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.

	Measurements for marine water stratification are routinely taken four times a year in the Georgia Basin and 12 times a year in the Puget Sound. Read more…
	A number of factors can decrease, produce or maintain stratification. Read more…
	The quality of the marine environment affects everything from the quality of the food you put on your table, our economy, and the quality of our community life. Read more…
	We cannot change or influence our oceanographic features, tides and winds, but we can minimize pollution from humans that is more likely to affect stratified (and more sensitive) marine waters. Read more…

Introduction to the Marine Water Quality Indicator

The marine waters of the Puget Sound Georgia Basin are home to a diverse array of plant, fish and animal life, all of which have intrinsic, legacy and commercial value. In many ways, this life is not only indelibly linked to our own, but anchors us in ancient traditions and our collective cultural heritage.

There are numerous properties that can be used to assess marine water quality, among them pH, temperature, salinity, dissolved oxygen, nutrients and fecal coliform. However, an important influence on many of these is seawater stratification. Seawater can be layered similar to the way an oil and vinegar salad dressing separates. Stratification is a measure of this layering. It affects the overall sensitivity of the water to pressures from humans, such as sewage, excessive use of plant fertilizers (nutrients) and chemicals from paved land surfaces. In a way, the intensity and persistence of stratification can tip the balance to favor either better or worse marine water quality.

How Seawater Stratification is Measured

Seawater stratification is characterized by its intensity and persistence.

Stratification intensity is defined as the change in seawater density (sigma-t) between the surface and the bottom. It is characterized as:

• Weak (delta sigma-t < 0-0.5)
• Moderate (delta sigma-t >0.5-2.0)
• Strong (delta sigma-t >2.0)

Sigma-t is an oceanographic convention used to represent density. For other definitions and units of measurement, visit the Washington State Department of Ecology Marine Water Quality Parameters.¹

Stratification persistence is defined as the relative frequency of occurrence of strong stratification. It is categorized as:

• Infrequent, or 0-33 percent of observations
• Intermittent, or >33-67 percent of observations
• Persistent, or >67-100 percent of observations out of all the time sampled

The degree of density stratification is labeled according to both its intensity and persistence: (1) Strong persistent; (2) strong intermittent; (3) moderate infrequent and; (4) weak infrequent. Water density is affected by both temperature and salinity, increasing with decreasing temperature or increasing salinity.²

Stratification patterns make our marine waters more sensitive or vulnerable to the impacts of human activities. Please see the How Does This Affect Me? section of the indicator for further information.

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