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A mystery in Lake of the Woods

Man wearing winter clothes and sunglasses stands in front of large lake with a wide streak of bright green algae that runs to the horizon.

Lake of the Woods is a big lake with a big problem caused by one of nature’s smallest organisms: algae. Under the right conditions, algae in the lake grow rapidly into blooms so large their full extent can only be seen from space. Just why these massive blooms occur, and how they might be reduced, has to some extent remained a mystery.

But thanks to new technology and cooperative studies involving state, local, international, and tribal partnerships, scientists are learning that some answers to these questions may be blowing in the wind.

It’s long been known wind has an outsized effect on recreation in this large, shallow lake. Any breeze above 10 mph makes it difficult to fish due to large waves, which is why the greatest fishing pressure on the lake is from ice fishing during winter months. But it wasn’t until recently it was learned that wind, or rather the lack of it, can also play a significant role in algae production.

The algae blooms are being fed by the nutrient phosphorus. Scientists from the MPCA and the Science Museum of Minnesota are working together to better understand a major source of phosphorus in the lake — internal loading, which is phosphorus collected in bottom sediments from decades of pollution entering the lake from tributaries that becomes re-suspended in the lake under certain conditions.

A few years ago, crews installed a system of buoys that support strings of sensors that take temperature and dissolved oxygen readings throughout the water column (from the lake bottom to the surface) over time. With the new data, the scientists were able to confirm that during extended periods (7-10 days) of little-to-no wind in the summer, the lake becomes “stratified” in terms of temperature, with lower temps near the bottom and higher temps near the surface. This stratification results in lower dissolved oxygen levels near the bottom, which draws phosphorus from the sediment and re-suspends it in the water column.

“We discovered the rate of release of phosphorus from sediments is 45 times greater during these periods of stillness than at other times,” says Adam Heathcote, senior scientist for the Science Museum. “It is a major cause of phosphorus becoming suspended in the water column that leads to the massive blooms we see later in the season.”

What research is also showing, says MPCA watershed project manager Cary Hernandez, is re-suspended phosphorus is slowly being flushed from the lake. “Our recent TMDL report for Lake of the Woods says phosphorus loading to the lake needs to decrease by 17.3% to meet water quality standards and reduce algal blooms that affect recreation. The report shows that cleaner water entering the lake from sources like the Rainy River is flushing the lake of phosphorus at a rate of about 1% per year.” Over time, he says, the lake should eventually meet water quality standards as long as no new sources of phosphorus develop and there are no increases from current sources. A TMDL (total maximum daily load study) establishes the amount of a pollutant a water body can accept and still meet water quality standards, and the amount of reductions needed from various sources for the water body to meet standards.

Other similar research projects are showing that Lake of the Woods is not the only body of water that experiences phosphorus re-suspension due to stratification. For example, the Science Museum is also working with tribal members and other local partners to study how this process is affecting phosphorus levels in Upper and Lower Red Lakes. “We know this is affecting other lakes as well,” says Mark Edlund, another senior scientist for the Science Museum. “And the impacts will only increase due to climate change bringing more periods of warm, still weather during the summer months.”

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