Minnesota's lakes are legacies of the glaciers that scoured the landscape over 10,000 years ago.
As the glaciers receded, they left behind a wide variety of depressions in the land: some shallow, some deep, some carved out of bedrock, others surrounded by sand. Many of these depressions were filled by rain and snow, and water that ran off the land.
This glacial legacy can be seen today in the lakes around the state. From the forest lakes of northeastern Minnesota, to the prairie ponds of the southwest, lakes are as varied as the three biomes in which they occur.
Coniferous Forest Lakes:
Cool, deep, clear waters, and rocky (photo left).
- Unproductive, although clear water seems a perfect environment in which fish would thrive.
- Nutrients necessary to support plant growth are in short supply.
Shallower, surrounded by rich prairie soils (photo right).
- The lakes support more fish and plant life, including occasional algal blooms.
- Agricultural practices often increase the addition of soils and nutrients such as phosphorus and nitrogen.
Deciduous Forest Lakes:
Lakes in central Minnesota occur in a transition between the rocky north and the rich-prairie south:
- Wider variety of types. Many occur on sand, gravel, and other glacial deposits.
- Relatively rich in aquatic and fish life.
- Tend to be deeper than lakes found in the prairie region.
Most Midwestern lakes are said to be "phosphorus limited".
Phosphorus - a nutrient - directly determines algae growth because it is the nutrient in the lowest supply. Lakes can be very sensitive to additions of phosphorus.
Human activities add phosphorus to lakes in many ways. Storm sewers, ditches, tile lines, and overland runoff carry extra phosphorus off the land and into a lake. When the concentration of nutrients in a lake reaches too high a level, growth of algae increases and the lake's water clarity tends to decrease. This condition can result in a loss of submersed aquatic plants.
Human activities also accelerate erosion — increasing the amount of soil that washes from the land into a lake. Soil limits transparency (the clarity of the water) while it is suspended. Once it settles and covers the lake bottom, soil degrades habitat for fish spawning and for bottom-dwelling creatures.
Excess phosphorus and sediment in a lake result in visible impacts to lake water quality. There are many other pollutants that affect water quality and lake ecology in a less obvious way. These include pollutants such as pesticides, bacteria, and salt that may enter lakes as a result of human activities like farming, lawn care, failing septic systems, and road maintenance.
In some lakes, few aquatic plants grow. Wave action or steep drop-offs may make the shoreline poor habitat. Wave action washes away the finer particles that provide nutrients to the plants, leaving gravel or rubble lake bottoms with few or no nutrients. In other lakes, murky water may shade out aquatic plants except in the shallowest nearshore areas.
However, aquatic plants are a natural part of lake communities. They vary with lake size, bottom slope, water clarity, and fluctuations in water level. Areas of the lake with a gradually sloping bottom, clear water, and protection from wind and waves often have the most abundant plant growth. Identifying the characteristics of a lake or stream that influence aquatic plant growth is an important part of shoreland buffer design.
Lake and vegetation history - check with government agencies, university, and private organizations for more information (See Plant Guide and References and Resources). This will be helpful when you develop your own Plant List.
Other sources of information: