By 2020, human-related contributions of nitrogen do not result in more than 0.2 mg/L reductions in dissolved oxygen levels anywhere in Puget Sound.
Low dissolved oxygen in marine waters can create significant problems, such as extensive fish kills in extreme conditions. Excessive nitrogen and carbon from human activities can create or exacerbate the conditions that lead to low oxygen in Puget Sound. Too much nitrogen fuels excessive marine algae growth. When the algae dies and decomposes along with excess organic carbon from human sources, oxygen is consumed and drops to levels that cause stress on fish and bottom-dwelling organisms. Processes leading to low oxygen can also create other problems such more acidic waters in some areas.
Human sources of nutrients include wastewater treatment plants and stormwater runoff that reaches Puget Sound from developed and agricultural lands, industrial discharges, and onsite septic systems. Learn more about nitrogen in Puget Sound from Ecology’s Story Map.
Washington Department of Ecology, Environmental Assessment Program
Washington Department of Ecology, Environmental Assessment Program, Modeling and TMDL Unit
Washington State Department of Ecology (Ecology) monitors dissolved oxygen and related parameters monthly throughout Puget Sound as part of the ambient monitoring program. Dissolved oxygen data are also part of the Marine Water Condition Index, the other indicator under the Marine Water Quality Vital Sign.
Much of Puget Sound science work involves the Salish Sea Model. The Pacific Northwest National Laboratory and Ecology worked in collaboration to develop this peer-reviewed, state-of-the-science computer modeling tool that simulates hydrodynamic and water quality processes. The model accounts for both human and natural sources of nutrients to Puget Sound. The sources of nutrients that are not directly from humans represent the expected “natural” loading of nutrients. The Salish Sea Model uses this information to predict a reference condition that is absent of local human nutrient sources in Puget Sound. The indicator focuses on reducing the impact of human sources so that dissolved oxygen does not decrease by more than 0.2 mg/L.
The first phase of modeling to support the Marine Water Quality Implementation Strategy (MWQ IS) and the Puget Sound Nutrient Source Reduction Project (PSNSRP) was completed and published in January 2019. The modeling work established that nutrients from a combination of human sources in the Puget Sound region cause depletions in dissolved oxygen more than the indicator target of 0.2 mg/L. The results of that report are available in an online webmap and the full report is available here: Ahmed et al. 2019. Puget Sound Nutrient Source Reduction Project. Volume 1: Model Updates and Bounding Scenarios. Dept. of Ecoloyg Publication No. 19-03-001.
Marine point sources include all facilities with outfalls in marine waters (or in close proximity to marine waters), such as wastewater treatment plants and industrial facilities. From: Ahmed et al. 2019. Puget Sound Nutrient Source Reduction Project. Volume 1: Model Updates and Bounding Scenarios. Dept. of Ecology Publication No. 19-03-001.
Watershed sources of nutrients include both point and non-point sources. Rivers and streams deliver nutrients to Puget Sound from the watersheds they drain. Rivers are pathways for nitrogen present in plant fertilizers, livestock manure, and septic systems. The Salish Sea Model includes loads from rivers, streams, and their watersheds, as well as flows from shoreline fringes. From: Ahmed et al. 2019. Puget Sound Nutrient Source Reduction Project. Volume 1: Model Updates and Bounding Scenarios. Dept. of Ecology Publication No. 19-03-001 and Nitrogen in Puget Sound Story Map.
Anthropogenic sources (human sources) of nutrients include both point and non-point sources such as wastewater treatment plants discharging to Puget Sound and human activities within watersheds delivered via rivers. From: Nitrogen in Puget Sound Story Map.
River/streams are freshwater pathways that deliver nutrients and drains watershed areas. "River inputs” and “river inflows” are used interchangeably with “watersheds,” “watershed inputs,” and “watershed inflows” to represent the delivery of flow and nutrient inputs into the Salish Sea Model. In the model, these estimates are for the mouth of each river, stream, or watershed and represent loading at the point at which the freshwater inflow enters the Salish Sea. These estimates include but do not distinguish between various upstream point and nonpoint sources in the watersheds that contribute to the loading at the mouth. From: Ahmed et al. 2019. Puget Sound Nutrient Source Reduction Project. Volume 1: Model Updates and Bounding Scenarios. Dept. of Ecology Publication No. 19-03-001.
0.2 mg/l oxygen depletion rate was adopted as an element of Washington State's marine DO criteria as a small allowable variation from natural oxygen regimes. The 0.2 mg/L limit is based on restricting changes in the DO regime and does not represent a biologically based value but rather, a detectable or measurable change in water quality. From: Washington State’s Marine Dissolved Oxygen Criteria: Application to Nutrients (2018), Washington State Department of Ecology Water Quality Program.
The latest Salish Sea modeling work evaluated three different years: 2006, 2008, and 2014. The goal of this work was not to evaluate trends between each of the years, but rather to understand the variability in hydrodynamic processes and water quality in Puget Sound. Understanding the current range of conditions provides a baseline, from which we can work towards identifying reductions needed from different human sources of nutrients. The three years (2006, 2008, and 2014) were specifically modeled because of distinct differences in the climate and ocean conditions in each year. These years represent different overall residence times relative to a 16-year baseline. The residence time index for 2014 is closer to the baseline, while 2008 is slightly higher and 2006 is much higher than the baseline value. In order to evaluate a trend in the impact of human sources on the decrease in dissolved oxygen levels, different questions need to be asked for the model and many more years of data would need to be modeled. So far, work has focused on "what are the human impacts on dissolved oxygen?" Focusing on the question of "are the human impacts on dissolved oxygen getter better or worse over time?" requires a different modeling project and funding.
