Oxygen & nutrients in Puget Sound

Many parts of Puget Sound and the Salish Sea have oxygen levels that are below the levels needed for marine life to thrive. In some parts of Puget Sound, low levels of oxygen persist for most of the year.

Many physical, chemical, and biological factors influence dissolved oxygen levels in Puget Sound. These include:

The quality and timing of water entering Puget Sound from the Pacific Ocean
Local circulation patterns
Air and water temperature
Salinity
The timing and size of river flows
Factors influencing nutrient retention as well as nutrient loading from various sources

When water quality standards are not met, we are mandated by the federal Clean Water Act to initiate a study. Our water quality assessment of state waters shows that Puget Sound dissolved oxgen levels are not meeting standards. We are working on reducing excess nutrients through the Puget Sound Nutrient Reduction Project.

Icon displays map of Puget Sound and the words

Nitrogen in Puget Sound - A story map

We're involved in various efforts to understand how, and to what extent, excess nitrogen and other nutrients are a problem. This involves identifying nitrogen sources, monitoring nitrogen levels, analyzing how things are changing, and determining what we need to do to improve water quality.

This Nitrogen in Puget Sound story map, created by our scientists, allows you to explore what we know about nitrogen in Puget Sound.

View from above Puget Sound from sea plane. The water is turquiose and dark blue in streaks. — High nutrient levels can cause extra algae to grow in Puget Sound.

Nutrients contribute to low oxygen levels

Both nitrogen and organic carbon contribute to dissolved oxygen depletions. Too much nitrogen fuels excessive marine algae growth. When the algae dies and decomposes, oxygen is consumed.

Nitrogen sources

Nitrogen is a nutrient that is naturally present in marine waters, along with other nutrients such as phosphorus and carbon. While marine life does need some nitrogen to grow, excess nitrogen can fuel the excessive growth of algae. These algae eventually die, sink to greater depth, and decompose — a process that uses up oxygen in the water. In this way, too much nitrogen can decrease oxygen levels in Puget Sound.

See the list below to learn more about several sources of nitrogen in Puget Sound and how they are changing:

Human wastewater contains nitrogen. Most wastewater treatment plants remove solids and pathogens, but do not typically remove nitrogen, though there are a few exceptions, such as LOTT in Thurston County. Nitrogen from municipal wastewater treatment plants enters Puget Sound via marine outfalls. This source of nitrogen does not vary greatly with the seasons. In Puget Sound, wastewater nitrogen loading on a yearly basis is generally greater than nitrogen loading from rivers.

Grayscale cross-section of an ocean bay with landscape. Clockwise from the left are a farm with septic system, a mountain and river, an ocean inlet, a wastewater treatment plant, a road, and a city. The wastewater treatment plant is colored in, and red arrows show effluent running into the bay.

How is it changing?

Nitrogen loading from wastewater treatment plants generally increases as the population increases, and the population of Puget Sound is expected to double by 2070. However, implementation of nitrogen-removal treatment technologies (currently not required) has the potential to offset future increases in nitrogen loading due to population growth.

Nitrogen is present in plant fertilizers, livestock manure, and septic systems. This nitrogen enters Puget Sound via rivers, streams, and stormwater runoff. Nitrogen inputs from these sources vary seasonally, with largest inputs occurring during the wetter winter months. Nitrogen loading from rivers in summer is lower than that from wastewater facilities.

How is it changing?

Monitoring of nitrogen levels in rivers shows that, in some rivers, nitrogen levels are increasing, and in others they are decreasing. More land use dedicated to agricultural or urban activities/development and fewer forested lands generally results in higher nitrogen loading. Managing urban growth and changing land-use practices and human activities can reduce some of the nitrogen loading to Puget Sound.

Upwelled water from the Pacific Ocean off the continental shelf is rich in nitrogen and low in oxygen. This water enters Puget Sound from the Strait of Juan de Fuca and Admiralty Inlet. This upwelled ocean water is the largest source of nitrogen to Puget Sound, but it is also the source that we have little to no influence over since it is affected by processes in the Pacific Ocean.

How is it changing?

Nitrogen concentrations in the Pacific Ocean water vary greatly from the surface to greater depth. Changes in the intensity and duration of upwelling in response to climate, as well as the concentrations of nitrogen in upwelled water, have large impacts on nitrogen levels entering Puget Sound. We have limited monitoring data on the continental shelf to understand how upwelling processes and nitrogen concentrations are changing, so long-term trends are currently difficult to identify.

Nitrogen emissions from vehicles and other industrial activities end up in the air and can enter Puget Sound as a component of wet and dry atmospheric deposition.

How is it changing?

EPA has documented a decreasing regional trend of nitrogen oxide levels in the western U.S. However, estimates of atmospheric nitrogen deposition have remained relatively stable over the past decade with higher deposition during specific periods, potentially due to forest fires. Nitrogen deposition in the north Pacific Ocean, on the other hand, is reported to have increased, potentially due to the rapid rate of industrialization in Asia in recent decades.

Measuring changes

We have been monitoring the marine waters of Puget Sound for more than 20 years. Changes in marine water quality show that nitrogen concentrations are increasing within Puget Sound, and the cycling of nutrients is changing. In addition to lowering oxygen levels, changes in nitrogen can also affect food-web dynamics and, in turn, affect aquatic plants, fish, benthic organisms, and other marine life, resulting in other changes to the marine ecosystem.

What we are doing?

Our marine monitoring and freshwater monitoring efforts provide us with valuable data to track changes in dissolved oxygen and nitrogen. In addition, we are developing and using the Salish Sea Model, a computer-modeling tool, to better understand this complex system and the relative influence of different factors that contribute to low dissolved oxygen in Puget Sound and the Salish Sea. This model is used to identify what needs to be done to reduce nutrients entering Puget Sound and guide management actions. One regulatory action we are taking is developing a Puget Sound Nutrient General Permit for wastewater treatment plants that discharge to Puget Sound.

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Oxygen & nutrients in Puget Sound

Nitrogen in Puget Sound - A story map

Nutrients contribute to low oxygen levels

Nitrogen sources

Measuring changes

What we are doing?

Related links