Lakes are in no way eternal. Their status can fluctuate over thousands of years. Devil’s Lake in the Wisconsin Dells region of Wisconsin began its life as an oxbow lake formed by the flow of the Baraboo River. Glaciations caused a natural moraine, or earthen dam, to block off the body of water, turning it endorheic. Such changes will also modify the lake’s classification in the trophic system.
The trophic system defines the level of nutrients and oxygen. The levels are important, as they are the building blocks for microscopic life forms such as algae and zooplankton. The life forms support smaller fish. Smaller fish, in turn, support populations of larger fish. Larger fish support larger varieties of birds.
On one end of the classification systems sit the eutrophic lakes. Eutrophic lakes contain the nutrients necessary to support algae, phytoplankton and zooplankton. These creatures in turn support a comparable level of small fish. Eutrophic lakes typically have limited visibility. In addition, the additional organic matter settles and slowly decomposes on the bottom of the lake. In the right conditions, this sediment can continue the nutrient cycle or create a deadly gas buildup. Lakes that are more on the eutrophic end are desired by fishermen and outdoorsman.
At the opposite end of the trophic classification is oligotrophic lake. In contrast to the eutrophic, these lakes contain little or no nutrients. This lack of nutrition breeds little organic material, making the water exceptionally clear. Often the watersheds that feed these lakes, if any, are equally rocky and nutrient free. These lakes make for poor fishing, as even the fish that are available are too small or undernourished.
It’s important to note that the trophic classification system is a graduated system of measurement. Limnologists find that lakes are often in transitive states of eutrophism or oligotrophism. It is important to remember that these two classifications are different ends of the spectrum, with eutrophic being the “(e)utopia” of lake ecosystems, as it were.
It is important to understand the role of thermoclines. In deep bodies of water, the varying temperatures will actually cause the water to separate. Cold water sinks to the bottom while warm water sits on the surface. These separations are referred to as thermoclines.
The cold water at the bottom is often devoid of any oxygen, a necessary component to supporting life in a lake ecosystem. Furthermore, the organic matter at the bottom of a deep lake requires oxygen in order to decompose. In a temperate climate, cold temperatures cause the top layer of the thermocline to drop. This temperature drop will cause the lake waters to mix, distributing oxygen and nutrients throughout all layers until they separate again in the summer and fall months.
At extreme depths, even winter cannot cause oxygen-starved water to rise to the surface. An excellent example of this is the recovery of wooden logs at the bottom of Lake Superior by the Superior Lumber Company. In the 1890, heavy logging activities occurred in the then dense forests around Ashland, Wisconsin. During this time, many logs were lost, sunken into Lake Superior. At a depth of 60 feet, the logs found water that was completely without oxygen. Because of this, they survived decomposition for over 100 years.
Variations in the surrounding ecosystems can cause lakes to fluctuate within the trophic classification. A prime example is hypertrophic lakes which have been enriched with nutrients. The nutrient enrichments are often caused by human intervention, such as agricultural runoff, and can cause algal blooms that decrease the dissolved oxygen. This creates a poor environment for humans and fish species.