverse food web, the Puget Sound has almost 3,000 species of marine invertebrates, is home to the North Pacific giant octopus (the largest octopus in the world), and has 165 species of birds that make their living on the water, to just name a few examples.

Also like many estuaries, the Puget Sound is surrounded by a growing population that is putting an extreme stress on the marine environment. In recognition of the impacts that human activity has on this fragile resource, numerous regional organizations from state government efforts like the Puget Sound Partnership to non-profits such as People for Puget Sound are focused on protecting and restoring this vital ecosystem.

The South Sound Estuary Association
The name South Sound Estuary Association stems from an overall desire to engage our community in a dialogue about the 2,800 square miles of inland marine waters that we are connected to through our watershed.

As an organization with the word "estuary" in our name, however, we sometimes get pointed comments regarding Capitol Lake removing the lake's dam continues to be a polarizing community issue. SSEA does not take a position on Capitol Lake: we have members who are on both sides of the controversy.

As an educational organization, SSEA's mission is to create learning opportunities. It's through education that people become more aware of their unique and special environment, and becoming informed helps them realize how their everyday actions affect the health and well-being of Puget Sound...the estuary.

If you don't know much about it, it's harder to care about it. If you don't care much about it, you won't be likely to protect it.

For information about the South Sound Estuary Association, go to http://www.sseacenter.org or call (360) 918-8708.

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Updated 2015/01/07 21:14:22