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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 temperatures, salinity, the timing and size of river flows, factors influencing nutrient retention, as well as nutrient loading from various sources.

There are many natural and human sources of nutrients, including: Pacific Ocean water entering Puget Sound, plant material, wild animal waste, livestock manure, agriculture, urban runoff, wastewater treatment plant effluent, and the atmosphere.

We are mandated by the federal Clean Water Act to initiate a study when a water body is not in compliance with the state's water quality standards. Puget Sound dissolved oxygen levels are violating these standards in many places, based on our water quality assessment of state waters.

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.

Nitrogen source Description How is it changing?
Pacific Ocean 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. 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.
Natural sources Soil, plant material, nitrogen-fixing plants, and wild animal waste all release nitrogen which can enter rivers that discharge into Puget Sound. Natural sources currently make up the smallest proportion of nitrogen entering Puget Sound and have not changed much.
Human land-use activities 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.

Monitoring of nitrogen levels in rivers show 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.

Wastewater 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. 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.
Atmosphere Nitrogen emissions from vehicles and other industrial activities ends up in the air and can enter Puget Sound as a component of wet and dry atmospheric deposition. 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 within Puget Sound, nitrogen concentrations are increasing, 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 will be used to identify what needs to be done to reduce nutrients entering Puget Sound and guide management actions in the near future.