Study: Water clarity in lakes affecting loons’ success

A loon floats just above the water.
A common loon floats on Lake Minnetonka.
Evan Frost | MPR News 2019

A new research paper says reduced water clarity of lakes in mid summer, linked to increased rainfall caused by climate change, is affecting the survival of common loons in northern Wisconsin.

Walter Piper, a biology professor at California’s Chapman University, has studied Wisconsin loons for three decades. Three years ago, he expanded his research to north-central Minnesota.

Piper has noticed in recent years that loon chicks in Wisconsin aren’t growing as fast or as large, and are less likely to survive to adulthood. He honed in on a possible contributing factor: increased rainfall associated with climate change that washes organic matter and sediment into lakes. 

Piper’s team used satellite imagery to calculate water clarity for 127 lakes in northern Wisconsin from 1995 to 2021. They found an overall decline in lake clarity over the past 25 years, although it fluctuates from year to year.

Because loons are diving birds with keen underwater vision, water clarity is important when they’re hunting for food, Piper said.

“Adults have to see clearly,” he said. “If they’re not seeing clearly they are not going to be able to find as many fish as they did previously for their young.”

People in life jackets work on a boat on a lake.
Walter Piper, a biology professor at Chapman University in southern California, uses binoculars to scan the water of Middle Whitefish Lake for loons.
Kirsti Marohn | MPR News

The researchers found a positive correlation between water clarity in the month of July and the body mass of loon chicks. July is a critical month of growth for loon chicks, Piper said, and smaller chicks are less likely to survive to adulthood.

The study was published in March in the journal Ecology.

With climate change expected to cause more weather extremes and rainfall events, Piper’s findings don’t bode well for loons.

“As time goes by, with climate change and warming temperatures, we can expect that loon chicks will continue to suffer,” he said.

However, the findings do indicate that reducing sediment and nutrient runoff into lakes can have a positive impact on loons’ success, Piper said.

“This should be a wake-up call that water clarity is critical to loons, and that humans are having a negative impact on water clarity, and that’s hurting loons,” he said.

The findings focused on the short-term water clarity of lakes in July. Surprisingly, long-term clarity had an inverse relationship to chick size. Piper speculated that loon families living on cloudy lakes might forage on prey they can capture with reduced visibility, so they’re less affected by short-term changes in water quality.

Piper said he plans to continue collecting data from loons on the Whitefish Chain of Lakes north of Brainerd this summer. He said it’s too early to tell if Minnesota loons are experiencing a similar pattern as Wisconsin’s.

The link between loons and climate change isn’t a new concern. In a 2019 report, the National Audubon Society said 389 species of North American birds are highly or moderately vulnerable to climate change due to changes in temperature, precipitation and habitat. 

A loon carries a 2-week-old chick on its back
A loon carries a 2-week-old chick on its back on Upper Gull Lake.
Courtesy of Sheila Johnston

Common loons are in the “moderate” category, said Brooke Bateman, senior director of climate and community science at the Audubon Society.

“We found, particularly in summer, they’re going to lose a large part of the southern edge of their range — pretty much losing all of their breeding range in the lower 48 states,” Bateman said.

In Minnesota, the Department of Natural Resources relies on citizen volunteers to count and report the number of adult and juvenile loons.

The DNR’s 2023 report says monitoring data from the past 30 years suggest that Minnesota’s statewide adult loon population is stable. However, juveniles appear to be experiencing a small decline of less than 1 percent per year.

Four regions — including Aitkin/Crow Wing, Becker, Kandiyohi and Otter Tail counties — are likely experiencing an increase in adult loons, the report states. Two other areas — Cook/Lake and Itasca counties — seem to be experiencing a declining trend.

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