15 Sediment Covers and Surface Shading for Macrophyte Control 15.1 INTRODUCTION Rooted aquatic vegetation control with sand, gravel, or clay has been mostly unsuccessful because root sys
Trang 115 Sediment Covers and
Surface Shading for Macrophyte Control
15.1 INTRODUCTION
Rooted aquatic vegetation control with sand, gravel, or clay has been mostly unsuccessful because root systems remain to produce shoots that penetrate earthen covers, and because many aquatic plants reestablish through fragments carried to the treated site from other lake areas, or by seeds, turions, and rhizomes that survive the treatment Another option is synthetic sheeting and screening materials Their effectiveness is higher because shoots may be unable to penetrate them, but they are expensive, application is labor intensive, effectiveness is correlated with application techniques and types of material, and most materials manufactured for this purpose are not widely available Some are no longer manufactured
Engel (1982) listed these advantages of sediment covers:
1 Their use is confined to specific lake areas
2 Screens are usually out of sight and thus create no disturbance on shore
3 They can be installed in places where harvesters or sprayer boats cannot gain access
4 No toxic substances are released
5 They usually require no permit or license
6 They are easy to install over small areas
7 They can be removed
These are their disadvantages (Engel, 1982):
1 They fail to correct the cause of the problem
2 They are expensive
3 They are difficult to apply over large areas or on sites with obstructions
4 They may slip on steep grades or float to the surface after trapping gases beneath them
5 They can be difficult to remove or relocate
6 They may rip during application
7 Some materials are degraded by sunlight
Reviews include Armour et al (1979), Cooke (1980), Nichols and Shaw (1983), Perkins (1984), and Newroth and Truelson (1984)
15.2 COMPARISON OF SYNTHETIC SEDIMENT COVERS
15.2.1 POLYETHYLENE
One of the earliest uses of sediment covers was the treatment of 10 ha of Marion Millpond, Wisconsin with 0.1 mm (4 ml) impermeable black polyethylene sheeting (Born et al., 1973; Peterson
Trang 2et al., 1974; Nichols, 1974; Engel, 1982; and Engel and Nichols, 1984) In 1969 to 1970 the pond was drained and the basin cleared of stumps and debris Sheeting was laid over the sediments and covered with 7 to 15 cm of sand and gravel Application in soft mud was tedious, so enough water
was added to just cover the treatment area and allowed to freeze Sheeting and the sand and gravel
cover were placed on the ice and water withdrawn, cracking the ice The materials fell to the pond’s bottom The pond was refilled in 1971, and control of macrophytes was achieved in 1971–1972
Chara and filamentous algae covered the treated area in 1973 (Nichols, 1974) The screened areas
were covered with macrophytes in 1978, although biomass was about half that of untreated areas The polyethylene sediment cover exerted little control of macrophyte biomass after 1973 because
plants like Najas, Myriophyllum, and some species of Potamogeton colonized the sand and gravel
layer over the sheeting (Engel, 1982; Engel and Nichols, 1984) The cost was $371 per ha (2002 prices) for materials (Born et al., 1973) Labor was donated by lake users
An area of 0.43 ha in Skaha Lake, British Columbia was treated, but most of the polyethylene sheeting washed up during storms Treatments at some sites were effective but plants grew through holes punched for gas escape, or grew on accumulated sediment (Armour et al., 1979)
The Skaha Lake application cost $38,960 per ha (2002 prices), including labor and materials Negative features of polyethylene sheeting include:
1 It is difficult to apply over an irregular bottom or over a high density of weeds
2 Gas forms under the sheets, even with 1.2 cm holes
3 It is not feasible to move and relocate the sheets
4 It slides down steep inclines
5 It is deteriorated by sunlight (about 1 year in direct sun)
6 Its buoyancy makes it difficult to handle
Perhaps one of the greatest negative features of polyethylene sheeting, as well as some of the other covering materials, is that all plants can be eliminated, but regrowth after sheeting removal includes the target exotic plant For example, polyethylene sheeting, in place for 4–6 weeks in a Wisconsin reservoir, eliminated all species In the next year, native plants regrew on 40% of the treated area
while milfoil (Myriophyllum spicatum) appeared on 60% In contrast, areas treated with 2,4-D
(Chapter 16) had native plant recovery in 10–12 weeks over 95% of the area, while milfoil covered only 5% (Helsel et al., 1996)
15.2.2 POLYPROPYLENE
Polypropylene is a black, woven, semi-permeable sheeting often used as a soil stabilizer or “geo-technical” material It has a specific gravity less than one, thus requiring anchoring to prevent bulges or floating It is permeable to gases and does not need slits or holes It was effective in
controlling M spicatum, but plant fragments grew on the sediments accumulating over the screen,
and also penetrated the screen (Armour et al., 1979) Polypropylene anchored with concrete blocks,
did not allow root penetration by M spicatum over three summers, and a plant-free water column
was evident, even though plant fragments did appear on the sediments that accumulated on the screen (Lewis et al., 1983) Anchored polypropylene (cement blocks) was completely effective in
preventing the growth of Najas flexilis, Potamogeton gramineus, P crispus, P foliosus, and P pusillus for 1 year in an area that previously had been drawn down and exposed to freezing Small growths of N flexilis and filamentous algae were evident on the screens (Cooke and Gorman, 1980).
