Hood Canal report compiles oxygen studiesSeptember 20th, 2012 by cdunagan
Despite millions of dollars spent on research in Hood Canal, the precise causes of low-oxygen problems in Southern Hood Canal are still not fully understood, according to a report released this week by the U.S. Environmental Protection Agency and the Washington Department of Ecology.
News articles about the report have created some confusion, and I’ll get to that in a moment.
As I reported in Tuesday’s Kitsap Sun, research has not proven that nitrogen from human sources is responsible for a decline in oxygen levels greater than 0.2 milligrams per liter anywhere in Hood Canal. That number is important, because it is the regulatory threshold for action under the Clean Water Act.
Mindy Roberts, one of the authors of the report, told me that scientists who have worked on the low-oxygen problem have gained an appreciation for Hood Canal’s exceedingly complex physical and biological systems. So far, they have not come to consensus about how much human inputs of nitrogen contribute to the low-oxygen problems in Lower Hood Canal.
The report, which examined the complexity and scientific uncertainty about these systems, seems to have generated some confusion, even among news reporters. I think it is important to understand two fundamental issues:
1. The deep main channel of Hood Canal is almost like a separate body of water from Lower Hood Canal (also called Lynch Cove in some reports). This area is generally defined as the waters between Sisters Point and Belfair. Because Lower Hood Canal does not flush well, low-oxygen conditions there are an ongoing and very serious problem.
2. Fish kills around Hoodsport cannot be equated or even closely correlated with the low-oxygen conditions in Lower Hood Canal. The cause of these fish kills was not well understood a decade ago, but now researchers generally agree that heavy seawater coming in from the ocean pushes up a layer of low-oxygen water. When winds from the south blow away the surface waters, the low-oxygen water rises to the surface, leaving fish no place to go.
I’m not aware that researchers were blaming nitrogen from septic systems for the massive episodic fish kills, as Craig Welch reports in the Seattle Times. At least in recent years, most researchers have understood that this was largely a natural phenomenon and that human sources of nitrogen played a small role, if any, during a fish kill.
The question still being debated is how much (or how little) humans contribute to the low-oxygen level in the water that is pushed to the surface during a fish kill and whether there is a significant flow of low-oxygen water out of Lower Hood Canal, where oxygen conditions are often deadly at the bottom.
The new report, which was reviewed by experts from across the country, concludes that fish kills can be explained fully without considering any human sources of nitrogen. Evidence that low-oxygen water flows out of Lower Hood Canal in the fall is weak, the report says, though it remains a subject of some debate.
“We have not demonstrated that mechanism to their satisfaction,” Jan Newton of the Hood Canal Dissolved Oxygen Program told me in an interview. “We never said it caused the fish kill, only that it can reduce the oxygen level below what it was. In some years, it wouldn’t matter, but in some years it would make it worse.”
A cover letter (PDF 83 kb) to the EPA/Ecology reports includes this:
“While the draft report concludes that although human-caused pollution does not cause or contribute to the fish kills near Hoodsport, our agencies strongly support additional protections to ensure that nitrogen and bacteria loadings from human development are minimized.
“Water quality concerns extend beyond low dissolved oxygen and include bacteria and other pathogens that limit shellfish health. Overall, human impacts to Hood Canal water quality vary from place to place and at different times of year. Hood Canal is a very sensitive water body and people living in the watershed should continue their efforts to minimize human sources of pollution.”
One of the most confounding factors is the large amount of nitrogen born by ocean water that flows along the bottom of Hood Canal. An unresolved but critical questions is: How much of that nitrogen reaches the surface layer, where it can trigger plankton growth in the presence of sunlight?
Plankton growth is a major factor in the decline of oxygen levels, because plankton eventually die and decay, consuming oxygen in the process.
Human sources of nitrogen often enter Hood Canal at the surface, but researchers disagree on how much of the low-oxygen problem can be attributed to heavy seawater that reaches the sunny euphotic zone near the surface.
Here are the principal findings in the EPA/Ecology report, “Review and Synthesis of Available Information to Estimate Human Impacts to Dissolved Oxygen in Hood Canal” (PDF 3.8 mb).
Science Review Summary
This review incorporates information from published documents including draft and final reports, journal articles, and memoranda (see full citations in the References). Supplemental information was provided by Hood Canal researchers in meetings, conference calls, and email communications. Based on the best available information, we conclude the following:
1. Sediment cores establish that hypoxia occurred before European settlement, and oxygen levels were lower before 1900 than between 1900 and 2000. Shifts in organic matter associated with human watershed activities did not coincide with shifts in low oxygen conditions in Hood Canal. On the contrary, prior to the 1900s, low oxygen conditions prevailed in a decadal pattern that is consistent with climate influences (Brandenberger et al., 2008 and 2011).
2. There is no compelling evidence that humans have caused decreasing trends in dissolved oxygen in Hood Canal. Climate effects cannot be ruled out, the decreasing trends are not unique to Hood Canal, and dissolved oxygen in Lynch Cove, where relative human influences are the largest, have increased and not decreased (Bassin et al., 2009).
a. While the Hood Canal dissolved oxygen inventory shows a decline between the 1950s and 2000s, this pattern is also found in the Strait of Juan de Fuca and in other parts of Puget Sound and is not unique to Hood Canal. The decline is consistent with the decades-to-centuries climatic pattern that Brandenberger et al. (2008 and 2011) found in sediment cores.
b. Lynch Cove, the area east of Sisters Point, is the part of Hood Canal where human contributions are most likely to influence dissolved oxygen concentrations. Lynch Cove dissolved oxygen concentrations show a statistically significant increase in concentrations between the 1950s and 2000s, counter to the decline in the Hood Canal dissolved oxygen inventory and counter to declines in other areas of Puget Sound.
