Re-published from Field Notes, the newsletter of the St. Croix Watershed Research Station.
Lake of the Woods is the vast body of water that makes up most of the spur on Minnesota’s northern border. Sprawled across the U.S.-Canada border, it is 70 miles north-south and 60 miles east-west, contains more than 14,552 islands, and boasts 65,000 miles of shoreline. It is the size of Rhode Island and a good candidate as the “sixth Great Lake.”
Like Lake Erie and other Great Lakes, Lake of the Woods is also plagued by harmful algal blooms. Even after nutrient pollution was reduced, Lake of the Woods seems to keep getting greener.
Enormous amounts of waste discharged from paper mills on the Rainy River poured into the lake for decades, carrying phosphorus, which fed the algae. City wastewater also went straight into the river. After the Clean Water Act was passed in 1972, the waste discharges were significantly lowered and the overall cleanliness of the water entering the lake was improved.
“There has been enough monitoring in Lake of the Woods that we can detect efforts of things like the Clean Water Act,” says senior scientist Mark Edlund. “We’ve seen significant decreases in nutrient loading since the 1970s.”
Yet algae still covers more of the lake for longer periods every summer. In satellite imagery, the lake disappears into the surrounding boreal forest during the summer, turning the same shade of green as the trees.
Lake of the Woods’ size means it is of great importance to many people. It offers world-class fishing, boating, camping and other recreation in a remote landscape. Because it drains 27,000 square miles of the U.S.A. and Canada – larger than the island of Haiti – spread across two nations, management and stewardship is no simple matter, and demands scientific foundation.
Growing interest from both Canadians and American authorities into understanding why the lake is still plagued by algae, and what if anything can be done about it, has brought St. Croix Watershed Research Station scientists to the huge body of water that comprises the lower 48’s northernmost point.
The team travels by boat and snowmobile across the lake, collecting data and answering key questions about how sediment, erosion, and climate change could affect its algae problem.
Paying for past sins
For many decades, sewage was pumped directly into the Rainy River from the border cities of International Falls and Fort Frances. Starting in about the 1930s, and through the 1960s, paper plants also dumped mountains of sawdust and other waste into the river. Much of that waste was carried down the Rainy to Lake of the Woods, where it settled on the bottom.
If it had stayed there, it might not have been an issue. But, the research has discovered that the nutrients in the waste have been cycling between the sediments and the algae growing in the water ever since. That recycling process, called “internal loading,” means that no matter how much new pollution is prevented from entering the lake, it may take a long time for all the available phosphorus to be consumed by algae and removed from the sediments.
“There was extremely high nutrient loading from the 1950s to the 1970s, and much of that phosphorus ended up in the sediment. But Lake of the Woods doesn’t accumulate sediment very fast, and therefore the phosphorus wasn’t buried. It’s still mobile,” says Edlund.
Lake of the Woods and the Rainy River are ideal subjects to study to better understand the challenge of internal loading. The river is by far the lake’s largest tributary, and it has been monitored for decades, so relatively precise data is available about what has been going into the lake. Those inputs don’t line up with what the scientists are seeing in the sediment cores.
Phosphorus levels going into Lake of the Woods from the Rainy River. Nutrients being discharged into Lake of the Woods have dropped dramatically, but water quality has not followed.
Unlike other water bodies they’ve studied, there isn’t much phosphorus in the sediment. Instead, the phosphorus is getting stirred up on the bottom and entering the water, where it is consumed by algae. When the algae dies, it falls to the bottom again, where the phosphorus is again stirred up into the water column.
The upside is that the phosphorus is slowly being flushed out of the lake as algae, and the available supply is dwindling. Water stays in the lake for about a year, which means a slow but steady process.
“That pool of phosphorus is declining. The lake is on a trajectory to meet some new steady state,” Edlund says.
At the same time this history is playing out, shifting weather patterns are also at work.
Changing climate, changing lake
The warming planet is having a complex effect on Lake of the Woods. Not only are average water temperatures higher, but the lake is less windy, both of which may be playing a part in its algae problem.
