Plumbing the Depths

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“In lakes, water quality relates directly to water clarity,” says Patrick Brezonik. He is the Director of the Water Resources Center at the University of Minnesota and has been testing the waters in the Upper Great Lakes region for the past 20 years. He says that in scientific terms a lake with high water quality is known as an oligotrophic lake. Such a lake has clean, clear water with little algae. It’s what most of us think of when we imagine a pristine lake in an untouched pine forest. At the other end of the spectrum are hypereutrophic lakes. These polluted lakes are typically murky, smelly, and overgrown with algae and exotic plants.


Photographs Comparing Oligotrophic and Hypereutrophic Lakes


Brezonik explains that lakes in the Upper Midwest typically deteriorate due to runoff from farms, urban areas, industry, and construction sites. The storm water runoff will often carry chemicals and sediments from these locations into nearby lakes. Sediments can cloud a lake, and nutrients in the sediments, particularly phosphorous, can cause unwanted algae to grow. While the degradation is gradual, once a lake has become hypereutrophic, it is very difficult to reverse the damage.

A large drop in water quality can impact both the wildlife and the people in and around the lake. Rough fish such as catfish and carp multiply and replace higher quality fish such as small mouth bass and pike. Algae, bacteria, and chemicals can affect the health of people and animals who fish and swim in the lakes. In addition, many small towns throughout the Upper Great Lakes region depend on tourist dollars to keep their economies afloat, and an algae-filled lake turns away summer vacationers.

“It’s important for both the surrounding communities and the environment that lakes are monitored on a regular basis,” says Brezonik. For the past century, the Secchi disk has been the standard instrument for monitoring water quality. Pioneered by Italian physicist Pietro Angelo Secchi in 1865, the Secchi disk is an 8-inch disk painted in an alternating black and white pattern. A person in a boat lowers the disk into the lake and records the depth at which it disappears from sight. This depth, known as the Secchi depth, is the measure of water clarity. In Minnesota, oligotrophic lakes have a Secchi depth of 16 feet or greater, and hypereutrophic lakes have a Secchi depth of only a few feet or less. If a lake is hypereutrophic, it’s then up to the researcher or resource manager to determine the cause.

  Hallet Lake (left) and Schwanz Lake (right), both in Minnesota, are representative of very good and very bad water quality. Hallet lake is oligotrophic, with very clear water. On the other hand Schwanz Lake is hypereutrophic, with murky water that is choked with algae. (Photographs courtesy Upper Great Lakes Regional Earth Science Applications Center)
  Photographs of Secchi Disk Measurements

Although Secchi depths provide an excellent classification system for limnologists, taking such measurements is labor intensive. The person making the measurements has to take a boat to the open part of the lake, slowly drop in the Secchi disk, record its readings, and then repeat the process at several other locations. While Secchi disk measurements work well for small numbers of lakes, they are not practical for monitoring the over 30,000 lakes in Michigan, Minnesota, and Wisconsin.

Tom Lillesand, the director of the Environmental Remote Sensing Center at the University of Wisconsin, explains that the states of the Upper Great Lakes region have modest budgets with which to monitor lakes. “In Wisconsin we have a budget to sample lakes, but it allows agencies such as the Wisconsin Department of Natural Resources to perform a complete ‘water quality physical exam’ on 50 to 100 lakes, which is less than one percent of the state’s lakes,” he says. With such minimal coverage scientists cannot obtain a comprehensive picture of water quality across the state, and they cannot follow trends in water quality across the state over time.

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  Water quality measurements are typically done by hand with a Secchi disk. Although the measurements are accurate, it would be very expensive to conduct them by hand on a large scale. The photographs show decreasing water quality from left to right. (Photographs courtesy Minnesota Pollution Control Agency)
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