Polypropylene may be difficult to remove and clean, so plants can grow on sediment deposits There can be gas accumulation and “ballooning” beneath polypropylene screens (Engel, 1984) Costs of materials (2002 prices) range from US$14,715 per ha to $65,900 per ha Commercial availability for use in lakes and ponds is uncertain
Trang 315.2.3 AQUASCREEN
Aquascreen (Menardi-Criswell, Augusta, GA 30913) is a fiberglass screen coated with polyvinyl chloride It is flexible and dense (specific gravity = 2.54), with a mesh size of 62 apertures/cm2
(400/in.2) The standard roll size is 7 × 100 ft
Aquascreen can be effective in eliminating aquatic plants, at least for the application period Sediment accumulation on the screen eventually allows new plants to root This is a problem typical
of all synthetic screening materials, and users are advised to remove and clean the screens annually Another problem with Aquascreen, and with similar screens (including window screen), is the devel-opment of a community of attached organisms, thus eliminating or severely reducing gas permeability (Pullman, 1990) This promotes gas accumulation and the screen may lift off the sediments
The relationship between Aquascreen coverage time and effective control of M spicatum was
tested in 9 × 24 m plots in shallow (0.5 to 2.0 m) and deep (2 to 3 m) areas of Union Bay, Lake Washington (Perkins et al., 1980; Boston and Perkins, 1982) Panels were removed at 1-month, 2-month, and 3-month intervals One month of coverage produced decreases of 25% and 35% for shallow and deep plots, respectively One month after panel removal, plant regrowth was small Panels in place for 2 months produced decreases of 78% in shallow plots and 56% in deep, with minimal regrowth Plant biomass and regrowth after 3 months of coverage was small The screen was most effective where there was good contact with the lake bottom Plant death in test aquaria was slow enough to prevent DO loss and P accumulation under the typical field conditions (Boston and Perkins, 1982) Aquascreen’s effectiveness, as well as its impact on benthic macroinvertebrates, was investi-gated in Cox Hollow Lake, Wisconsin The screen was easily applied, removed, cleaned, and stored Macrophyte growth was prevented regardless of when Aquascreen was installed during the summer, and few plants could be found under firmly anchored screen, although there was considerable
growth under loosely applied screen Eichler et al (1995) also found that plants such as M spicatum
grew through the screen, or grew under it There was little control in the second season after application unless the screens were removed, cleaned, and repositioned This is easily accomplished with Aquascreen (Engel, 1982, 1984)
Macroinvertebrates were eliminated by Aquascreen panels in Cox Hollow Lake, apparently because of poor circulation and low DO during the 1-year application period (Engel, 1982, 1984) Other screening materials also reduce or eliminate macroinvertebrates beneath them (e.g., Bartodz-iej, 1992; Ussery et al., 1997)
15.2.4 BURLAP
Burlap (340 g/m2) was applied to two sites in Lake Rockwell Reservoir, Ohio Burlap at one site was lightly pretreated with Netset (Nichols Net and Twine Co., East St Louis, Illinois), a sealant and preservative Despite burlap’s porosity, “ballooning” occurred at the site with unconsolidated organic muck sediments due to high benthic metabolism and to difficulty in securing the material
in the highly fluid mud At both locations, plant growth was controlled over the growing season, but treated and untreated burlap rotted during the 3 months of placement (Jones and Cooke, 1984) Untreated burlap applications in British Columbia were cost effective, and plant growth was controlled for 2 to 3 years, after which rotting and sediment accumulation over the burlap curtailed effectiveness (Newroth and Truelson, 1984) The difference with regard to rotting in these studies may be the highly organic, biologically active, warm sediments of the Ohio Lake, where decom-position is faster Rotting of a bottom barrier could be an advantage Burlap’s cost is lower than most other materials ($7,900 per ha, 2002 prices), plus installation
Benthic barriers are effective for macrophyte control where they are firmly positioned directly
on the sediments at depths where they cannot be dislodged by waves or boat propellers “Ballooning”
of barrier material is a major problem (Gunnison and Barko, 1992), but this can be minimized by placement prior to biomass development Assuming adequate gas escape, plant control for several
Trang 4years could be possible before new plants root on the accumulated material on top of the barrier Because of cost, most uses will be for small areas
15.3 APPLICATION PROCEDURES FOR SEDIMENT COVERS
Application technique is important Barriers should be applied close to the sediments, without
“ballooning” or pockets after installation In muck-type unconsolidated sediments, this ideal cannot
be met because stakes may never be held by the soil, and bricks or cement blocks will anchor only areas where they have been placed Plant infestations also may prevent application close to the sediment There should be no gaps between barrier strips, since plants will grow there, and the option of barrier removal and repositioning should be kept in mind
The first step is to survey the lake bottom for obstructions and to test sediment ability to hold stakes SCUBA equipment is essential In fluid, unconsolidated sediment, long stakes will be required, and these should be tested for their holding ability prior to application If the sediments are too flocculent, bricks or cement blocks can be used, or link chain can be sewn into the edges