An analysis of ice-out records shows that the northern part of the lake is having an average 28 more days of open water each year. “If there are 28 more days, you have another month to grow algae every year,” Edlund says. “Ask any gardener or farmer if they’d like four more weeks of growing season.”
Inland regions like central North America are expected to be less windy as a result of climate change. Less wind means a calmer lake, which means the top and bottom waters don’t mix together as much. When this happens, colder, low oxygen water settles on the bottom – ideal for absorbing phosphorus from the sediment.
Confoundingly, the southern part of the lake is staying ice-covered longer, but that’s also where most of the big algae blooms are seen. The team has not solved that part of the puzzle yet, but they’re still working on that and other questions.
A growing lake
Lake of the Woods is a naturally-formed body of water, but was slightly expanded in the late 1800s by the construction of a dam on its outlet at Kenora, Ontario. As the water went up about three feet, it also expanded horizontally, and shoreline is still slowly being eaten by the lake.
That soil being washed into the lake could be another factor in its algae blooms, as it too carries nutrients from the land into the water.
“One hundred years later, it’s still in play, it’s contributing to the nutrient budget of the lake,” Edlund says.
Although thirty-six inches more water might not seem like a lot, it has likely had an effect on sediment and phosphorus. The research station’s studies identified areas of the lake that had never accumulated sediment before the dam and that now are.
It’s not thought to be a major contributor of nutrients though, compared to other sources. Nonetheless, the research must account for all the phosphorus in the lake, and the erosion can’t be ignored.
Innovative aquatic science
Lake of the Woods is a puzzle, and the scientists are using all the tools in their toolboxes to understand it – and also inventing some new tools while they’re at it.
The first step of the study was a trip around the lake on a boat equipped with sonar. This let them see where sediments were deposited and come up with places they could study that would represent the lake pretty well. Then they went back and took sediment cores. From a sample of the muck on the bottom of the lake, they can reconstruct the history of the whole watershed.
“The sediment provides a perfect history of what’s happened in the lake over time,” Edlund says.
To construct this history, several analyses were performed. They dated the cores by analyzing lead molecules, so they could then say that, for example, four inches below the top of a sediment core is 50-year-old muck.
Then they analyzed the amount of phosphorus in different eras, and looked for fossilized diatoms – algae that produce a glass shell that is preserved even after they die – to understand the lake’s chemistry at different points in time. The many species of diatoms all prefer slightly different ecology, and thus provide clear clues as to whether a body of water was more or less biologically productive, higher or lower in nutrients, and other key factors.
Lastly, they looked at the pigments of algae preserved in the cores. There, evidence of toxic blue-green algae was apparent in the 1960s and 1970s, when nutrient loading peaked.
This year, the team will deploy buoys with data loggers on board to take real-time measurements of water temperature and oxygen levels. They will also use sediment traps to capture samples of what is actually in the water. When they retrieve the buoys at the end of the season, they will also collect a boatload of important information.
These techniques are being used because the lake changes rapidly, yet Lake of the Woods is immense. One person in a boat sampling the water once a month is not going to catch many of the changes, like a brief period of stratified water or a localized algal bloom. The buoys will work around the clock, all season long.
The buoys will help show what happens in late July, when the winds typically calm down, raising the potential for temperature stratification and phosphorus movement. They might even show that there is one key location on the lake where most of the big algae blooms are being born, before being blown elsewhere.
The Lake of the Woods study will hopefully help managers and stewards better address its big, complex problems. It will also provide valuable insights for watershed management elsewhere. The “internal loading” challenge facing Lake of the Woods, and the ineffectiveness so far of reductions in nutrients could be a major factor in many lakes and rivers.
“Reducing nutrient loads to a watershed is usually our first tool for improving water quality,” Edlund says. But this fundamental tool might not be up to the challenge of internal loading.
Of course, the fact remains that, even if reduction of phosphorus going into the lake hasn’t yet led to improved water quality, the lake could not start to slowly heal itself until the pollution stopped.