Oxygen for life: Understanding how low dissolved oxygen in Puget Sound affects marine ecosystems
Introduction
This publication grew out of a series of workshops exploring the science of dissolved oxygen in Puget Sound. It comes at a time when managers and policy makers are faced with tough questions about the future of one of marine life’s fundamental needs. Studies show that impacts such as global warming will gradually change the amount of dissolved oxygen available to marine life in Puget Sound. Mathematical models also suggest that other human impacts such as the release of nitrogen from wastewater
treatment plants, agriculture, and stormwater will have an impact as well.
Part of our goal with the workshop series is to simply understand what we know and don’t know about these conditions. The articles in these pages, especially section one, explore some of the fundamentals. They are designed to help non-experts understand the causes of low dissolved oxygen—such as algal blooms fed by excess nitrogen—as well as when, where and why oxygen levels are changing.
We also explore questions that may be harder to answer. Questions that are puzzling scientists. Chief among them is how much humans might tip the balance. Will excess nitrogen released from human wastewater, for example, cause a significant impact on Puget Sound’s species? The answers may affect state and federal regulations and could lead to billions of dollars in retrofits to wastewater treatment plants.
At the heart of these questions is a fundamental fact about Puget Sound: it is already naturally low in oxygen. Even under the best of circumstances, at certain times of the year, parts of Puget Sound will be in violation of dissolved oxygen requirements. That is because of Puget Sound’s natural shape—there are areas where water doesn’t circulate as well—and because of the input of water from the ocean that also carries with it loads of oxygen-depleting nitrogen. In fact, 88% of all nitrogen in Puget Sound comes from ocean sources, including waters from outside Washington. Another 3% of the nitrogen comes from natural watershed sources.
That leaves a remaining 9% from human sources including agriculture, wastewater treatment plants, and stormwater runoff. Modeling suggests those extra nutrients may further decrease dissolved oxygen in parts of Puget Sound, but we know that this is highly variable in space and time. There may be pockets of Puget Sound that experience low dissolved oxygen levels, but not others. Whether significant changes to dissolved oxygen will harm marine life depends upon when and where different species occur and how they are able to respond. Section two of this booklet includes a framework prepared by University of Washington biologist Tim Essington by which to analyze these potential impacts.
Finally, in sections three and four we look at regulatory concerns and analyze some of the tradeoffs facing policy makers as they weigh the potential needs of future nutrient management. We close with the inevitable concerns posed by global warming. As our workshop series continues, the University of Washington Puget Sound Institute is collaborating with experts locally and globally to advance the science to inform these important decisions.
This publication was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality. Its content does not necessarily represent the views of King County or its employees.