Cindy Hagley
In the middle of a typical Minnesota winter, global warming sometimes doesn’t sound so bad, but even small changes in temperature or precipitation could cause significant changes in the lakes we love. In a previous From Shore to Shore article, we explained that lakes in our part of the world have predictable temperature cycles that impact everything else about the lakes, including the plants, fish, and other aquatic creatures that live there. These days we hear a lot about global warming and its potential impacts on our climate, but we hear very little about how our lakes might be affected. Although scientists are still working to find agreement on predictions about the rate and effects of global warming on our Midwestern environment, there are some observations of changes already occurring that can help us understand potential impacts.
David Schindler, a world-famous limnologist (person who studies lakes and streams), has shown that, over the last 20 years in northwest Ontario, lake temperatures have increased by 2°C (nearly 4°F) and on average the lakes remain open (ice free) three weeks longer each year. More recently, scientists have reviewed ice cover historical records from across the globe and have confirmed this general conclusion (estimating a 2- week longer ice-free period from 1845 to 1995) for lakes in all northern latitudes.
At the same time that temperatures are expected to continue to increase in the future, precipitation in the Midwest is also expected to increase, with a larger proportion of the precipitation falling in heavy, extreme rain events. Despite higher precipitation, the higher temperatures and longer ice-free season will result in higher rates of evaporation, probably leading to lower lake and stream levels.
Let’s think about other potential changes to lakes. Warm water holds less oxygen than cold water. More frequent and extreme rainfalls is likely to mean more erosion of sediments and nutrients into lakes. These two factors combined could lead to increased algal growth. Lower oxygen levels and warmer temperatures could also make some of our lakes unsuitable for cold water fishes such as the trout that we love to catch. You’ve probably heard the expression, “one man’s poison is another man’s pleasure.” That phrase applies well to the potential impacts of global warming on biological populations. A change that is harmful to one species is likely to encourage another species to expand its range to fill the void left by the unlucky species. Scientists expect to see a northward shift in the distribution of coldwater fishes and an increase in warmwater fishes in our region.
Lake Trout
It is important to remember that there is much we don’t know about how lakes will be impacted by global warming. Lakes are complex environments with intricate relationships among the many species of plants and animals making lakes their home. An example of the complex reactions that can occur is a recent study of a small European bird, the great tit (Parus major). This bird is related to our chickadee and relies on a type of short-lived caterpillar to feed its young. As a result of many years of warmer springs, scientists found that the caterpillars were hatching nine days earlier in 1995 than they did in 1973, while the birds were hatching their young at approximately the same time as always. Because the caterpillars live only a few days, the great tits have lost an important food source for their young. In lakes, changes in the timing of temperature stratification, hatching of aquatic organisms such as zooplankton, emergence of aquatic insects, and timing of algal blooms could have impacts on other organisms, including fish and waterfowl. This all might sound somewhat discouraging, but take heart. There are many steps we can take to make a difference. Make sure your shoreline is well vegetated to reduce the sediments and nutrients that enter your lake from heavy rainfalls. Take good care of your septic system to ensure that it is not contributing nutrients or contaminants to the lake. Help reduce the greenhouse gases that lead to global warming by reducing vehicle use and using a more fuel-efficient vehicle. Carpool. Lower your home’s thermostat a few degrees. Plant trees that soak up excess carbon dioxide. Purchase green power.
Minnesota law requires the state’s electric utilities to offer customers voluntary options to purchase power generated from renewable sources (contact your electrical power company). Finally, take the time to get involved by learning more about climate change and teaching others about it.
References
National Assessment Synthesis Team, US Global Change Research Program. 2000. Climate change impacts on the United States – The potential consequences of climate variability and change. http://www.usgcrp.gov/usgcrp/Library/nationalassessment/overview.htm Perkins, S. 2003. Spring forward – Warmer climate accelerates life cycle of plants, animals.
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