Lake Tips: Aquatic Plant Management - Part I

Nuisance Vegetation
Aquatic plants and algae are an integral part of the lake's ecosystem, and that is important to keep in mind as we address nuisance problems, such as algal blooms and aquatic weeds. Inlake techniques for controlling algal blooms and aquatic weeds differ, but the conditions that support their growth and watershed measures used to prevent such conditions are similar. This Lake Tips will help identify when aquatic vegetation becomes a problem and factors that support the growth of these nuisances. (For information about nuisance vegetation control measures, see Lake Tips: Aquatic Plant Management, Part 2).

A System Balance
Aquatic plants, algae (found on rocks, etc.) and free-floating algae (called phytoplankton) are the primary producers in a lake. This aquatic vegetation provides a food source for  microscopic animals called zooplankton and small fish. The smaller fish provide food for larger fish, etc. All these living organisms are part of the lake ecosystem food web. Any action at one level ultimately will affect all other levels of the system, so management must be tailored to the lake and its watershed conditions.

Problem Signs
Aquatic vegetation often becomes a problem when it tends to dominate the system. At this point, it usually interferes with some intended lake usage. For example, it may impair recreational activities like boating, swimming or fishing. It may affect irrigation or hydropower generation or decrease lake shore, and area property values. Algal blooms can lower dissolved oxygen levels enough to kill fish, create offensive tastes and odors, and even be toxic to wildlife, domestic animals and humans.

Plant species that are not native to a particular environment, so-called exotic species, also can become a problem by out-competing native plants for available nutrients and habitat. Eurasian watermilfoil, purple loosestrife and southern naiad are among the exotic plants that have found their way into lakes. (Though not a plant, zebra mussels are yet another example of an exotic species that has become a lake nuisance in recent years.)

Who is creating the problem? We all are! If you live in a watershed (as we all do), you are part of the problem and, more importantly, part of the solution!

Have a Plan
The solution begins with awareness of the interconnected nature of humans and their activities, and a knowledge of how these activities impact the natural environment. The next step is to develop a successful aquatic plant management plan. The plan should include three major components:

  1. Understanding the source(s) of the problem;
  2. Prioritizing realistic goals and objectives to correct the problem; and
  3. Implementing the necessary actions to meet your goals and objectives.

This plan will be influenced by the watershed and lake ecology, socio-economic factors, and regulatory considerations. Identifying the sources of the pollution problem and determining feasibility of alternative measures to address these sources are components of ecological evaluation. Identifying all lake uses and user groups, forming working partnerships, and anticipating and resolving conflicts are social elements in the overall plan. Economic factors include creating a budget, and securing funding sources. To avoid costly delays, local, state, and federal regulations that apply to environmental projects should be identified early in the process.

A broad watershed perspective is needed for successful aquatic plant management in a lake. This land area that drains into a lake directly relates to the productivity of the lake. Inputs to the lake from both point sources (municipal and industrial discharges) and nonpoint sources (runoff from cropland, roadways) can carry nutrients and organic matter that spur the growth of aquatic vegetation.

What Feeds Nuisance Vegetation?
The nutrient that is usually most crucial for the growth of lake vegetation is phosphorus. In addition to watershed sources of phosphorus, in-lake sources of phosphorus loading also can promote plant and algae growth. Usually in-lake phosphorus remains bound to sediment on the bottom of the lake, but under certain conditions, this nutrient is released and becomes available to support plant production or algae blooms. For instance, underwater currents from outboard motors can stir up bottom sediments in shallow lakes and release nutrients to be available for algae. When bottom-dwelling plants are cut or die off, phosphorus from the plants is released into the water column to be used by suspended algae or phytoplankton and support algae blooms.

Because nuisance growths of both plants and algae can results from excessive nutrient inputs from the watershed as well as from inside the lake, control strategies need to target all potential sources. A nutrient budget, which assesses phosphorus sources and sinks in the watershed and lake, can help pinpoint those areas to target for nutrient control. Although the cost of a detailed nutrient budget may seem high, in the long run, this up-front investigation can save time and money.