Native Freshwater Plants

Aquatic Plants and Lakes

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.

Acknowledgments:
This information was taken from a brochure called About Aquatic Plants written by the Municipality of Metropolitan Seattle's (METRO) Water Resource Section (now King County).

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 near-shore 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.

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:

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.

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:

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.