Benthic Barriers

What is a Benthic Barrier? The benthic screen limits light to the bottom of the lake, thus helping to preclude the growth of aquatic vegetation. Barriers can be porous or solid. Benthic barriers, also known as benthic mats or bottom screens, are mats that are installed on the bottom of a lake to inhibit the growth of invasive aquatic plants. Barriers consist of dark, light blocking fabric that is held at the bottom of the lake by weights. The benthic screen limits light to the bottom of the lake, thus helping to preclude the growth of aquatic vegetation. Barriers can be porous or solid.

Appropriate Locations Benthic barriers are most appropriate for small areas, such as around docks or boat launches or in swimming areas. This technique is not suitable for controlling widespread invasive plant infestations.

Advantages   Highly flexible control   Reduces turbidity from soft bottom sediments   Can cover undesirable substrate   Can improve fish habitat by creating edge effects

Disadvantages   May cause anoxia at sediment-water interface   May limit benthic invertebrates   Non-selective interference with plants in target area   May inhibit spawning/feeding by some fish species

  Porous Barrier - subject to less billowing, but will allow settling plant fragments to root and grow; annual maintenance is therefore essential   Solid Barrier - generally prevent rooting in the absence of sediment accumulations, but will billow after enough gases accumulate; venting and strong anchoring are essential in most cases

Information for Proper Application   Mapping of area to be covered by barrier, with information on plant types and density   Knowledge of sediment features, along with any obstructions or other interference factors   Inventory of biological features of the target area, especially the presence of any protected species   Plan for installation and maintenance

Installation Process Proper application requires that the screens be placed on the sediment surface and staked or securely anchored. This may be difficult to accomplish over dense plant growth, and a winter drawdown can provide an ideal opportunity for application in exposed areas. Late spring application has also been effective, despite the presence of plant growths at that time, and barriers applied in early May have been removed in mid-June with no substantial plant growth through the summer. Scuba divers normally apply the covers in deeper water, which greatly increases labor costs.

Installation Considerations   The size of the area to be treated   Bottom features and possible obstructions   Costs: product, application and maintenance   Possible impacts to non-target organisms in the installation area.   Deployment & Anchoring Method

Permitting  Depending on size of installation, will require filing of Request for Determination of Applicability or a full Notice of Intent

Material Costs  Aquatic Control Technologies -.85 per square foot (price includes installation)  Lake Bottom Blanket – 10 x 40’ lake bottom blanket $200  Typar 12 x 300 foot (3,600 sq feet) roll approx $350, or about 10 cents per square foot.  Aquascreen, a product marketed for the purpose of benthic barrier applications, is sold for about 50 cents per square foot. Screen

Material Costs  There are a variety of methods to anchor the screening; Cinder Blocks Rebar Wood Frame Anchor Material

Permit Fees & Installation  At a minimum a RDA with the local conservation commission is required; in some cases Notice of Intent may be required  If installed professionally, labor costs must be factored  In many installations; homeowner can install  If diver services needed due to depth; diver costs must be factored

Case Study of Benthic Barrier Installation

Lake Holland, Belchertown, MA 2007  Much of the area treated was being choked by invasive milfoil stands. The water column and growing along on the surface. The water is hot and stagnant, it does not circulate, and is a noxious habitat favorable to insect breeding.  The area was about 3,000 sq ft. (1/15th of an acre).  The area is bounded by a thick wetland, deep water.

Materials  The fabric utilized was a non-woven polypropylene that is water and gas permeable but opaque. It is sold by Dupont under the brand name Typar and is commonly used for house-wrap in construction, riprap (shoreline structuring), sandbags and road construction to separate aggregate from soil. It is very durable and could be moved and re-used. There is much experience with Typar as a benthic barrier. The following is from the Lake Sutherland (Washington State) Association Newsletter regarding Typar: –“The new barriers have been the key to our success. Made of porous, non-woven fabric, not plastic, the material is rolled out underwater and secured with gravel-filled bags….The old barriers were made of burlap which degraded within a year, sometimes less, any stray milfoil readily established on top. The fabric barrier is much better. By preventing fragments from getting established, the new barriers have allowed us the breathing space to return and remove stray plants as they re- grow or try to re-established.”  Typar is slightly buoyant with a gravity rating of.9, technique requires secure anchoring. As weeds decompose gases will accumulate and cause billowing to some extent even though the fabric is gas permeable, and this must be managed. This is the same challenge for any fabric.

Work Plan  Crew - Homeowner & 2 individuals.  Most of the target area is very shallow but scuba will likely be necessary for some of the placement work. (Diving to be done by property owner who is a PADI Rescue certified diver with much experience in various conditions including lakes. )  In preparation, some of the weeds to be removed by hand. This step is not necessary to the technique but pre-clearing some of this biomass would make it easier to install the barriers in the shallows and it will reduce the amount of rotting matter during treatment, which may reduce maintenance later.  The fabric will be cut from the bolt into tailored pieces. These will be re-rolled, taken to the site, unrolled and sunk, pinned at the corners and along the sides with sandbags. A number of overlapping pieces will be assembled to cover a contiguous area.  Optionally, instead of covering one contiguous area, depending on circumstances, gaps may be left to allow hand-pull after installation. The idea is that these gaps, if cleared quickly, would help restore the area post treatment, and would add to the amount of edge effects that is a side benefit of the barriers to fish.  From the shore or from a row boat, one person will hold the rolled fabric and dole it out while a second person in a row boat or canoe pulls the fabric across the water. The four corners of the section will be weighted and the weights dropped into approximate position. Then a sandbag is dropped in the center of the barrier. Then the fabric will be pressed down, flattening it outwards from the center sandbag, crushing the plants as much as possible. This would be done either from the boats using hands and canoe paddles and/or a swimmer with snorkel and mask in shallows or scuba diver in deeper water. More sand bags are positioned as the barrier is flattened, and adjusted as necessary.  While the barriers are in position site to be checked daily for billowing, which can either be pushed to the sides or the fabric can be cut to vent the gas without affecting the treatment.  The fabric can be removed or moved to an adjacent location after 4 to 8 weeks. However, because the fabric is porous, does not degrade quickly and is inexpensive, it is possible to leave some pieces in place. This is similar to the use of Typar as a separator in road construction and riprap. In some cases this might be desirable, e.g. where the muck layer is shown to be the source of nutrient loading in the water column, or for longer term control with less maintenance requirements.

Installation Images

Before Pictures Taken October 8, 2007

After Pictures

Project Costs  A 12 x 300 foot (3,600 sq feet) roll of Typar cost approx $350, or about 10 cents per square foot. Aquascreen, a product marketed for the purpose of benthic barrier applications, is sold for about 50 cents per square foot. Aquascreen has one advantage over Typar: A specific gravity of 2.5 as compared to.9 for Typar, which means that Typar is slightly bouyant.

Sources  2004 Practical Guide to Lake Management in Massachusetts, Executive Office of Environmental Affairs  Aquatic Control Technologies tments.html tments.html tments.html  Lake Project Net – Lake Holland Project  Instructions for building a benthic barrier; wood frame construction nthic_barriers.pdf nthic_barriers.pdf nthic_barriers.pdf  Lake Bottom Blanket  Lycott Environmental