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Sources and Pathways of the Nitrogen in Puget Sound

Many natural and human sources contribute nitrogen within the Puget Sound ecosystem, and these sources are delivered to marine and freshwaters through various pathways, as illustrated below.  Ongoing studies by Ecology are exploring the effects of nitrogen loading to the Puget Sound ecosystem. Though we do have estimates of how much nitrogen is from human vs. natural sources, we still don’t know whether human sources are contributing to low dissolved oxygen in Puget Sound.

Forms of Nitrogen
Typical Range of Nitrogen Concentrations
Nitrogen Contributions to Puget Sound and the Greater Salish Sea
Seasonality of Nitrogen Loading from Wastewater Treatment Plants and Rivers

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We know that the Pacific Ocean contributes the largest amount of nitrogen to Puget Sound through Admiralty Inlet. Nitrogen loading from the ocean cannot be reduced by local source control/management activities.

After the Pacific Ocean, the second largest sources are municipal wastewater treatment plants discharging directly to Puget Sound via marine outfalls, followed by upstream watershed sources delivered to Puget Sound via rivers and streams. Some of this nitrogen loading may be influenced by human management practices.

Breakdown of the percent nitrogen load contributions to Puget Sound from different sources, adapted from Mohamedali et al. (2011a).

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Forms of Nitrogen

Nitrogen has many components, and is present in the environment in inorganic and organic forms as well as in dissolved and particulate forms. The graphic below outlines the partitioning of total nitrogen into the various forms that are found in the environment.

Many of the estimates and plots presented in this website which describe how much nitrogen enters Puget Sound focus on dissolved inorganic nitrogen (DIN). As shown above, DIN is the sum of nitrate, nitrite, and ammonium—three different forms of inorganic nitrogen. Of all forms of nitrogen, DIN is the form of greatest interest, since it is the most bio-available form of nitrogen used by marine algae. Organic forms of nitrogen are also present, but in much smaller amounts.


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Typical Range of Nitrogen Concentrations

The figure below illustrates the typical range of nitrogen concentrations we see in different components of the Puget Sound ecosystem. Note that the y-axis of the figure is on a log scale, so septic system effluent and WWTP effluent concentrations are one to two magnitudes higher than most other sources.

Typical range of nitrogen concentrations found in the Puget Sound ecosystem (note: the y-axis is on a log scale).

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Nitrogen Contributions to Puget Sound and the Salish Sea

Most of the information presented on this website focuses on nitrogen loading to Puget Sound south of Admiralty  Inlet.  The northern geographical boundary of Puget Sound begins at Admiralty Inlet and includes all the bays, inlets and basins to its south. Puget Sound is part of the greater Salish Sea, which also includes the Strait of Juan de Fuca and the Strait of Georgia, parts of which are within Canadian borders, as illustrated below.

The Puget Sound and Salish Sea ecosystem, including the watersheds that drain into these marine areas (Source: Mohamedali et al., 2011a).

Nitrogen from rivers and wastewater treatment plants enters the Straits and other parts of the Salish Sea from U.S. and Canadian sources. The largest Canadian source is the Fraser River, which discharges to the Strait of Georgia. Nitrogen contributions to the Straits have a lesser influence on Puget Sound.

The bar chart below illustrates nitrogen loading contributions from rivers and wastewater treatment plants to Puget Sound and the Salish Sea from both U.S. and Canadian Sources.

Bar chart comparing relative annual and summer contributions of dissolved inorganic nitrogen (DIN) loads from U.S. and Canadian rivers (pink) and WWTPs (blue) into Puget Sound, the Strait of Juan de Fuca and the Strait of Georgia (Source: Mohamedali et al., 2011a).

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Seasonality of Nitrogen Loading from Wastewater Treatment Plants and Rivers

Wastewater treatment plants and rivers, including all upstream watershed sources discharging directly to Puget Sound, are the two largest local sources of nutrients to Puget Sound.  The relative contribution of nitrogen from these two sources varies by location and by season. Wastewater treatment plants serving the largest population centers produce the highest wastewater nitrogen loads, while the rivers with the largest flow generally contribute the highest river loads.

Concentrations, Loads, and Yields explains some of the ways that we describe the amount of nitrogen from various sources.

As illustrated below, wastewater treatment plants and river dissolved inorganic oxygen (DIN) loads to Puget Sound are similar in magnitude on an annual basis, but summer wastewater treatment plant loads are much greater than summer river loads. During the summer, streamflows are at their lowest, and river DIN loads reach an annual minimum.

The seasonality in loads is important to Puget Sound water quality because the lowest dissolved oxygen concentrations in Puget Sound have been measured in the late summer season. Low dissolved oxygen conditions in Puget Sound in the late summer may mean that the ecosystem is more sensitive to nitrogen loading in the summer months.

Comparison of dissolved inorganic nitrogen (DIN) load from rivers (pink) and wastewater treatment plants (blue) into Puget Sound (Source: Mohamedali et al., 2011a).

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