Introduction

Plants growing in our lakes, ponds, and streams are called macrophytes. These aquatic plants appear in many shapes and sizes. Some have leaves that float on the water surface, while others grow completely underwater. In moderation, aquatic plants are aesthetically pleasing and desirable environmentally. Their presence is natural and normal in lakes, and in fact they are an important link in a lake's life system. In large quantities, plants can interfere with some water uses and may be seen as a problem.

When lake users are confronted with too many plants in the wrong places, a common reaction is to remove the entire problem. Such hasty decisions are often made with little regard for the important role plants play in the water environment. Neglecting to see these interrelations often results in unintended impacts to wildlife, fish, and other forms of life. Unfortunately, the information required to make environmentally sound decisions is not always easily available. Without this information, how can anyone know what is best over the long term to do about aquatic plants?

The purpose of this information is to provide some general but pertinent information and insights regarding aquatic plants: (1) how they came to be where they are, (2) what beneficial contributions that they make to life in the lake, (3) how our activities can affect them, and (4) some things to consider if plant removal is planned.

Life History of a Lake

A question frequently raised regarding aquatic plants is why some lakes have them in abundance and others do not. An answer to this question lies in the explanation of the lake's aging cycle.

Most of our local lakes came into being as a result of activity of glaciers in the most recent ice age, approximately 10,000 years ago. A lake bed is a natural depression or low spot in the terrain. In the Puget Sound lowlands they were often gouged out by movement of glacial ice. These depressions then became holding basins for water of the drainage area as it flowed toward sea level. As a lake detains water on its way downstream, it also becomes a settling pond for sediment. Part of the sediment that settles in lakes is carried in by the flow of streams or other runoff, and part comes from the accumulation of the remains of organisms in the water and near the shoreline. Aquatic life includes visible plants and animals and also multitudes of microscopic plants and animals that can, over time, add significantly to the accumulation of sediment in the lake by dying and settling to the bottom. The microscopic plants in the water are nourished by plant nutrients (phosphorus and nitrogen) that originate in the watershed and are washed into the lake. Within the lake, a portion of these nutrients can be recycled indefinitely, while more continue to be washed in from the lake basin. Over time sediment accumulates in the lake as productivity gradually increases.

When first formed, many of the lake beds were deep and "clean" (mostly free of sediment). Sediments that were first deposited were silty and had little organic material because there was little life in or around the lakes. Over time, hundreds or thousands of years, the sediment deposits became deeper and more favorable to the growth of rooted, aquatic plants (macrophytes). As these plants (and the microscopic life in the water, now also more abundant) died back at the end of each growing season or life cycle, they enriched the sediments with organic material. Since macrophytes tend to grow better in organically richer sediments, this process set up a cycle of more growth in the lake causing richer sediments and these in turn favored even more growth of macrophytes.

As both microscopic and rooted plants continue to grow in increasing volumes and to enrich the bottom sediments of the lake, the lake ultimately fills in with silt and partially decomposed plant and animal material, forming a wetland and eventually a meadow. This occurrence marks the end of a lake's life.

A lake's life generally extends over several hundreds or thousands of years under natural conditions. It can, however, be significantly shortened by human activities such as improper construction practices that disturb the soil and make it susceptible to erosional processes. As erosion increases in the watershed, quantities of sediment transported to the lake also increase. Consequently, the lake fills in faster and its life expectancy is reduced. The rate at which the process occurs varies with the size, shape, and flushing rate of the lake. Human beings introduce to the watershed nutrient-rich materials such as fertilizers and wastes. Installing hard surfaces (roofs, roads, pavement) makes it easy for these nutrients to be washed into streams and lakes. These activities boost the continuing production of organisms that contribute to the filling of the lake as they die and become part of the organic sediment that favors macrophyte growth.

Lake Aging and Macrophytes

Aquatic plants are an integral part of the lake and its aging process. Although they may appear in deep, clear lakes, they generally grow most abundantly in shallow, productive lakes---old lakes that have accumulated thick sediments. An abundance of aquatic plants represents a sign of "middle" or "old" age.

Conditions that favor macrophyte growth are fairly well documented. Many observations and studies have shown that conditions favoring their growth include an accumulation of organically enriched sediment and water that is shallow enough and clear enough for sufficient light to penetrate to the bottom. Algae, silt, or a natural amber or brown color in the water can interfere with transparency. Of course, the temperature range has to be acceptable to the plants, and there has to be enough stability in the sediments to permit plants to gain a foothold. Thus, the nearshore area cannot be too steep.

Although considerable research has been done to determine specific characteristics of sediment and water where macrophytes grow, many fundamental questions remain unanswered. However, it has been documented that macrophytes are able to get most of their nutrition from the sediment and can grow abundantly even when the lake water is not high in nutrients.