of the fabric Steel stakes usually cannot be used in gravel, hard clay, or rock bottoms, because they cannot be pushed deeply enough by the diver to hold the screen
Stakes can be made from 6- to 7-mm diameter steel reinforcing bar Bend the bar at one end into an L-shaped “handle.” Stake length varies with sediment softness Sharpen the long end and drive it through a doubled layer of screen until the “handle” end is flush with the sediments Screens are efficiently applied from a reel in the stern of a rowboat The material is unrolled
by two applicators, one on each side, and staked on opposite sides every 1 to 2 m If the screening
is applied directly over vegetation, stakes should be placed at 1 m intervals to prevent lifting In deeper water, where SCUBA is used, another helper in addition to the rower will be needed to hand stakes to the divers and to assist in diving emergencies Divers will disturb sediments and visibility will be low Apply barriers perpendicular to the shoreline
Application can be improved by returning to the site to flatten bulges in 15 to 20 days, when plants beneath the screen have started to decompose This will increase application costs The ideal application time is prior to plant growth One technique is to place them on top of the ice With proper weighting, the covering material will sink at ice-out, although it is possible that
“rafts” of ice will displace the screens Placement on ice is much more effective if done with water level drawdown (Chapter 13) Screens in place for about 2 months can be removed and used elsewhere
in the lake (Perkins et al., 1980; Engel, 1982), meaning that sites covered in May and June can be uncovered and the screen moved for July and August coverage in another location Application can
be facilitated by lake-level drawdown followed by application of screen to frozen lake sediments,
or by harvesting and screen placement Complete coverage of a dense infestation of a small pond could produce a sharp decline in DO if the screening process is done over a short period
Sand “blankets” in shallow beach areas create a better bottom surface for wading, and can inhibit some macrophyte growth
15.4 SHADING OF MACROPHYTES WITH SURFACE COVERS
Reduction of macrophyte biomass through shading has received little attention because a surface cover denies use of the treated area, and because covers can be easily dislodged
Black polyethylene sheeting was used as a surface cover to control plants in a pond (Mayhew and Runkel, 1962) Polyethylene sheets were floated (specific gravity = 0.92) over 186-m2 plots, and the corners were anchored to prevent shifting Eight similar plots, populated by different
dominant species, were studied All species of Potamogeton were controlled for the entire summer
if a cover period of 15 to 21 days occurred before the plants matured (May in north temperate
latitudes of the U.S.) Ceratophyllum demersum was controlled by continuous cover of 18 to 28
Trang 5days Where plants were controlled, filamentous algae invaded and revegetated the plots The covers
did not control Chara vulgaris, Sagittaria latifolia, and emergent species.
This procedure needs further evaluation For example, swimming areas could be covered in early May and the covers allowed to remain in place for 25 to 35 days This would probably not deny the area to swimming, since water temperatures may not reach the comfortable range until June in northern latitudes If the sheeting was removed carefully, it could be reused in subsequent seasons Shade created by riparian vegetation, especially trees, can reduce littoral submersed plant growth Property owners should be discouraged from cutting trees along the shoreline
Dyes to suppress plant growth have been suggested (Eicher, 1947) Aquashade (Aquashade, Inc., Eldrod, New York) is designed specifically to shade plants in hydrologically closed systems such as ponds The active ingredients are Acid Blue 9 and Acid Yellow, and these dyes control submersed plants by filtering wave lengths of light critical to photosynthesis (Madsen et al., 1999) Aquashade is added as a concentrate, and winds disperse it throughout the pond, turning the pond
blue The manufacturer claims that the material is effective against Elodea, Potamogeton, Najas, Myriophyllum, Hydrilla, Chara, and various filamentous algae, without toxicity to aquatic life, but
effectiveness would likely be poor in water depths less than 1 m Swimming is permitted immedi-ately after application, but the material cannot be used in a potable water source Aquashade does not reduce transparency to the point that safe swimming standards are violated (Madsen et al., 1999) There is insufficient published information at this time to evaluate commercial dyes The mode of action is light limitation and not direct toxicity to the plants (Spencer, 1984; Manker and Martin, 1984)
REFERENCES
Armour, G.D., D.W Brown and K.T Marsden 1979 Studies on Aquatic Macrophytes Part XV An Evaluation
of Bottom Barriers for Control of Eurasian Watermilfoil in British Columbia Water Investigations
Branch, Vancouver
Bartodziej, W 1992 Effects of a weed barrier on benthic macroinvertebrates Aquatics 14(1): 14–16 Born, S.M., T.L Wirth, E.M Brick and J.P Peterson 1973 Restoring the Recreational Potential of Small
Impoundments The Marion Millpond Experience Tech Bull No 71 Wisconsin Department of
Natural Resources, Madison
Boston, H.L and M.A Perkins 1982 Water column impacts of macrophyte decomposition beneath fiberglass
screens Aquatic Bot 14: 15–27.
