Reprinted from: http://el.erdc.usace.army.mil/aqua/apis/mechanical/html/benthic_.html

Benthic Barrier

Benthic barriers or other bottom-covering approaches are another physical management technique that has been in use for a substantial period of time. The basic idea is that the plants are covered over with a layer of a growth-inhibiting substance. Many materials have been used, including sheets or screens of organic, inorganic and synthetic materials, sediments such as dredge sediment, sand, silt or clay, fly ash, and combinations of the above (Cooke 1980b; Nichols 1974; Perkins 1984; Truelson 1984). The problem with using sediments is that new plants establish on top of the added layer (Engel and Nichols 1984). The problem with synthetic sheeting is that the gasses evolved from decomposition of plants and normal decomposition activity of the sediments underneath the barrier collect under the barrier, lifting it (Gunnison and Barko 1992). Benthic barriers will typically kill plants under them within 1 to 2 months, after which they maybe removed (Engel 1984). Sheet color is relatively unimportant; opaque (particularly black) barriers work best, but even clear plastic barriers will work effectively (Carter et al. 1994). Sites from which barriers are removed will be rapidly re-colonized (Eichler et al. 1995). In addition, synthetic barriers may be left in place for multi-year control but will eventually become sediment-covered and will allow colonization by plants. Benthic barriers, effective and fairly low-cost control techniques for limited areas (e.g., <1 acre), may be best suited to high-intensity use areas such as docks, boat launch areas, and swimming areas. However, they are too expensive to use over widespread areas, and heavily effect benthic communities.

Description

Bottom barriers are materials that are laid across sections of lake or river bottoms infested with this noxious weed. These barriers are attached to the bottom by pins or sandbags. Common bottom barrier materials are geotextile ground cover cloth or erosion control materials. A number of commercial bottom barriers have been marketed over the years. Aquascreen and Texel are two that are commonly used. These, however, carry a premium price as they are advertised for aquatic use. Similar or identical materials can be obtained from erosion control suppliers at a fraction of the cost.

Bottom barriers are brought to the shoreline where installation is to occur. They are then cut to fit the treatment site and rolled onto a length of pipe. Divers carry the roll into the water at the start of the treatment site and pin the leading edge of the material. The divers then roll about 3 to 6 feet of the material out and pin it again. This process is repeated until the plants are covered.

This technology has proven to be very effective against pioneering colonies of noxious weeds. In large lakes in British Columbia, the provincial government has eradicated pioneering infestations of this weed by covering the biomass and root crowns. This prevents spread by fragmentation and kills the root crowns by starving them. Bottom barriers can be moved from site to site as one area is cleared of the weed, or they can remain in place to prevent weed growth.

Operational Considerations

Many states require permits for the application of materials to lake or river bottoms. These materials cover the substrate and limit the movement of benthic organisms from the sediment to the water column. If applied lake wide, there would be potential for impacting these organisms. Bottom barriers are not generally considered for lake wide application, since costs alone will preclude this application. These are tools that are used for covering small pioneering colonies of the weed or to clear high-use areas such as swimming beaches, boat launches, and dock areas. The user should check with the appropriate state or local agency to determine if permits are required.

Gas buildup under these barriers can be a problem during the first year or two of installation. Covering sediments that normally exchange gases with the water column will trap those gases. Covering clay or sand substrates where this type of gas generation is not extensive will limit that type of problem. Covering highly organic sediments will required that the operator consider this and develops a maintenance program to deal with it. In addition, if the barrier is placed over actively growing weeds, those plants will die and decompose under the mat. This will also create gas problems in the short term. Gas buildup can be dealt with fairly easily. The operator should have divers periodically inspect the mats and push gas bubbles to the edge of the mat, where they are released. Divers can also cut small slits in the material to vent this gas. Pinning the material to the bottom will also help.

Ongoing maintenance can be required to help increase the long-term benefit of this tool. If the bottom barriers are applied in areas with high levels of silt accumulation, the buildup of silt can render the mats useless over time. Divers using water hoses can easily wash accumulated silt off the mats during regular inspections.

Expected Results

Bottom barriers provide 100% control of this weed in areas where they are installed. They also provide long-term control. An ongoing maintenance operation is required to inspect the bottom barrier and clear the mats of sediment buildup.