Low oxygen dead zones are threatening ocean life off coasts in the Pacific Northwest according to researchers at OSU.

Dead zones are areas of hypoxic, or non-oxygenated, areas of water within the ocean, that stunt the growth of organism. These dead zones have been speculated to be caused by ocean warming, however evidence backing that claim has proved to be limited.

“We started seeing ‘dead zones’ of hypoxic water offshore Oregon in the summer starting in 2002,” said research assistant in the College of Earth, Ocean and Atmospheric Sciences Maureen Walczak. “They kill almost all marine life that can’t get out of the way fast enough, and as time goes on they’re getting bigger and worse.”

According to principle investigator of the study Alan Mix, a few dead zones have appeared off the coast of Oregon, which prompted the research team to investigate their causes.

“There has been some speculation that these dead zones have been related to climate change, but it’s really hard to prove what are random events and what has direct cause,” Mix said.

However, this study established a clear connection between two prehistoric instances of sudden ocean warming and an increase of marine plankton sinking to the ocean floor, ultimately leading to dead zones, according to associate professor of integrative biology Francis Chan.

“In this case, the researchers found that once oxygen starts to decline in the layer of the ocean that is poorest in oxygen, it appears to have triggered the release of nutrients that further amplified the decline in oxygen,” Chan said.

According to Mix, the first step in this research was determining the extent of previous ocean warming through the use of paleoclimatology and paleoceanography, both of which use past occurrences to explain present issues, using sediments in the ocean floor.

“You can think of the sediments as tree rings—each layer accumulates over time and marks what was happening at that time period. It tells time, just like reading a book,” Mix said.

Mix and his research team developed chemical tracers to determine the temperature, oxygen levels and organism life during these prehistoric instances of sudden ocean warming.

“What we came up with is a record of how temperature has changed through time, which is done through some fairly exotic chemistry using chemical tracers that are left in the sediments which accumulate on the seafloor,” Mix said.

Through the use of these chemical tracers, the research team we able to discover an increase in water temperature, but not enough to create a dead zone on its own. However, they used this record of temperature change to determine the amount of oxygen in the water. This is another factor contributing to dead zones.

“It is just the same idea as if you open a pop bottle—it has a lot of gas in it. As the pop gets warmer, the bubbles of gas comes out,” Mix said. “The same is true for ocean water, it holds more oxygen when it’s cold and less when it’s warm. We could figure out the oxygen change just based on the temperatures.”

This change in temperature was warmer, but it was not a sufficient increase to create a dead zone on its own. This led the researchers to analyze other factors that could have caused the dead zones, including plankton and single-celled organism activity.

In 2006, Walczak and other researchers developed the stable isotopic records, which allowed them to mark organism life and salinity. These records were housed in the bodies of foraminifera—single-celled organisms that make a tiny calcium-carbonate shell.

“The oxygen isotopes in the foraminifera are sensitive to both temperature and seawater salinity, which force the record in the same direction—increasing temperatures and decreasing salinity,” Walczak said.

The combination of temperature, oxygen levels and organism life all conspire to trigger the system. Once the ecosystem is triggered to moving toward a zero oxygen state, it continues to cycle until it reaches that point, according to Mix.

“You can think of it as riding a rollercoaster. Every rollercoaster has a tipping point, a point where it stops climbing upward, finally tips over and runs away,” Mix said. “This is a series of mechanisms that keeps going once triggered. In this case, once the system was set up to move into this oxygen deprived environment, it kept going until it got there.”

This tipping point will have a big impact on the Pacific Northwest, according to Walczak.

“This research is of particular importance in the Pacific Northwest and (off the coast of) Alaska because of the recreationally and commercially significant fisheries in those regions,” Walczak said. “The link between rapidly increasing water temperatures and hypoxia is of particular concern, as very few aquatic species can survive in water in which the oxygen concentrations have become very low.”

According to Chan, although this work is discovering links between past and present conditions, it will also be used to help predict future conditions.

“This is about understanding if there might be surprises ahead,” Chan said. “For Oregonians, we live on an exceptionally productive coast, but one that is also vulnerable to low oxygen zones. This work reinforces the key point that we have a lot at stake in how we address climate change.”

Prompting the discussion and action toward climate change is another potential consequence of this discovery, according to Walczak.

“As we improve our understanding of the ecological and economic consequences of changing the temperature and chemistry of our oceans, it is increasingly clear that as a society we need to take mitigating action.

This study of dead zones increases researchers understanding of the consequences of changing temperature and chemistry of the oceans and can lead to further action, according to Walczak.

“It is very frustrating as a scientist to see the very serious issue of climate change being deliberately confused and politicized,” Walczak said. “What remains to be seen is what this generation will do about it.”