Mat Forming Macrophyte Species | Region 10 | US EPA

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Mat Forming Macrophyte Species

An example of one of the mat-forming macrophyte species, Trapa Natansor the European water chestnut. Photo courtesy of Leslie Mehrhoff, U. CN,

Freshwater weeds occur in a variety of shapes and types, but most generally have one thing in common: where they occur, they often create dense mats of floating vegetation that can affect both wildlife habitat and beneficial uses of the water. Thick weed mats destroy recreational fishing, are a hazard to swimming, and cause navigation and boating nightmares. This often also directly translates to lost recreational revenue. Weed mats also affect the aquatic habitat: they decrease light penetration, leading to death of the underlying native vegetation, which in turn leads to death of many of the associated animals. Vegetation mats also affect water quality in several ways; they prevent wind-mixing of the water layers, leading to increased temperature in the upper water layers and decreased oxygen in the lower water and sediments. This, in turn, alters pH, causing release of phosphorus from the sediments, that then causes noxious algal blooms. The human management response to these blooms is often herbicide use.

Pathways for these plants variously include the pet, horticulture, and nursery trades, as well the internet. Some of these plants are still sold by nurseries that justify their sale by stating the climate is not conducive to their establishment. Others are sold by biological supply houses for ecological studies. State biologists are working hard to close these pathways.

Status in the Pacific Northwest: A variety of weeds are of particular concern to the Pacific Northwest. Populations of African waterweed (Lagarosiphon major) and European Water chestnut (Trapa natans) are not yet established in the Pacific Northwest. Control/eradication programs for Brazilian elodea (Egeria densa) and Eurasian watermilfoil (Myriophyllum spicatum), are variously underway in a number of locations in WA, OR and ID. Hydrilla (Hydrilla verticilliata) has been found and hopefully eradicated in Washington, and a small population was recently found in Idaho. See the individual links for more information on these species.

What can you do? Do not transfer plants from one waterbody to another, either intentionally or through carelessness. Do not plant or transfer plants from a nursery or somewhere else into your local lake because you think they would “be pretty”. Clean your boats, motors, trailers and other recreational gear of all weed fragments before moving from one lake, river or wetland area to another. If you own a water garden, ensure it is located where flood events will not wash the plants out into the environment. Do not dump aquariums into the environment; dispose of any unwanted water garden or aquarium plants and plant fragments in the garbage, where they will not be able to have contact with water. Do not assume that something is “safe”, just because it is sold in a local nursery, or because it is normally found in warmer conditions.

If you have questions about the safety of any particular plant, contact the appropriate state for additional information:
In Washington: 1-360-902-2700 (ANS Coordinator)
In Oregon: 1-866 INVADER
In Idaho: 1-208-332-8686 (ANS Coordinator)
In Alaska: 1-877-INVASIVE

The links provided may be outside the domain.

Lagarosiphon major - African waterweed

Fact Sheets

Trapa natans - European water chestnut

Fact Sheets

MD Management plan: Water Chestnut

Brazilian elodea- Egeria densa

Fact Sheets

Portland State University Project

Eurasian watermilfoil - Myriophyllum spicatum

Fact Sheets

Eurasian watermilfoil Biocontrol

Hydrilla - Hydrilla verticilliata

Fact Sheets

Hydrilla Biocontrol

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