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.

What is happening?

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.

The following links may provide helpful information and are located outside the EPA.gov domain.

Puget Sound

Between 1998 and 2004, a series of 46 stations in greater Puget Sound were monitored monthly for water quality and to measure density stratification. This work on Marine Water Quality Monitoring³ is conducted by the Washington Department of Ecology as part of the Puget Sound Ambient Monitoring Program.

The stratification patterns of these stations are shown in Figure 1. The majority of the stations (23) show moderate-infrequent stratification. These are located throughout the Puget Sound and reflect the strong tidal mixing of the area. Eleven stations show strong-persistent stratification. These are typically located near river mouths (e.g. Budd Inlet, Commencement Bay, Port Susan, Possession Sound, Skagit Bay), near river influence (Penn Cove, Saratoga Passage) or where mixing processes are weak (Hood Canal).

Figure 1: Seawater (Salinity) Stratification Patterns in Greater Puget Sound and the Straits of Georgia and Juan de Fuca⁴,⁶

Click on the image at right to view a larger version.
Data Methodology: Marine Water Quality Technical Background (PDF, 3pp., 25KB)

Georgia Basin

Between 1999-2004, a series of 13 stations, extending from the mouth of Juan de Fuca Strait up to the northern end of the Strait of Georgia, have been visited seasonally. Each year, surveys are taken in April, June, September and December to capture seasonal variations. This work is conducted by the Department of Fisheries and Oceans Canada and further information can be found at the Department of Fisheries and Oceans Canada: Monitoring Southern BC Coastal Waters.⁵

The stratification patterns of these stations are shown in Figure 1. The majority of the stations show strong-persistent stratification due to the influence of freshwater from the Fraser River. However, stations located in strong tidally induced mixing areas, such as Boundary Pass, Rosario Strait, and the northern end of the Strait of Georgia, show moderate-infrequent stratification.

Is Seawater Stratification Constant?

The stratification pattern data presented here are annual averages. It should be noted that the intensity and duration of stratification can vary greatly over time, due to weather events, seasonality and interannual climatic differences. But are there any long term changes? Since seawater density is determined by temperature and salinity, we can look for directional changes in these variables.

Long-term data records are scarce, but temperature records over the last 35 years exist for the Strait of Georgia, collected by the Canadian Department of National Defense. The deep water temperature in the central Strait of Georgia is shown for the period 1970-2004 in Figure 2.

Over this period, the data indicate a warming trend as well as the occurrence of pronounced interannual variability. Since 1980, this variability is dominated by the signature of El Niño events and, accordingly, over this period, the time series are well correlated with the Southern Oscillation Index (SOI). However, the strongest event, the marked cold event of winter 1978-1979, is of local origin and not linked to larger scale variability. Continued monitoring of seawater properties will help us follow long-term changes.

What is the Southern Oscillation Index?

El Niño-Southern Oscillation or "ENSO," is a coupled ocean-atmosphere phenomenon centered in and over the tropical Pacific. It involves large-scale fluctuations in a number of oceanic and atmospheric variables such as sea surface temperature, sea level pressure, etc. El Niño (warm phase) and La Niña (cold phase) episodes are the opposite extremes of the ENSO phenomena. During an El Niño, above normal sea surface temperatures (SST) extend across the central and eastern tropical Pacific Ocean. Learn more at AIRWeather.7

Figure 2. Deep water temperature in the central Strait of Georgia, 1970-2004²⁵

Analysis Source: D. Masson, Institute of Ocean Sciences, DFO, Canada.

Consequences of Stratification: Dissolved Oxygen in Hood Canal

One outcome of sensitive marine waters is that the problems associated with low dissolved oxygen, or "DO," may become more pronounced. Strong-persistent stratification forms a barrier to mixing, which brings surface oxygen down to depth. Hood Canal is one location in Puget Sound that has strong-persistent stratification and has had historically low DO also. However, over the last decade or so, the amount of DO is lower than the historical average, as shown in Figure 3. This pattern could be caused by several factors, natural and human, and is under active study by the Hood Canal Dissolved Oxygen Program.⁸

Changes in density stratification are an important factor being addressed. Long-term data records of water quality are scarce. The University of Washington collected this record in the 1950-60s and by joint efforts of UW's PRISM (Puget Sound Regional Synthesis Model) program and the Washington Department of Ecology during the 1990s and 2000s.⁹

Figure 3. The average amount of dissolved oxygen in the deep waters (>20 m) of Southern Hood Canal between Dabob Bay and the Great Bend.

Analysis Source: M. Warner, J. Newton, U. Washington. (see the Hood Canal Dissolved Oxygen Program Historical Comparison for background for this chart).¹⁰

Click on the image at left to view a larger version.

Dissolved oxygen in water, as shown in Figure 3, is often measured in milligrams per liter or parts per million (ppm).

To understand the significance of the numbers on the figure, we offer this explanation from the HCDOP. The program uses marbles as a metaphor for the amount of oxygen in water or air.

Fish, and many other aquatic critters, live in their watery world with the oxygen dissolved in water measured between 5 and 20 ppm. Translated, fish only need 5 to 20 marbles of air out of every million marbles of water, to survive. Humans require much higher levels of oxygen – at levels approximating 200,000 marbles of oxygen out of every million marbles of air. This is one good explanation for why we don't breathe well under water. Below 5 ppm fish and many other aquatic animals begin to stress, and below 3 ppm many can't survive.¹¹

Read an HCDOP brochure with more explanations of oxygen and the issue in Hood Canal.

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