Local View: Warming lakes squeezing cold-water fish

Minnesota lakes and their fisheries are changing because of land use, invasive species, and, increasingly, climate change. Climate change is accelerating. The average air temperature for 2011 through 2016 measured at Duluth International Airport ...

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Minnesota lakes and their fisheries are changing because of land use, invasive species, and, increasingly, climate change.

David Gerhart

Climate change is accelerating. The average air temperature for 2011 through 2016 measured at Duluth International Airport was 1.5 degrees higher than for the 30-year average calculated for 1981 through 2010. But it was a whopping 3.2 degrees higher than for the average calculated for 1951 through 1980.

Surprisingly, surface water temperatures of the largest lakes of North America and Europe are increasing even more rapidly than air temperatures. Over the past 25 years, the temperature gain in many lakes was more than twice that of air temperature.

In our own backyard, summer surface water temperatures in Lake Superior have increased by 5 degrees over the past 30 years, one of the largest measured increases of any ecosystem worldwide. Globally, the temperatures of lakes that freeze in winter rose by nearly 0.9 degree per decade between 1985 and 2009.


But we all know that water warms more slowly than air, so how can this be? Lakes actually integrate climate changes over time, taking into account not only air temperature but also ice cover, cloud cover, snow melt, and other climate variables. Therefore, changes in lakes are good indicators of climate trends.

Warmer air increases water temperatures, and this extra heat stored in lakes is one reason ice-free season lasts longer into autumn. But ice-out also is occurring earlier. In 2012, many Minnesota lakes set new ice-out records. Long-term records have documented this reduction in ice cover. One bay in Lake of the Woods has seen a 28-day reduction since 1970.

Most deeper lakes stratify in the summer. After ice-out in the spring, the sun's rays heat surface water which, being less dense, begins to float on top of colder, deeper water. This heating results in an upper layer of well-oxygenated warm water and a colder, deeper layer that may have significant and progressive oxygen depletion. These two layers are separated by a thermocline in which temperature and dissolved oxygen decrease rapidly with depth.

Several things happen as lake surface temperatures become warmer. Because of the increased temperature difference between the upper and lower layers, the thermal layering of lakes becomes more stable. The thermocline then becomes a more effective barrier to winds that might otherwise bring oxygen to deeper waters. These effects are compounded by longer ice-free seasons, resulting in deep waters that are isolated for longer periods. Consequently, oxygen levels may be reduced significantly.

The result is that, as summer progresses, fish like cisco, walleye, and trout become squeezed into a diminishing habitat space that supplies both cooler temperatures and adequate oxygen. Ciscos are food for sport fish and are an important part of the food web of many lakes.

In addition to oxygen depletion and heat stress, warmer lakes can cause decreased reproductive success, slower growth, increased hooking mortality, and changes in prey species for fish.

Of 2,148 lakes studied by the Wisconsin Department of Natural Resources and the U.S. Geological Survey, 86 percent are projected to support primarily largemouth bass rather than walleye by mid-century, up from 58 percent today. Other scientists warn that rising water temperatures will degrade spawning and nursery habitats for lake sturgeon.

Warmer waters also affect plants and plankton at the base of the food chain. Shifts in diatom species and more frequent algal blooms, some containing toxic bacteria, have been documented.


In addition, climate change has brought heavier rains to Minnesota. These storms flush more nutrients into lakes and fuel plant growth, exacerbating oxygen depletion at night and when plants die and decompose.

It is always hard to imagine changes in nature that we can't observe in a short time frame. But our grandfathers' lakes are not the lakes we experience today. And our children and grandchildren are expected to see even more dramatic changes in their lifetimes.


David Gerhart of Duluth has a doctorate in aquatic ecology from Cornell University and has published and reviewed manuscripts for scientific journals in the fields of ecology and biochemistry. This commentary was reviewed and edited by Richard Axler, a limnologist and aquatic ecologist at the Natural Resources Research Institute at the University of Minnesota Duluth, before it was submitted to and edited by the News Tribune. Additional information was contributed by the Minnesota Pollution Control Agency, the National Weather Service in Duluth, and KBJR-TV Channel 6 Weather.

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