New research led by University of Nevada, Reno Associate Professor Joanna Blaszczak shows that hypoxia in rivers and streams is generally more widespread around the world than previously thought. Hypoxia is low or depleted levels of oxygen in surface waters that can be harmful to aquatic species and can in some cases increase production of harmful greenhouse gases from rivers.
Research recently published in the journal Lake Science and Oceanographycollects more than 118 million dissolved oxygen and temperature readings taken from more than 125,000 sites in rivers across six continents and 93 countries and spans more than 100 years, from 1900 to 2018. Hypoxia, defined in this study as dissolved oxygen concentrations less than 2 milligrams per liter, detected in rivers and streams in 53 countries, with 12.6% of all sites showing at least one measurement of hypoxia.
“Anoxia in coastal waters and lakes is widely recognized as a deleterious environmental problem, yet we lacked a similar understanding of hypoxia in rivers,” said Blaszczak of the university’s College of Agriculture, Biotechnology and Natural Resources. “Although 12.6% may not seem like a lot, it was previously generally thought that the incidence of hypoxia in rivers and streams was extremely rare. Showing that hypoxia is present in one in eight river sites with data is definitely a rule changer. The game in terms of how we need to think about and care about hypoxia in rivers and streams.”
Blaszczak says that advances in measuring hypoxia over the past 15 years or so using field-deployable dissolved oxygen sensor technology that allows for continuous, such as hourly, monitoring have given researchers the tools to get a better handle on the presence of hypoxia. Previously, readings were taken manually with a hand-held probe or by collecting water samples mostly during the day, when oxygen levels are naturally higher, due to photosynthesis occurring during daylight hours.
“Photosynthesis by algae produces oxygen that is released into the water column,” Blaszczak explains. “So, you don’t get a true picture of hypoxia occurring if you only measure during the day. Hypoxias are most likely to develop in the early morning hours, after an overnight lack of photosynthesis. Newer technology allows us to capture that data.”
In fact, the research showed a significant difference in results between using older methods and newer technology, due to times of daily measurements being taken.
“We found that if we sampled just during the day each day, between 8 am and 5 pm, we would detect the number of river sites where we observe hypoxia by about 25 percent,” she said.
Blaszczak says much of the data from the study is newer, and was taken using the latest technology.
“There wasn’t a lot of testing going on, especially before the 1950s,” she said. “Even all the way back to 2005 or so, data on a global scale is very scarce.”
Research shows both natural and human influences
As expected, the data showed that river hypoxia was predominantly present in warmer, smaller, lower sloping, and calmer waters, where oxygen in the water is not ‘driven’ by turbulence. Blaszczak says the research found that these conditions occur most often in rivers that drain naturally occurring wetlands, as well as in rivers that drain some urban areas, where human actions have led to hypoxic conditions.
“We as people who manage water often influence these conditions – by extracting water, building dams and increasing the amount of organic matter released into rivers, for example,” she says. “We could create these conditions from very low flows. Rivers surrounded by wetlands and urban areas had the same potential for hypoxia. This shows that while it can occur naturally, we can also create these conditions that lead to hypoxia.”
Blaszczak says hypoxia often isn’t recognized until it’s too late.
“You’ll see a report in the media of fish dying from hypoxia — from flows that are too low from dams, something being released into the river, or anything else we can control,” she says. “If we can put in more sensors to monitor the conditions, we can get early warning signals, and take action to mediate the problem, such as modulating the flow in some way.”
Newer sensor technology that provides continuous monitoring is also important, Blaszczak says, because the duration of hypoxic states affects the damage they may do.
“Fish, in some cases, have avoidance strategies. So if it’s only for a few hours, they can try to manage themselves, by staying near the surface, for example,” she says. “But if it’s more extended, they’re going to have a more difficult time. And there are other factors. An organism’s size may affect its ability to survive, and some organisms are not mobile, so they can’t adapt to survive a prolonged hypoxia event.”
Besides the harm that hypoxia can cause to aquatic life, it can also stimulate the production of the greenhouse gases methane and nitrous oxide.
“There is nothing good about hypoxia in rivers or other bodies of water,” says Blaschczak. “We can’t completely eradicate it, but we can certainly do a better job of monitoring it and taking action to prevent it, rather than stimulate it.”
Take a global view
On a more positive note, Blaszczak says that looking at data from 1950 to 2018, the research shows that, globally, there does not appear to be an overall increase in instances of anoxia in rivers. However, in some locations, there are increases. She says being on some sites is extreme. The researchers compiled a global map featured in the research article, which symbolizes the incidence of hypoxia they found by region, with blue being the rarest occurrence and red being the highest incidence of hypoxia.
“You’ll notice that Florida is completely red, off the charts,” Blaszczak noted.
Much of the United States is yellow or medium. Blaszczak says there is more data available from the United States in general than from other countries. She and a team of researchers compiled data from government-backed work and other published data to create a data set that spans all continents except Antarctica, but is dominated by data from North America. Each location must have a geographic coordinate, at least one dissolved oxygen measurement, a corresponding water temperature, and a date and time stamp. Blaszczak then analyzed the data, as part of her research at her college’s Department of Natural Resources and Environmental Sciences and Experiments Station.
I set out to lead the study after attending a September 2018 workshop in Switzerland conducted by Jim Heffernan and Tom Patten of Duke University. The workshop aims to advance global understanding of the dynamics of hypoxia in rivers and streams, and was supported by the Swiss National Science Foundation and the US National Science Foundation.
Going forward, Blaszczak says more monitoring using the latest technology is key to mitigating the harmful effects of hypoxia in rivers.
“We need to further develop our ability to identify when and where rivers are vulnerable to the development of anoxic conditions, so that we can guide river management in the face of ongoing climate change and land use. Support ongoing monitoring and expand monitoring to regions where data scarcity is essential.”
Blaszczak is grateful to the researchers and governments who provided the data used in the study and to the study co-authors, including Lauren E. Koenig, Francine H. Mejia, Lluís Gómez-Gener, Christopher L. Dutton, Alice M., Nancy Grimm, Judson W. Harvey, Ashley M. Hilton and Matthew J. Cohen.