Cooke, G.D 1980 Covering bottom sediments as a lake restoration technique Water Res Bull 16: 921–926.
Cooke, G.D and M.E Gorman 1980 Effectiveness of DuPont Typar sheeting in controlling macrophyte
regrowth after overwinter drawdown Water Res Bull 16: 353–355.
Eicher, G 1947 Aniline dye in aquatic weed control J Wildlife Manage 11: 193–197.
Eichler, L.W., R.T Bombard, J.W Sutherland and C.W Boylen 1995 Recolonization of the littoral zone by
macrophytes following the removal of benthic barrier material J Aquatic Plant Manage 33: 51–54.
Engel, S 1982 Evaluating Sediment Blankets and a Screen for Macrophyte Control in Lakes Office of Inland Lake Renewal, Wisconsin Dept Nat Res., Madison, WI
Engel, S 1984 Evaluating stationary blankets and removable screens for macrophyte control in lakes
J Aquatic Plant Manage 22: 43–48.
Engel, S and S.A Nichols 1984 Lake sediment alteration for macrophyte control J Aquatic Plant Manage.
22: 38–41
Gunnison, D and J.W Barko 1992 Factors influencing gas evolution beneath a benthic barrier J Aquatic
Plant Manage 30: 23–28.
Helsel, D.R., D.T Gerber and S Engel 1996 Comparing spring treatments of 2,4-D with bottom fabrics to
control a new infestation of Eurasian Watermilfoil J Aquatic Plant Manage 34: 68–71.
Jones, G.B and G.D Cooke 1984 Control of nuisance aquatic plants with burlap screen Ohio J Sci 84:
248–251
Trang 6Lewis, D.H., I Wile and D.S Painter 1983 Evaluation of Terratrack and Aquascreen for control of
macro-phytes J Aquatic Plant Manage 21: 103–104.
Madsen, J.D., K.D Getsinger, R.M Stewart, J.G Skogerboe, D.R Honnell and C.S Owens 1998 Evaluation
of transparency and light attenuation by Aquashade Lake and Reservoir Manage 15: 142–147.
Manker, D.C and D.F Martin 1984 Investigation of two possible modes of action on the inert dye Aquashade
on hydrilla J Environ Sci Health A 19(b): 725–753.
Mayhew, J.K and S.T Runkel 1962 The control of nuisance aquatic vegetation with black polyethylene
plastic Proc Iowa Acad Sci 69: 302–307.
Newroth, P.R and R.L Truelson 1984 Bottom barriers to control rooted macrophytes LakeLine 4(5): 8–10 Nichols, S.A 1974 Mechanical and Habitat Manipulation for Aquatic Plant Management A Review of
Techniques Wisconsin Dept Nat Res., Madison.
Nichols, S.A and B.H Shaw 1983 Review of management tactics for integrated aquatic weed management
of Eurasian watermilfoil (Myriophyllum spicatum) curly-leaf pondweed (Potamogeton crispus) and elodea (Elodea canadensis) In: Lake Restoration, Protection and Management
USEPA-440/5-83-001 pp 181–192
Perkins, M.A 1984 An evaluation of pigmented nylon film for use in aquatic plant management In: Lake
and Reservoir Management USEPA 440/5-84-001 pp 467–471.
Perkins, M.A., H.L Boston and E.F Curren 1980 The use of fiberglass screens for control of Eurasian
watermilfoil J Aquatic Plant Manage 18: 13–19.
Petersen, J.O., S Born and R.C Dunst 1974 Lake rehabilitation techniques and experiences Water Res Bull.
10: 1228–1245
Pullman, G.D 1990 Benthic barriers tested LakeLine 10(4): 4,8.
Spencer, D.F 1984 Influence of Aquashade on growth, photosynthesis, and P uptake of microalgae, J Aquatic
Plant Manage 22: 80–84.
Ussery, T.A., H.L Eakin, B.S Payne, A.C Miller and J.W Barko 1997 Effects of benthic barriers on aquatic
habitat conditions and macroinvertebrate communities J Aquatic Plant Manage 35: 69–73.