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Saving Puget Sound

South Puget Sound Dissolved Oxygen Study

About the dissolved oxygen study

Ecology is conducting a water quality study on low dissolved oxygen levels in South Puget Sound. This study is evaluating how human activities, along with natural factors, affect low dissolved oxygen levels in South Puget Sound.

Project Status

Ecology has completed the report South Puget Sound Dissolved Oxygen Study: Water Quality Model Calibration and Scenarios.

The study found that low oxygen concentrations naturally occur through much of South and Central Puget Sound. However, human contributions from marine point sources and within watershed inflows decrease oxygen as much as 0.2 to 0.4 mg/L below natural conditions in portions of Totten, Eld, Budd, Carr, and Case Inlets, and East Passage in Central Puget Sound.

We will improve and refine our computer prediction models over the next few years, in concert with the Salish Sea dissolved oxygen modeling assessment. We will continue to coordinate with stakeholders through the Advisory Committee.

Overview presentation of the South Sound study:

Fish need oxygen: In areas with low levels of dissolved oxygen, fish and other marine life become stressed and die or are forced to flee their habitat. There are many areas in Puget Sound with very low levels of dissolved oxygen.

Nitrogen is the main pollutant that causes low dissolved oxygen levels: Discharges from wastewater treatment plants, septic systems and other sources add nitrogen to Puget Sound. Excess nitrogen causes excess algae growth. As the algae dies and decays, they rob the water of dissolved oxygen. Once released into Puget Sound, nitrogen moves around. Nitrogen discharged at one spot may cause low dissolved oxygen levels many miles away.

We need to study the effects of nitrogen discharges: The purpose of this study is to determine how nitrogen from a variety of sources affects dissolved oxygen levels in South Puget Sound. This ongoing study is a critical first step in determining what might need to be done to improve water quality. The results of the study may show that human-related sources of nitrogen need to be reduced to keep South Puget Sound healthy. If reductions are needed, the study will also help determine where reductions might need to occur.

To learn more, see What is the problem?