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 Aquatic Plants: Why those "weeds" are important to your lake

An introduction
(adapted from Through the Looking Glass and the WI DNR )

The plants growing in your lake fit into one of the following categories:

  • Emergent- in the shallowest zone to about knee-deep. Here you'll find cattails, bur-reed, blue-flag iris, bulrush, spikebrush, and sedges. These plants can tolerate fluctuating water levels and dense stands can dampen wave action.
  • Floating-leaf- going from knee-deep water out up to your waist and deeper. The plants you'll find are white and yellow water lilies, watershield, floating-leaved pondweeds, duckweeds, and some submergent species like coontail, bladderwort and water milfoil.
  • Open water submergents- now you'll need to start swimming. These underwater plants grow from the shallowest zones out to several meters deep. There's a great diversity of plants out here; water milfoil, coontail, bladderwort, water marigold, large-leafed pondweeds

Data Update 3/15/01: Hennepin Parks has conducted aquatic plant composition surveys. Click here for a map showing the lakes that were surveyed.

Aquatic plants perform some of the same roles in your lake as trees do in a forest. While most people are aware of the consequences of poor logging practices in a forest, most are unaware of what changes in the aquatic plant community will do to their lake. Like trees in a forest, aquatic plants provide structure and food for other organisms. They also stabilize soft lake bottoms and minimize shoreline erosion by dampening the effect of waves like trees hold the soil and block the wind in a forest. These plants also absorb and use nutrients like phosphorus and nitrogen, making less available for algae. A healthy plant community in your lake also makes the lake less susceptible to the spread of exotic plants like Eurasian water milfoil (see Exotics section).

Let's go through these benefits in a bit more detail:

Structure and food-
Plants provide a refuge for small animals against predation by bigger ones. Zooplankton and invertebrates use the plants to escape predation, as do small fish. Lake vegetation can have a strong influence on the types of fish and invertebrate communities present, different communities and relationships exist between vegetated and unvegetated parts of the lake.

Stands of aquatic plants tend to have a much richer invertebrate (aquatic insects and zooplanton) than unvegetated stands, both in terms of number of species (diversity) and total numbers of bugs (biomass). This is not only because fish have a harder time catching bugs in plant beds but also because the plants provide food.

Aquatic plants are usually coated with periphyton (attached algae), which is an important food source for invertebrates. Decaying plant parts are also consumed. In addition, the seeds/fruits of some plants and the invertebrates that live within plant beds are important sources of food for waterfowl and shorebirds. The plants often serve as nesting areas.

A number of mammals also need aquatic plants. Muskrats and beavers feed on plant tubers and often use plants for their construction projects.

Stabilizing sediments-
Many aquatic plants are rooted and their roots help keep soft sediments from being resuspended. Light limitation is one of the main problems for submerged vegetation in many lakes. The resuspension of sediments by wind, waves and boat traffic can cause turbidity in the water, which limits the amount of sunlight that can reach the plant leaves. Once the plants grow, the water clears up and they grow even better. Turbid (cloudy) unvegetated shallow lakes tend to remain that way because wave action and bottom feeding fish tend to keep resuspending sediments. Vegetated waters however tend to stay vegetated because the resulting water clarity promotes more plant growth, but also because the sediment is stable, the fish community is shifted more towards piscivores instead of bottom feeder like suckers and carp (Sheffer 1998). Predatory fish like northerns are highly dependant on sight to catch their prey and obviously do much better in clearer waters.

Shoreline protection-
Beds of aquatic vegetation often help dampen wave action and currents. Water flow is slowed and wave energy absorbed by the stems and leaves of the plants. Emergent vegetation near shorelines also keeps the lake bottom more stable there.

Aquatic plants and algae-
Observations in many shallow lakes have shown that when vegetation reaches its peak in the summer, algae or phytoplankton abundance declines. So it becomes a trade-off. As long as nutrients and sunlight are available either algae or aquatic plants or both are going to use them. If you remove aquatic plants from a lake, algal growth will increase. This is an important ingredient in determining how to approach plant management.

Prevent alien invasions-
One reason exotic species are able to take over an ecosystem is because that system has been perturbed. There is room for that new organism. Usually, but not always, a healthy, unperturbed ecosystem can resist the invasion of an exotic plant or animal. This provides another reason to try and maintain a healthy aquatic plant community in your lake.

How do I Protect the plants in my lake?
From Wisconsin DNR:

Aquatic plant protection begins with us. We need to work to maintain good water clarity and healthy native aquatic plant communities. How can we do it? The first step is to limit the amount of nutrients and sediment that enter the lake. There are other important ways to safeguard a lake's native aquatic plant community. They may include developing motorboat ordinances that prevent the destruction of native plant beds, limiting aquatic plant removal activities, designating certain plant beds as sensitive areas and preventing the spread of exotic plants, such as Eurasian watermilfoil. Most aquatic plant communities have a dynamic balance. If plant management is needed, it is usually in lakes significantly altered by humans. If we discover how to use aquatic plant management techniques that blend with natural processes rather than resist them, the forecast for healthy lake ecosystems looks bright.

Chadde, Steve W. 1998. A Great Lakes Wetland Flora. Pocketflora Press, Calumet, MI.
Sheffer, Martin. 1998. Ecology of Shallow Lakes. Chapman and Hall, London, UK.

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