A refined mathematical model is now capable of predicting carbon inputs and outputs for freshwater lakes around the world, according to new research from INSTAAR’s Isabella Oleksy and collaborators. Their work could help scientists understand the role of freshwater lakes in the global carbon cycle.

Oleksy's most recent paper, which was published, tests and revises an equation that allows scientists to estimate the overall biological activity in a lake from limited data.

The equation  by a group of scientists, including Oleksy's co-author , in 2018. It’s a mathematical formulation of  — a longstanding theory in the field. Basically, the theory posits that you can estimate the total growth of phytoplankton in a lake from the color of the water and measurements of a few key nutrients. Phytoplankton is the basis of the marine food web, which makes it a good stand-in for lake productivity on the whole.

  Further refinements of a model like this might be used to generate estimates of how much carbon is being fixed by lakes annually

-- Isabella Oleksy

“This is a way to potentially be able to understand what algal biomass and water quality might look like in a bunch of different lakes, even when you can’t necessarily get out there and measure it,” Solomon said.

Oleksy's study is the first to test the model against real world data — quite a bit of it. Back in 2019, when Oleksy was a postdoctoral researcher at the , she put out a call for data at a meeting of the .

“I asked people, ‘Hey. I want to test this model, but we need observations from lakes around the world,’” she said.

Collaborators were eager to help out. With the help of 30 scientists at many different institutions, Oleksy gathered detailed measurements from 58 different freshwater lakes around the world. Then, she tested the model’s predictions against the data. The initial test was encouraging.

“The results were pretty realistic,” she said.

The next step was to make the predictions even better. Through a process called Monte Carlo analysis, Oleksy pitted the model’s predictions against the on-the-ground data. Where the model faltered, she added new parameters to improve it.

In the end, Oleksy and her collaborators created a model capable of estimating the conditions of freshwater lakes in a diverse range of locations and ecosystems.

According to Oleksy, the new model could have implications far beyond freshwater lakes. It elucidates one small element of the global carbon cycle — a cycle that has become a priority for scientists in the era of global warming.

Policy makers and researchers rely on global-scale models of the carbon sources and sinks to predict the Earth’s future climate and inform large-scale solutions for climate change. These models are vast and complex — they must take into account the inputs and outputs of diverse human activities, ecosystems and geologic phenomena. Oleksy and her collaborators hope that their new study can be used to eliminate some uncertainty from these estimates.

“There is a lot of uncertainty about the role of inland waters,” Oleksy said. “Further refinements of a model like this might be used to generate estimates of how much carbon is being fixed by lakes annually.”

If you have questions about this story, or would like to reach out to INSTAAR for further comment, you can contact Senior Communications Specialist Gabe Allen at gabriel.allen@colorado.edu.

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As winters warm and lake ice duration shortens, algal blooms are interfering with water quality in lakes

I’m motivated because we’re seeing, in these cold regions of the world, fast rates of change. We’re losing lake ice cover in ŷ���ڱ���Ƶ at twice the rates of other temperate lakes in the U.S. There’s so much we don’t know about the ecological impacts. Getting a full, year-round picture of ecosystem processes with give us a better idea of what’s coming - Isabella Oleksy

Lakes represent some of the most vital natural resources on Earth. While they only hold a small percentage of the planet’s overall water supply, they provide most of the fresh water people depend on daily. Supported by a new five-year, $2.5 million grant from the National Science Foundation, a group of researchers at five universities will examine how rapidly warming temperatures and shorter winters can influence the growth and toxicity of lake algae. Isabella Oleksy, who studies aquatic ecosystems and recently joined INSTAAR, is leading the ŷ���ڱ���Ƶ contingent at ŷ���ڱ���Ƶ Boulder.

The researchers will use predictive modeling to forecast future outcomes on samples taken from more than 30 lakes across the country to measure the impact of climate change. They will study how year-round algal growth and increasingly warmer winter temperatures could negatively affect lake ecosystems and water quality.

“We're looking to make future projections of what to expect out of water quality, specifically levels of phytoplankton and the cyanobacteria that are harmful algal blooms, and then potential increases or decreases in toxin concentrations,” said University of Missouri researcher Rebecca North. “To do that, we’ll collect data from a whole suite of lakes all the way from alpine lakes in the Rocky Mountains in ŷ���ڱ���Ƶ and ice-covered lakes up in Vermont and New York State, down to lakes in Missouri and some in Florida. These are the endpoints in terms of what we can expect from a changing climate.”

Researchers will deploy several instruments in the lakes to collect data throughout the year, including during times when lakes are covered in ice. Wildlife cameras and remote sensing imagery will show when ice appears and disappears on the lakes.

Researchers auger a hole through the ice of the Loch, a subalpine lake in Rocky Mountain National Park, ŷ���ڱ���Ƶ.  They used the hole to collect water samples. This lake will be closely monitored for the first three years of the five-year project.

“Ice duration and ice cover set the stage for all kinds of biological and ecological processes in lakes,” said Oleksy. “We’re seeing winter conditions—specifically ice cover duration—change very rapidly all over the world, including ŷ���ڱ���Ƶ. But because we mostly take measurements from lakes in the summer, we have a pretty poor understanding of what this loss of winter means for ecosystem processes in lakes.”

Most of the data the researchers will collect is about lake productivity: broadly defined as a lake’s ability to support plant and animal life, or as how much carbon the lake is cycling. More is productivity is not necessarily better—just different. Different levels of productivity determine the types of insects, fish, and other creatures that can live in the lake. Algae is the foundation layer of productivity.

“Algae are the base of the lake food web,” said Oleksy. “Most organisms rely on algae. But if the species and abundance of algae changes, that changes what insects can thrive there. And that changes what fish can live there. The changes cascade up.”

“We want a more holistic understanding of what’s happening,” added Oleksy.

Oleksy’s field sites include lakes in Rocky Mountain National Park and in the Green Lakes Valley, near the city of Boulder. Her ŷ���ڱ���Ƶ-based research team documented dramatic changes to productivity of Rocky Mountain lakes. They have found increases in the abundance of filamentous green algae—basically big mats of algae—on the margins of lakes, which is unusual for these lakes.

Mountain lakes are usually thought of as having low productivity, with few nutrients. “But we know now that Rocky Mountain lakes have become much more productive since the 1850s, with an accelerating trend in the last few decades,” said Oleksy. Researchers aren’t sure exactly what is driving the change. Increased temperatures? Nitrogen deposition? Less ice cover? All of the above? Oleksy wants to find what she describes as the “dominant lever.”

“I’m motivated because we’re seeing, in these cold regions of the world, fast rates of change. We’re losing lake ice cover in ŷ���ڱ���Ƶ at twice the rates of other temperate lakes in the U.S. There’s so much we don’t know about the ecological impacts. Getting a full, year-round picture of ecosystem processes with give us a better idea of what’s coming.”

Joining Oleksy and North on the project are Ana Morales at the University of Vermont, Meredith Holgerson at Cornell University, and Dave Richardson at State University of New York at New Paltz. .

Isabella Oleksy takes measurements from a raft on The Loch, a subalpine lake in Rocky Mountain National Park, ŷ���ڱ���Ƶ. This lake will be closely monitored for the first three years of the five-year project.

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