Oxygen & Nutrients
Nutrients contribute to low oxygen levels
Both nitrogen and organic carbon contribute to dissolved oxygen depletion. Too much nitrogen fuels excessive marine algae growth. When the algae die 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:
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.
Soil, plant material, nitrogen-fixing plants, and wild animal waste all release nitrogen, which can enter rivers that discharge into Puget Sound.
How is it changing?
Natural sources currently make up the smallest proportion of nitrogen entering Puget Sound and have not changed much.
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 the 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.
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.
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 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.