Marine Water Quality
This indicator measures water quality based on seawater density stratification from 1998-2004 in Puget Sound and 1999-2004 in the Georgia Basin. Seawater density stratification is an indicator of: (1) the degree of mixing within the water column; (2) its resilience to mixing; and (3) the likelihood that aspects of poor water quality will develop due to human-induced pressures.

Why is this happening?

The various degrees of seawater density stratification presented in Figure 1 are normal within the Puget Sound Georgia Basin. Stratification means there are differences in the density of layers in the water. Density gradients (differences) in the water indicate stratification.

Factors that influence stratification include:

• Ambient air temperature
• Solar radiation
• Fresh water input from river flows and precipitation
• Surface winds
• Internal waves
• Tidal circulation

Factors that decrease stratification include:

• Winds
• Tidal circulation
• Any other factor that mixes water masses

Factors that produce or maintain stratification include:

• Freshwater flows into marine waters
• High solar radiation

Both freshwater (reducing salinity) and solar radiation (increasing temperature) cause the density difference between the surface layer and the cold, salty deep waters to be greater.¹²

The Effect of Freshwater Flows

In coastal estuaries and in many parts of Puget Sound, differences in salinity have a stronger influence on density stratification than differences in temperature.¹³ The layer of water where density increases rapidly with depth is called the pycnocline. In Puget Sound, sometimes the pycnocline extends to the surface and sometimes there is a mixed layer above the pycnocline.

Puget Sound Circulation Model

Click on the image below to view a larger version

Source: Staubitz, W., Bortleson, G., Semans, S., Tesoriero, A., Black, R. "Water-Quality Assessment of the Puget Sound Basin, Washington, Environmental Setting and Its Implications for Water Quality and Aquatic Biota." U.S. Geological Survey.

In the Georgia Basin, the flows from the Fraser River force a typical estuarine circulation, with freshwater from the estuary to flow onto the shelf at the surface, and salt water from the ocean shelf water to enter the estuary at depth.¹⁴ The resulting circulation is strongly modulated by tides and winds.

Our Geography

The Puget Sound and Georgia Basin encompasses the marine waters of the Puget Sound, the Strait of Georgia and the Strait of Juan de Fuca. A large portion of the marine waters in the basin ebb and flow through deep fjords. A fjord is a deep, steep-walled, U-shaped valley formed by glaciation, which has been flooded by seawater. These beautiful and unique features were formed millions of years ago as the glaciers carved our current geography. The waters of the Puget Sound Georgia Basin are largely well-mixed because of strong tidal currents interrupted by islands, underwater sills (mountains) and straits, many of which are extremely narrow. However, the waters at the ends of these deep fjords, and in the deep bottom waters behind shallow underwater sills, are somewhat isolated in terms of exchange with incoming Pacific Ocean waters. The implications are that pollutants, such as nutrients or toxics once discharged into these areas, may become entrapped and recycled within local areas over relatively long periods of time.¹⁵

   

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