Is an Invasion of Our Lakes Imminent?

Scott Williams, Executive Director
Maine Volunteer Lake Monitoring Program

Maine is one of the few States whose lakes have not experienced significant problems resulting from "exotic" or invasive aquatic plants. We have been more fortunate than our New England neighbors. Numerous lakes in New Hampshire, Massachusetts, Vermont and Connecticut have become choked with invasive plants that can interfere with boating and swimming and alter fishery habitat. Most of these plants are exotic or "foreign". Exotic may imply that a plant or animal is not native to a region or a country. It can also be used in a more narrow sense to describe an organism that has not occurred historically in a state, or even within an individual lake. For example, some plants that are native to North America may never have occurred in a particular lake. When newly introduced to a lake, many plants have the potential to become a disruptive nuisance.

Rooted aquatic plants like cattails, water lilies, and rushes are collectively referred to as macrophytes. They normally grow in diverse, balanced communities with other plant and animal residents of our lakes. Their habitat preference is shallow, sheltered areas with soft, nutrient-rich organic bottom sediments. The location and growth of lake plants is limited by these requirements. Their growth is also limited by competition with other species, by insects, fish and mammals that forage these plants, and by disease. Lake plant populations also vary from one year to the next due to fluctuations in water levels, ice depth, and natural community successions.

Native aquatic plants play a number of important roles in lake ecology. However, their presence may, at times conflict with the ways in which we use our lakes. For example, dense stands of Pickerel Weed may interfere with swimming and boating, but fishermen recognize the excellent habitat that these plants provide for some species of fish. An increase in the number of naturally occurring plants in an individual lake may be caused by elevated nutrient levels in the lake and accumulations of sediment in shallow areas, both of which may be the result of watershed development. . The overall benefits of rooted lake plants to water quality and to all lake users far outweigh the occasional inconveniences they cause to swimmers and boaters.

 

Some of the benefits of lake plants include:

• Maintaining healthy biodiversity in the lake ecosystem
• Providing food and shelter for a wide variety of animals including insects, fish, birds, and mammals
• Minimizing bottom sediment resuspension in shallow areas. This helps to keep lake water clearer along shoreline areas
• Helping to reduce shoreline erosion by reducing the force of waves
• Protecting water quality by using nutrients that might otherwise be available to less desirable plants like some species of algae

When invasive plants from other geographic regions (or even from the lake next door) are introduced to a lake they may proliferate rapidly because the factors that control them in their native waters, like disease and competition, may not exist in their new habitat. Many of the most troublesome exotic aquatic plants in this country were brought here for use in private aquariums, or they were transplanted to a lake or pond because people admired their flowers.

Most invasive plants are hearty, resilient, and highly adaptive. They reproduce in a number of ways, including fragmentation (any fragment that breaks off from the plant is capable of becoming established as a separate plant), and some species have multiple variants, some of which are particularly aggressive, compared to native plants.

Once invasive lake plants become established in a lake, they may spread to nearby through natural processes. However, these plants can also be spread when small fragments become attached to boats, motors, trailers, fishing traps, nets, bait buckets, and other human contrivances. When a boat is transported from one lake to another, an attached fragment may break loose, creating the potential for a new population to develop in that lake.

When an invasive exotic plant is introduced into a lake, the following negative changes may take place in the ecosystem:

• The plant may rapidly out-compete native plants. The natural diversity of lake plants may be replaced by a monoculture
• Fishery predator-prey relationships may be changed due to dense stands of plants, resulting in changes in fishery species composition, and a reduction in the numbers and average size of the fish population.
• Stress to, or even mortality of fish populations, due to low dissolved oxygen concentrations in dense plant stands.
• An increase in the rate of organic enrichment of the lake (eutrophication), resulting from rapid, uncontrolled growth of the invasive plants. Nutrients can be rapidly cycled from the bottom sediments to the plant, and into the water when the plant dies. Those nutrients may then become available to algae or other rooted lake plants.
• Swimming and boating may be seriously impaired due to heavy plant growth along the shoreline
• Reduced aesthetic appeal
• It is likely that a relationship exists between the excess growth of exotic rooted plants and shorefront property values, similar to the relationship that has been documented between lake transparency and shorefront property values

Extensive research has been conducted on the subject of controlling invasive plants. Much of this research suggests that the most effective (economic and practical) approach to this problem is through prevention or early detection. If lake users are well-informed about the ways in which plants spread, infestations may be prevented. If one does occur, early detection can reduce the chances that the plant will spread throughout the lake.

This potential threat became very real on a small lake in southwestern Maine this past summer. Residents of Cushman Pond in Lovell first reported seeing an unusual plant growing very rapidly in the lake in 1996. The plant was readily identified as a member of the Milfoil family (genus Myriophyllum), but the species could not be determined until the plant flowered in July, 1997. It was then determined that the plant in Cushman Pond is M. heterophyllum. Although a native plant of North America, this species of Milfoil can be invasive in New England lakes where it has not grown in the past. Although generally not as aggressive as Eurasian Milfoil (M. spicatum), it is nonetheless capable of causing many of the problems described above. M. heterophyllum has also been reported in the Sebago Lake area, and the New Hampshire Department of Environmental Protection has reported that this plant has caused problems for some lakes in that state.

The Cushman Pond plant was most probably transported from a nearby New Hampshire lake. It may have been introduced by a boat, or by fishing bait traps. Because the plant was discovered before it spread extensively, an attempt was made to eradicate it from the lake. We will not know until next summer whether or not the substantial combined efforts of the residents of Cushman Pond, the Town of Lovell, the Kezar Lake Association, and the Maine DEP have been effective in eliminating this plant from the lake.

 

 

Reprinted from The Water Column, Vol. 2, No. 2, Fall 1997