Maximum dissolved oxygen depletions are predicted to occur in inlets where flushing is relatively poor compared to the main channel, such as Case, Carr, Dyes, Sinclair, Budd, and Henderson Inlets (spatial distribution shown above). Well-mixed basins are predicted to experience smaller dissolved oxygen depletions. Most of the central Main Basin, for instance, is predicted to experience close to, but less than, a 0.2 mg/L reduction in dissolved oxygen. The maximum dissolved oxygen depletions for the years 2006, 2008, and 2014 were –1.9 mg/L, –1.5 mg/L, and –2 mg/L, respectively, all occurring in the East Bay of Budd Inlet. The overall median dissolved oxygen depletions for 2006, 2008, and 2014 were –0.29 mg/L, –0.27 mg/L, and –0.28 mg/L, respectively.
The spatial distribution of the cumulative number of days that the dissolved oxygen concentrations were below the water quality standards for 2006, 2008, and 2014 are shown below. Various locations during 2006, such as Lynch Cove, Holmes Harbor, and parts of Skagit Bay, are predicted to have experienced a significantly higher number of days below the standard compared to 2008 or 2014. Other locations such as Penn Cove, portions of Port Susan, Quartermaster Harbor, Case, Carr, Sinclair and Dyes Inlets, and Liberty Bay are predicted to have experienced a cumulative three months or more of noncompliance with the water quality standard during each of the three years. The maximum number of cumulative noncompliant days occurred in Carr Inlet in 2006 and 2008, where for 250 and 216 days, respectively, water quality standards were not met. In 2014, however, the maximum number of cumulative noncompliant days (198) occurred in Quartermaster Harbor.
Cumulative number of days when dissolved oxygen did not meet the water quality standard.
The spatial distribution of the maximum dissolved oxygen depletions below the water quality standard in 2006 from (1) all anthropogenic sources, (2) only marine point sources, and (3) only watershed sources are shown below. Data from 2008 and 2014 are not shown because the distributions are similar. At every impacted location, the effect of all anthropogenic loads results in larger dissolved oxygen depletions than those due to either marine point sources or watershed sources alone. This points to the need for strategies that reduce nutrient loading from both types of sources. Human activities from watershed sources alone produce dissolved oxygen depletions in Bellingham Bay, Whidbey Basin, South Sound, Hood Canal, and Main Basin. Marine point sources alone produce some dissolved oxygen depletions in Whidbey Basin, and multiple depletions in South Sound and Main Basin. The combined effect of marine point and watershed sources can exacerbate dissolved oxygen depletions much more than either of the sources alone. Given the complexity of the Puget Sound system, the expected impacts of reductions from the two sources would not be linear. The next phase of Salish Sea modeling will begin to better define and understand the significance of watershed sources of nutrients on marine dissolved oxygen and inform how much reduction might be needed from human sources in watersheds in addition to reductions from marine sources.
Modeled oxygen depletion in Puget Sound based on human sources.
Please visit Ecology’s Puget Sound Nutrient Source Reduction Project website for the most up-to-date information on that project and the Salish Sea modeling effort.
Dissolved oxygen patterns reflect complex interactions of physics, chemistry, and biology. Concentrations vary tremendously around Puget Sound, near the surface or at the bottom, and during different seasons of the year. Using computer models and observational data, we have learned that much of this variation occurs due to conditions in the Pacific Ocean, climate and seasonal biogeochemical influences, and circulation patterns within Puget Sound.
Human nutrient inputs have increased over what would naturally flow to Puget Sound under forested conditions, and will continue to increase with our population. Understanding the impacts of local human activities on dissolved oxygen in Puget Sound is a long-term endeavor. We continue to improve our knowledge of what drives Puget Sound dissolved oxygen patterns through monitoring programs and modeling analyses in an effort to guide regional investments in nutrient reductions.
Ecology is leading the development of the Marine Water Quality Implementation Strategy (MWQ IS) that will inform actions intended to meet the dissolved oxygen indicator target. Development of the MWQ IS and Ecology’s Puget Sound Nutrient Source Reduction Project (PSNSRP) in parallel ensures that Puget Sound recovery actions are aligned with Ecology’s regulatory actions to reduce point and nonpoint nutrient sources. This is a collaborative effort with communities and Puget Sound stakeholders to address human sources of nutrients. This work uses the latest science and experts from tribes, the regulated community, and state and federal agencies to find the right solutions for regional investments to control nutrients from point and non-point sources.
To reduce the frequency and severity of low dissolved oxygen and associated problems in Puget Sound, the Leadership Council adopted a target intended to minimize local human contributions that worsen low dissolved oxygen in Puget Sound. However, the issue is complicated by the fact that dissolved oxygen naturally exhibits a high degree of variability in marine waters, changing almost continuously with time of day, location, season, tidal cycle, depth, the mixing and movement of different water sources, and many other factors. The fact that there are several large sources of nitrogen entering Puget Sound that are not directly from human sources also needs to be considered. These sources of nitrogen include the ocean (generally the largest overall source), natural watershed yield (from forests, for example), groundwater, and the atmosphere. Determining precisely the degree to which human inputs are responsible for a decline in dissolved oxygen is a complex issue. Addressing the issue requires a combination of good monitoring data, information on the sources of nitrogen, and sophisticated mathematical models like the Salish Sea Model.
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