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All lakes are not created equal!

Many factors influence the aquatic plants in a lake. Some of the most important include the lake’s size and shape, how protected a shoreline is from wind and waves, what activities occur in the lands that drain into it (the watershed), and the region in which the lake is located. Minnesota lies at a crossroads of regions with differing geologic histories, climates, and plant and animal communities. These differences have been used to divide Minnesota into six ecoregions, or broad areas of land, each having a unique combination of land use, climate, soil, land form (topography), and potential natural vegetation. The lakes within an ecoregion, because they occur in an area of similar land type, often have similar physical characteristics, water chemistry, and biological communities. If you’ve visited lakes across Minnesota, you probably recognize that northeastern lakes are often fairly small, clear, cold and relatively deep compared to the large, shallow lakes common in southern Minnesota. These differences can be understood by observing the different ecoregions in which the lakes occur (Figure 1).

Although there is a great deal of natural variation, in general, the deep, clear, cold lakes of the northeast have limited aquatic plant and algae growth (low productivity). In contrast, the shallow, warmer lakes of southern Minnesota tend to produce lush aquatic plant and algae growth (high productivity). Lakes in central Minnesota tend to fall between these two extremes.

Aquatic plant communities

The aquatic plants within a lake are often grouped by managers into three plant communities, or assemblages of species, according to their growth form and depth range within a lake. In practice, there are no hard divisions between plant communities. Some plants are adapted to growing in a wide variety of conditions and may be found associated with more than one plant community, while other plants are very specific to a narrow range of environmental conditions and occur in only one plant community.

The three plant communities—emergent, floating-leaf and submergent—are named according to the predominant plant form found in the community. They typically form concentric rings within our lakes (Figure 2). However, under certain conditions one or more plant communities may be absent from a portion of a lake.

The emergent plant community extends from the water’s edge to a depth of 2–4 feet. This area typically receives high nutrient inputs from runoff from the land. It supports predominantly emergent plants that have leaves and flowers extending above the water surface and a dense network of roots. These roots anchor plants (and soil) in the area right along the shoreline that typically is most exposed to the erosive power of waves and ice action.

The floating-leaf plant community extends from a water depth of about 2 feet out to 6–8 feet. It supports predominantly floating-leaf plant forms, but frequently includes emergent and submergent plant forms. This community is found on smaller lakes, in protected bays of larger lakes, or in backwaters of rivers with little wave action or current.

The submergent plant community extends from a water depth of about 6 feet out to 20 feet or more, depending upon water clarity. Leaves of submergent plants are often finely dissected and offer little resistance to water movement. Some are able to photosynthesize at very low light levels—as low as 5% of the light reaching plants above the water surface. Plants of this community also use a wide variety of reproductive and overwintering strategies to insure their survival—seeds, tubers, rhizomes, turions (compact stem structures having dormant buds), and evergreen vegetation.

What can cause a change in aquatic plant communities?

Some species of aquatic plants tend to occur within a relatively narrow range of conditions that may include specific water nutrient levels, clarity, pH, and/or depth; while other plants may tolerate a wide range of these conditions. Annual variations in temperature or precipitation may cause one or more of the plant communities to temporarily expand or reduce its range. On some occasions, climate variations can result in long-lasting changes to a plant community. For example, during the drought in the 1930s, bulrush seeds that had been dormant on the lake bottoms for years were suddenly exposed to air as the lake levels dropped. Once exposed, these seeds germinated, forming many of the extensive bulrush beds we have in our lakes today. However, under the same low-water conditions, other species may experience high mortality due to exposure.

Trends in abundance and diversity of aquatic plant species over several years that can not be explained by climate change are often the result of human actions within a lake and watershed. Mechanical removal and herbicide treatments of aquatic plants (both requiring a permit from the Minnesota DNR); addition of sediment, nutrients and/or other pollutants from upland runoff; addition of nutrients from failing septic systems; as well as introduction of non-native plant or animal species (prohibited in Minnesota) can alter the abundance and reduce the diversity of aquatic plants and the creatures that depend on them for survival—and ultimately compromise the health of the lake. Severe disruption of the native plant community can in some cases result in a lake dominated by dense, unpleasant, and occasionally toxic algae blooms.

Maintaining or restoring aquatic plant communities is one of the best ways to keep your lake healthy. Once compromised, plant communities are costly and time-consuming to restore. Restoration may take decades, and plant communities will likely never reach the original levels of diversity and abundance. This emphasizes the importance of protecting existing plant communities and early detection of any trends or changes in the plant community.

Who is responsible for monitoring and managing aquatic plants in our lakes?

Minnesota law states that aquatic plants growing in public waters are the property of the state. As such, the Minnesota Department of Natural Resources (MN DNR) is the main agency responsible for aquatic plant monitoring and for overseeing aquatic plant management. Most lakes are mapped by the MN DNR every five years. Aquatic plant mapping is done to produce a historical record of changes in plant species and location and size of plant communities.

Results from one day of sampling, or even one season, provide a snapshot of a lake at a given point in time. Just one year of observed excessive plant growth may not indicate a problem, it may simply reflect the natural response to growing conditions in a particular season. Plant communities in lakes are dynamic, not static, and so must be looked at over time. Several years of mapping are needed to determine trends or changes in aquatic vegetation.

In between MN DNR mappings, trained volunteers can play a vital role in monitoring the aquatic plant communities in a lake. Volunteers can learn to differentiate seasonal growth changes from long-term trends. They can detect early introductions of non-native plant species (critical for their elimination or control), along with follow-up monitoring of success of non-native species control efforts. Yet another role for volunteers is timely reporting of plant removal—before it becomes excessive (in violation of state law). Identifying and mapping existing vegetation is also a critical initial step to develop an aquatic vegetation management plan for a lake.

In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.