Warming waters likely contribute to decreasing dissolved oxygen levels in Puget Sound, adding another layer of complexity to efforts to understand the health of the estuary and ensure healthy conditions for aquatic life ranging from zooplankton and crabs to salmon and killer whales.
The finding emerges from a preliminary University of Washington analysis of temperature and dissolved oxygen measurements taken at multiple locations in Puget Sound over the past century. The work, which is available on ESS Open Archive but has not yet been peer-reviewed, suggests that increasing temperatures account for roughly half of the dissolved oxygen decline documented in central Puget Sound over the last 100 years.
Puget Sound is “a very complex system that has many other things going on,” says University of Washington doctoral student Dakota Mascarenas, the study’s first author. But, she says, “we've identified a mechanism that, on a century scale, might have about this level of impact.” Mascarenas also presented the unpublished data at an online Science of Puget Sound Water Quality workshop in February.
It’s well known that warmer water holds less dissolved oxygen and other dissolved gases. As water gets warmer, water molecules move faster and jostle against each other more, making it more likely that oxygen molecules will get pushed out of the surface of the water.
In fact, scientists have documented temperature-related oxygen decreases at a global scale. There’s widespread concern that climate change and continued increases in temperature could further reduce oxygen levels in marine ecosystems. And so, the look backwards: "The first step in trying to predict the future is to know what happened in the past," says University of Washington oceanographer Parker MacCready, a senior author of the new study.
The researchers analyzed more than 12,000 measurements of water quality parameters (temperature, salinity, and dissolved oxygen) dating all the way back to 1932. The measurements were taken near the bottom of the water column in late summer and early fall – where and when low-oxygen conditions are most likely to develop in Puget Sound.
They identified five locations with at least 60 years’ worth of the requisite data: Saratoga Passage in the Whidbey Basin, Lynch Cove in Hood Canal, Carr Inlet in South Sound, and Point Jefferson and a location near Seattle in the Main Basin of Puget Sound.
The long timeframe was necessary to discern patterns that transcend the variations in ocean conditions that happen seasonally, from year to year, and on longer cycles (such as the Pacific Decadal Oscillation and El Niño-Southern Oscillation) affecting Puget Sound. “There's a lot of different timescales of potential variation,” says Mascarenas. "We've done our best to look as long as we can."
The full series of measurements revealed that the five Puget Sound sites have warmed by about 1.4 °C over the past century. This is in line with regional trends in ocean and atmospheric warming.
Meanwhile, dissolved oxygen has decreased at a rate of 0.3-0.9 milligrams per liter per century at the two Main Basin sites. Outside of central Puget Sound, dissolved oxygen trends were more variable: declining at Carr Inlet, stable at Saratoga Passage, and slightly increasing at Lynch Cove.
The researchers then used a well-established equation to calculate the reduction of oxygen in the water, given how much it has warmed. This revealed an expected decrease in dissolved oxygen of 0.31 milligrams per liter per century on average, across all locations.
At Point Jefferson and the near-Seattle location in the Main Basin, this warming-related, expected decline explains 40-100% of the actual decrease in dissolved oxygen observed over the past century. At Carr Inlet, warming accounts for about 50% of the dissolved oxygen loss. Elsewhere, measurements were too variable to draw conclusions.
The marked effect of temperature in explaining oxygen decline was surprising, says MacCready. Although the relationship between temperature and dissolved oxygen is well known, the scientific conversation about dissolved oxygen in Puget Sound has been so focused on nutrient pollution that he hadn’t given temperature much thought, he says.
Globally, an estimated 15% of the ocean’s oxygen loss between 1960 and 2010 can be attributed to the effects of increasing temperature, according to a report from the International Union for the Conservation of Nature. Warming may explain roughly half of the oxygen loss in the upper 1,000 meters of the ocean, the report says.
Because Puget Sound’s Main Basin is deep, contains about 65% of the total volume of Puget Sound, and supplies water to other sub-basins, the Main Basin findings likely reflect changes affecting Puget Sound as a whole, the researchers argue. “That's probably the closest to the background trend that we're going to be able to see,” says Mascarenas.
Similar findings have been documented in the Chesapeake Bay – another large estuary adjacent to urban and agricultural development but with a very different structure compared to Puget Sound. This suggests that temperature-related oxygen declines may be a general phenomenon affecting estuaries around the world.
The new study does not make any predictions about how warming may affect dissolved oxygen in Puget Sound in the future, as climate change intensifies. The relationship may not be a linear one and the system could encounter “tipping points” that magnify the effects, Mascarenas says.
The policy implications of these changes are also uncertain. Will climate change make Puget Sound more sensitive to nutrient loadings and make action to control human-caused nutrient inputs more urgent? Or will warming simply overwhelm any proposed gains from controlling nutrients?
What’s predictable, for now, is further warming, says MacCready. "There's every reason to think that that trend will continue."
This article 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.