3. Hood Canal phytoplankton growth is likely
nitrogen-limited, so nitrogen mass loading to the euphotic zone
directly affects dissolved oxygen conditions. The
predominant overall source of nitrogen to Hood Canal is Pacific
Ocean nitrogen entering as deep waters and entraining in the
surface layer of Hood Canal. This dominant contribution of marine
throughout the year and throughout the main arm of Hood Canal and into Lynch Cove (Steinberg et al., 2010; Paulson et al., 2006; Devol et al., 2011a; and Kawase and Bahng, 2012).
4. Human activities have increased nitrogen loadings to portions of Hood Canal from tributaries and groundwater compared to natural conditions.
a. Most of the tributary loading occurs between November and May, whereas severe hypoxia and fish kills have occurred in the summer months. Humans add to natural nitrogen loads directly through residential development, primarily OSS, and indirectly via red alders. Red alders fix nitrogen from the atmosphere and exist today in higher numbers than past years due to logging activities and slow regrowth of conifers.
i. In Lynch Cove, tributaries contribute an estimated range of 40% to 80% of the total nitrogen loading from the watershed in the summer months. Red alders and residential development contribute approximately 50% and 33% of the tributary loading, respectively (Steinberg et al., 2010 and Richey et al., 2010).
b. Nitrogen loadings entering Hood Canal from groundwater and shoreline on-site septic systems (OSS) are difficult to quantify because of variability in the natural and human environment, limited sampling information, and uncertainties in fate and transport of subsurface wastewater.
i. Based on the best available estimates, loadings from shoreline OSS discharges in Lynch Cove range from 20% to 60% of the total nitrogen loadings from the watershed in the summer (see synthesis of Paulson et al., 2006, Georgeson et al., 2008, and Sheibley and Paulson, pers. comm., 2011).
5. Lynch Cove is the part of Hood Canal where human contributions have the greatest influence on marine dissolved oxygen. A synthesis of multiple studies indicates that human-caused discharges have an impact on dissolved oxygen levels in Lynch Cove that may approach the regulatory limit of 0.2 mg/L for waterbodies with naturally low dissolved oxygen. The available studies are inconclusive on compliance with water quality standards in this area due to substantial uncertainty in the methods, nitrogen sources, and averaging over time and space.
a. For Lynch Cove, model simulations and aggregated models coalesce around an average summer impact ranging from 0.03 to 0.3 mg/L (see synthesis of Steinberg et al., 2010; Richey et al., 2010; Paulson et al., 2006; Georgeson et al., 2008; Sheibley and Paulson, pers. comm., 2011; Devol et al., 2011a; and Kawase and Bahng, 2012).
b. Substantial uncertainty remains in the estimates of
human impact to dissolved oxygen, particularly in estimates of
marine nitrogen loadings to Lynch Cove. Other sources of
uncertainty include the inability of simple models to represent
complexity of the system, insufficient spatial and temporal data,
and factors omitted from analysis. In addition, the maximum impact
cannot be addressed quantitatively in Lynch Cove due to limitations
of currently available models. Due to the uncertainty and analysis
limitations, it is not possible to definitively assess whether the
quality standards are currently violated by human-caused nitrogen loads with the available information.
c. Variability in groundwater nitrogen concentrations is a key area of uncertainty and concern. Groundwater quality is generally good, but a small number of seeps have extremely high nitrogen concentrations. These seeps have a disproportional influence on the overall human loading of nitrogen. Pollution Identification Program (PIC) efforts have shown the ability to drastically reduce these concentrations through improved OSS management.
6. Researchers have unraveled the conditions that lead to fish kills. Fish kills occur due to a cascade of natural events. Dense marine water enters Hood Canal and lifts water with low oxygen levels toward the water surface. As river inflows decline during the dry season, the freshwater cap on the surface thins. Southwest wind events push this thin cap to the north, which allows low-oxygen water beneath it to surface rapidly. In a matter of hours oxygen levels rapidly decrease in the sensitive nearshore regions of southwestern Hood Canal. (Newton et al., 2011c, and Kawase and Bahng, 2012), which causes the fish kills at Sund Rock near Hoodsport.
a. Because of the large-scale nitrogen loadings from marine waters, human-caused nitrogen discharges have almost no effect on dissolved oxygen in central Hood Canal. In the main arm of Hood Canal (from Lilliwaup to Potlatch), where fish kills have been reported, initial model simulations indicate very small impacts (maximum impact less than 0.04 mg/L) to dissolved oxygen from human-caused nitrogen loadings (Kawase and Bahng, 2012).
b. In addition to the episodic events at Sund Rock near Hoodsport, chronically low dissolved oxygen in Lynch Cove (east of Sisters Point) in the summer can stress or kill marine organisms, particularly bottom-dwelling organisms (Newton et al., 2011c).
c. Best available information indicates that the subsurface seaward outflow has an insignificant effect on fish kills at Hoodsport.