Effects of Nitrogen

Algal Blooms and Eutrophication

Nitrogen entering Puget Sound generally does not represent a threat. However, excess nitrogen can fuel algae blooms that eventually decompose. This organic matter decomposition process decreases dissolved oxygen levels, typically near the bottom layers of Puget Sound waters. The figure below illustrates this process, also known as ‘eutrophication.’

Algae growth also depends on other factors than nitrogen. The following are examples of other factors which can enhance or inhibit algae growth and the extent to which algae decomposition leads decreases in dissolved oxygen:

  • Water circulation
  • Sunlight
  • Temperature
  • Phosphorus levels

Low dissolved oxygen conditions are sometimes referred to as hypoxia, which means that there may not be enough oxygen for marine organisms to thrive.

Low dissolved oxygen levels have been observed in Puget Sound for the past several years. The figures below illustrates areas of Puget Sound where have low dissolved oxygen concentrations have been measured. One of the questions that Ecology is trying to answer is how much of the observed dissolved oxygen problems are a result of human sources of nitrogen.


Illustration of the eutrophication process by which nitrogen entering marine waters create algal blooms, leading to low dissolved oxygen levels lower in the water column.


Low dissolved oxygen levels have been observed in Puget Sound for the past several years. The figures below illustrates areas of Puget Sound where have low dissolved oxygen concentrations have been measured. One of the questions that Ecology is trying to answer is how much of the observed dissolved oxygen problems are a result of human sources of nitrogen.

Frequency of low dissolved oxygen concentrations (hypoxia) measured in Puget Sound from 2003-2008. The larger the dot, the higher the occurrence of low dissolved oxygen concentrations (Source: Puget Sound Partnership, 2009)


Click to enlarge.

Results from the 2012 Marine Water Quality Assessment for dissolved oxygen in Puget Sound


Since both human and natural sources of nitrogen reach Puget Sound, it is difficult to tease out the effects of human sources of nitrogen on Puget Sound using only monitoring data that Ecology and other organizations have collected. Distinguishing between human and natural sources of nitrogen and evaluating effects on the Puget Sound ecosystem therefore requires a combination of monitoring data as well as statistical and modeling tools.

In addition to algal blooms resulting in low dissolved oxygen levels, some algal blooms are called harmful algal blooms (HABs) because they can be toxic and can affect human health either directly by swimming in the water, or indirectly by consuming shellfish that are grown in water which has been exposed to a harmful algae. Ecology has not assessed links between nitrogen contributions and harmful algal blooms in marine waters.

Ongoing Ecology Studies that are Evaluating Effects

Three ongoing studies are evaluating whether human nitrogen contributions are causing dissolved oxygen problems in Puget Sound. All three projects investigate the same question but at different scales and for slightly different purposes:

  • Deschutes River/Budd Inlet Total Maximum Daily Load Study – Ecology found that the combination of human point and nonpoint sources, together with the Capitol Lake Dam, cause dissolved oxygen violations in Lower Budd Inlet. The next step is to establish nutrient reduction targets to meet water quality standards.

  • South Puget Sound Dissolved Oxygen Study The purpose of this modeling study is to determine whether human contributions from wastewater treatment plants and river inputs cause violations of the dissolved oxygen standards, and if so, to establish nutrient reduction targets. Water quality model findings include:

    • Human sources of nitrogen, within and outside of the study area, are causing dissolved oxygen declines by as much as 0.4 mg/L in the following locations in South Puget Sound: Totten, Eld, Budd, Carr and Case Inlets and East Passage.
    • Nitrogen from marine point sources have a greater impact on the dissolved oxygen levels than nitrogen from human sources in watershed inflows.
    • Reducing human nitrogen loads would decrease the magnitude and extent of DO depletion.
    • Additional study is needed to refine the impact estimates.

  • Salish Sea Dissolved Oxygen Model – The purpose of this modeling study is to explore the effects of human contributions at the larger Puget Sound scale relative to the influence of the Pacific Ocean and climate change. Water quality model findings include:

    • Human sources of nitrogen decrease dissolved oxygen concentrations below natural conditions, and marine point sources (such as wastewater treatment plants) cause greater oxygen depletions than watershed inflows.
    • Future human nitrogen loads, due to increased population and changes in land use, would further decrease dissolved oxygen levels through 2070.
    • Human nitrogen contributions have the greatest impacts on DO in portions of South and Central Puget Sound.
    • The Pacific Ocean conditions have a greater influence on dissolved oxygen concentrations in Puget Sound than local nitrogen sources under both current and future conditions, though future ocean conditions are highly uncertain.
    • Most of the Salish Sea reflects a relatively low impact from human sources of nitrogen, though future human nitrogen contributions could worsen dissolved oxygen declines in some regions of Puget Sound.

  • Hood Canal Science Review – EPA and Ecology reviewed Hood Canal science related to dissolved oxygen, drawing from the work of scientists at the University of Washington, Pacific Northwest National Laboratory, U.S. Geological Survey, and Mason County. The report by Cope and Roberts (2013) concluded that human nitrogen loadings are not contributing substantially to low dissolved oxygen in Central Hood Canal, including the region where episodic fish kills have occurred. However, available information was insufficient to determine whether human sources of nitrogen were causing dissolved oxygen violations in Lynch Cove and Lower Hood Canal.

Ocean Acidification

The growth of marine algae also temporarily decreases marine carbon dioxide (CO2) concentrations, resulting in an increase in pH of surface waters. When the algae die, decomposition of algae at depth returns CO2 to the water column, driving down pH values in deeper waters. Low pH can also occur due to global atmospheric CO2 levels that increase the acidity of the Pacific Ocean and decrease the availability of calcium carbonate for shell-building organisms. Distinguishing the relative impact of local and global contributions to these processes can help identify management activities that might have the greatest potential for managing acidification in Puget Sound and the Salish Sea.

Ecology and its partners published a quality assurance project plan in May 2015 for modeling acidification in the Salish Sea. This model is still in early development and will be used to determine the relative impact of regional and global carbon and nutrient sources on acidification in the Salish Sea, including Puget Sound. The plan is to expand the existing Salish Sea Model (described above) with the parameters and processes needed to model acidification processes.