Macrophytes in a Lake Ecosystem

In many lakes macrophytes contribute to the aesthetically pleasing appearance of the setting and are enjoyable in their own right. But even more important, they are an essential element in the life systems of most lakes. They perform a number of useful functions in maintaining the food chain of life in the lake.

• Macrophyte leaves and stems provide a habitat or home for small attached plants and animals. Some are microscopic in size and some are larger.  These attached organisms are valuable in that they are food for animals higher in the food chain, for example, fish and birds. There is a vast variety of these attached organisms, which are often very particular in their habitat needs. The kinds of surfaces available for their attachment will help determine the kinds and amounts of organisms present. Logs, sticks, and rocks of various sizes, shapes, and textures provide different kinds of surfaces. Different kinds of macrophytes---because they have a variety of sizes, shapes, and textures of stems and leaves, and because they assume various positions in the water---also provide different kinds of surfaces for attachment.
  
• Many types of small organisms live in the sediment. There are insects that spend the immature stages of life in the sediments, leaving when they become adults. Decomposing plant life provides part of the food supply for these sediment-dwelling organisms and the emerging insects, in turn, are food for fish.
  
• The submerged portions of macrophytes provide shelter and cover for small or young fish from larger fish that would feed on them.
  
• Types of plants that extend above the water can provide cover for waterfowl and their young.
  
• Many plants can serve directly as food for certain types of waterfowl.
  
• Macrophytes may have an impact on the amount of floating algae in the lake. They can provide a growing surface for an enormous amount of algae.  These algae use up a portion of the nutrients dissolved in the water that might otherwise "fertilize" the floating algae and increase their growth. On the other hand, the macrophytes themselves, as they die back and decay, contribute nutrients to the water and thus can promote algal growth. So in many cases we cannot be sure (without very specific and detailed analyses) whether removing rooted plants would bring about more algae growth, or less, or no change.

Macrophytes as Nuisances

Thus far, we have described how macrophytes are necessary to life as we know it in lakes. The question then is one of degree: How much plant life is desirable?  Can there be too much? How much is too much? The answers will be different for different types of lake users and may vary from one lake to another. Some uses of a lake are more affected by macrophytes than other uses, and some types of plants interfere with recreational activities more than others do. Generally, plants are not considered to be a problem unless they interfere with desired or accustomed uses of a lake.

Some plants have special capabilities to become very abundant and are thus apt to become nuisances. An example of this type of plant is the nonnative noxious weed Eurasian watermilfoil (Myriophyllum spicatum). This plant:

• Has a broad tolerance in its environmental requirements and is capable of flourishing under what seems to be difficult conditions and is found in a wide range of environments.
• It is a nonnative species and does not have significant natural controls in Washington.
• Its reproduction and growth habits are very efficient.
  • Its highly effective process of vegetative reproduction by fragmentation enables it to spread rapidly.
  • In milder western Washington, it can overwinter in an evergreen state and start growing early in the spring before the native plants get started.

Stagnant Eurasian Watermilfoil Mat     

Recreational boaters unwittingly contribute to the spread of milfoil and other exotic waterweeds such as Brazilian elodea, by carrying  fragments of the plant on their boats, trailers, or fishing gear to an uninfested  waterbody. Some of these noxious weeds can spread rampantly from even a tiny sprig, creating an aquatic jungle within a couple of years.

Problems frequently attributed to the growth of macrophytes in lakes used primarily for recreation include:

• Interference with recreational activities such as boating, fishing, water skiing, and swimming.
• Production of fish populations consisting of many small fish rather than fewer large fish as a result of the plant's protecting small fish too well against predators.
• Unsightly and odoriferous accumulations of plant material on the water surface or beach.
• Oxygen deficiencies in the water (perhaps causing death to fish populations) due to decay of plants and plant respiration.
• Accelerated filling of lakes by accumulation of decomposing plant material.

Minimizing Macrophyte Nuisances

There are several ways (aside from direct control operations in the lake) that a homeowner and citizen can reduce the chances for macrophyte problems to develop or worsen. Since nutrients and sediments influence the presence and growth of macrophytes, curtailing their flow into a lake is important. Preventative maintenance or actions that can be taken include the following:

