Climate, dead zones and fish: Solving a ‘wicked problem’ in Lake Erie and beyond

[Link]A yellow perch. Image credit: Ken Hammond/USDA

Study: Water quality-fisheries tradeoffs in a changing climate underscore need for adaptive ecosystem-based management (DOI: 10.1073/pnas.2322595121)

There's a famous piece of advice from hockey, attributed to Wayne Gretzky, about how it's better to skate to where the puck is headed rather than where it is.

Research is now showing that regulations designed to protect Lake Erie's water quality are heeding the Great One's words when it comes to safeguarding the Great Lake's fisheries.

Specifically, the currently recommended limits on the flow of nutrients into Lake Erie from agriculture may be too restrictive for some species of fish. They are, however, suited to maintain healthy fisheries until the middle of this century amid a warming climate, according to a new study led by the University of Michigan.

[Link]Don Scavia

"Hitting the target for the future is the right thing to do now," said Don Scavia, professor emeritus at U-M's School for Environment and Sustainability. "Even though the proposed limits are too much for now, I don't think they should be relaxed because it takes a decade or longer to affect change."

This is but one takeaway from the study led by Scavia with collaborators from across the United States and in Asia.

In the project, the team linked forward-looking climate projections and nearly a century of fishery data with a mathematical model of Lake Erie's nutrient load and a water quality indicator-namely, the oxygen available in the lake's depths.

[Link]The dashed line shows where bottom waters in Lake Erie are experiencing low oxygen, or hypoxia, created by nutrients that enter from agriculture. New research is showing that ecosystem management decisions focused only on minimizing hypoxic regions, often called "dead zones," won't necessarily benefit all fish. Image credit: Don Scavia et al. Used under a CC BY-NC-ND 4.0 license.

With all these factors considered, the researchers showed that the relationships aren't as straightforward as the casual observer might expect. For example, while cutting back on nutrients does improve water quality, that doesn't necessarily benefit all fish.

"There are tradeoffs," Scavia said.

And, moving forward, warming temperatures-rather than nutrient load-will become the dominant influence over the oxygen in Lake Erie and similar ecosystems around the world.

Taken altogether, this means that there's no simple solution to managing these ecosystems. And the solutions rolled out today will need to be continuously evaluated and reassessed in the future. In the land management world, this is what's called a "wicked problem."

Fortunately, the team's study provides key insights on how to approach those.

It's complicated

The nutrients in fertilizers that farmers use to feed plants also feed aquatic microorganisms like algae and cyanobacteria. Nutrient-rich runoff water from fields helps fuel the infamous algal blooms in Lake Erie-as well as, for example, in the northern Gulf of Mexico and Chesapeake Bay.

When the algae and cyanobacteria in those blooms die, they sink to the bottom of the water and decompose, consuming enough oxygen in the process to render the water hypoxic. Even underwater, oxygen is essential for life and so hypoxic bottom water regions are often referred to as "dead zones."

Hypoxia is bad news for lake whitefish that prefer the cold bottom water of Lake Erie, said Stuart Ludsin, a professor in the Aquatic Ecology Laboratory at Ohio State University.

But, while the cyanobacteria in algal blooms thrive in fertile, nutrient-rich waters, so too do the plankton that feed the lake's larger fish. And not all of those fish live at depths where hypoxia is a problem. Yellow perch, for instance, actually benefit from water that's richer in nutrients, Ludsin said.

At present, the level of nutrient inputs is beneficial to yellow perch and walleye but limiting bottom habitat for whitefish. Finding the right balance between fishery and water quality considerations is a delicate and ever-moving target, Ludsin said, complicated by the fact that the lake's microbes proliferate faster in warmer temperatures.

"I don't want to see my research being used to say we should pollute to keep the fisheries highly productive," Ludsin said. "If there's no effort to curtail this hypoxia and water quality problem, with continued climate change, the lake will not be able to support vital fish."

The real message, he said, is more nuanced.

"We should try to find the right level of nutrients that will support our water quality goals while supporting the lake's suite of fisheries," Ludsin said.

And what "right" is should be determined by a water system's managers and stakeholders coming together, said Anna Michalak, founding director of Carnegie Science's Climate and Resilience Hub.

By bringing data and tools like the team's model into those discussions, all parties can understand how the best choices for, say, water quality may come at a cost to fisheries, and vice versa.

"The number one takeaway is the importance of approaching these management issues with systems-level thinking. What I mean by that is that there are always going to be tradeoffs depending on what your specific objectives are," said Michalak, who has been collaborating with Scavia for more than a decade studying Lake Erie and other aquatic ecosystems.

"We can't be playing whack-a-mole when it comes to environmental systems. It's only by thinking about them holistically that we can come up with solutions that work."

Although the data and decisions will look different for different ecosystems, this approach is generalizable, the team said.

"I think that this study provides a template and prior knowledge that can be applied to other systems," said Dan Obenour, associate professor of environmental engineering at North Carolina State University. Obenour earned his doctorate at the University of Michigan under the mentorship of Scavia and Michalak.

The study also highlighted that the team's climate-hypoxia model is robust and reliable over long time periods. Combined with the fact that it was designed to be "embarrassingly simple," Michalak said, the model is equipped to be an asset in informing best practices into the future.

"This is a wicked problem," Obenour said. "It's challenging to determine what the best management solution is and, even if you reach a consensus, it's going to change in the future. We need to stay on our toes."

​​Also contributing to the project were researchers from Heidelberg University in Ohio, Shanghai Jiao Tong University and the Chinese Academy of Sciences.

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