• Curtail fertilizer use. Apply no more than the amount recommended by the manufacturer. Use a phosphorus-free fertilizer on established lawns.
• Develop lake-friendly landscaping practices.
• Maintain septic tank systems to prevent failures and support enforcement of laws aimed at preventing construction of septic tanks in unsuitable soil types.
• Place garden and grass clippings in areas not draining directly into the lake. Allow drainage from this debris to flow over soil or grassy places so that nutrients from their decomposition can be absorbed.
• Use low-or no-phosphate soaps or cleaning/washing compounds when washing your car. If you have a septic tank, use low-or-no phosphate dishwasher detergents.
• Avoid using impervious ground covers (pavement, plastics) that will cause increased amounts of water to run off your property. More water tends to transport more nutrients and sediments and tends to cause erosion.
• Clean up after your pet. Animal wastes should be flushed down toilets connected to a sewer system or disposed of away from the lake.
• Keep livestock away from streams and the lake shore. Animal wastes introduce nutrients and hooves can cause bank collapse, which leads to heavy siltation.
• Don't feed the waterfowl. Canada geese can contribute a significant amount of nutrients to a lake.
• Support the adoption of ordinances designed to minimize surface water runoff and unnecessary land clearing during construction.
• Be on guard against introduction of new kinds of plants to your lake by fragments clinging to boats, trailers, or fishing gear, from aquarium plant discards, or by intentional "ornamental" plantings in the lake.
• Find ways to communicate your ideas on how the lake or lakes you are interested in ought to be managed. Form a lake association to communicate effectively with government agencies and other organized groups about how they feel lakes should be managed and protected. In some cases, these types of organizations have enough authority to share financial responsibility and thus participate directly in management activities.

Predicting the Results of Macrophyte Control

In spite of efforts to control aquatic plants, they may proliferate along shores and in embayments. In situations where heavy plant growth interferes seriously with the use of a lake, the lake users are likely to think about getting rid of part or all of the plants. Because plants play an important role in the lake ecosystem any disturbance or removal of them will have an impact on the lake. This impact cannot be predicted accurately because of the complexity and differences in physical and chemical conditions and communities of organisms from lake to lake. 

Also some effects may be subtle and not recognized immediately. Once a lake's ecosystem is disturbed, a new balance must be achieved. What the new balance will look like or how rapidly it will occur is not always easy to know. Naturally, there is less cause for concern if a small portion of the lake is to be treated rather than a large area.

Almost all methods of plant control have direct effects on plant life and organisms other than the targets of the control program. Depending on what is actually done to the lake, these side effects may or may not be extensive or long-lasting.

In cases where macrophyte control is planned, the probable effects of the proposed program can be predicted to some extent, if the lake is studied so that its physical, chemical, and biological systems are understood. Literature that reports the results of similar treatments on similar lakes can be helpful. Control methods should be chosen to minimize impacts. Hand pulling, for instance, has relatively limited side effects. However, there will generally be some uncertainties remaining as to how well the treatment will work and particularly on how extensive and significant its side effects may be.

Glossary

Algae - Single celled plants, most of which are microscopic. The kinds referred to here are either floating in the water or attached to solid surfaces of material in the water, such as rocks, logs, and plant stems and leaves.

Decomposition - Decay of once-living material that releases back into the water some of the basic substances the living material was composed of.  For example, decay of plants release plant nutrients back into the water.

Ecosystem - The interacting system of all the living things in a biological community and all the parts of their environments.

Food Chain - A succession of organisms starting with algae or other plants, followed by the animals that feed on them. Next come the animals that feed on the animals that feed on plants, and so forth. A simplified example would be algae followed by tiny floating animals that eat the algae; larger floating animals (still small) that eat the tiny animals; small fish that eat the larger animals; big fish that eat the little fish; and humans who eat the big fish.

Habitat - The environment in which a plant or animal lives. Different kinds of plants or animals have different requirements for their habitats and cannot survive in a habitat that provides too few of their required conditions.

Macrophytes - Large plants that grow in the water. Most of the types we have are rooted in the sediment.

Organic - Refers here to material that is living or once was part of a living organism.

Organism - Any plant or animal including those of microscopic size.

Plant Nutrients - Materials that plants use in growth such as phosphorus and nitrogen.

Predators - Animals that prey on other animals, that is, use them for food. A particular kind of predator uses only certain other types of animals as food or prey.

Respiration - The physical and chemical processes by which an organism supplies its body cells with needed oxygen and rids itself of waste carbon dioxide. In some kinds of animals, respiration is also known as breathing; oxygen is used up and carbon dioxide is given off. Plants give off oxygen as a product of photosynthesis during daylight hours. Plants also respire, however, and at night when photosynthesis stops, they use up oxygen from the water rather than produce it as they do during the day.

Sediment - Refers here to lake sediment, which is composed of (1) rock and soil particles and (2) particles of plant and animal materials. The first is the silt portion. and the second is the organic portion.

Last Updated May 04, 2006

American Waterweed | Bladderwort | Cattail | Coontail | Sago Pondweed | Spatterdock | Water Celery | Water Shield | The Uses and Benefits of Aquatic Plants | Aquatic Plants and Lakes

Return to the Table of Contents