A Primer on Aquatic Plant Management In
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Lakewide / Whole Lake Management Activities 1. Mechanical Harvesting • Principle Mechanical harvesting is the physical removal of rooted aquatic plants (macrophytes) from the lake using a mechanical machine to cut and transport the vegetation to shore for proper disposal. This is one of the most common methods of aquatic vegetation control in New York State. The physical removal of macrophytes serves to eliminate the symptom of excessive vegetation growth.. Immediately after harvesting, swimming and boating conditions are improved. Harvesting also serves to remove the nutrients, primarily phosphorus, stored in the plant structure thereby addressing one contributor to the cause of excessive rooted vegetation growth. There are two different types of mechanical harvesting operations, single-stage harvesting and multistage harvesting. Typically single-stage mechanical harvester cuts a swath of aquatic plants from six to ten feet in width and from six to eight feet in depth. The harvester usually has two upright cutting bars and a vertical cutting bar. The cut vegetation is transported up a conveyer belt and stored on the harvester. The maximum capacity of the harvesting barge is usually between 6,000 to 8,000 pounds (wet weight) of aquatic plants. The harvester transports the plants to shore where they are unloaded via a shore conveyer to a truck for disposal. The multistage harvester refers to two or more specialized pieces of equipment. The first machine moves through the lake with cutting bars similar to the single stage harvester, cutting the vegetation and allowing the plant's natural buoyancy to bring it to the surface. A second machine follows the cutter and rakes up the cut fragments for disposal. The cutting capabilities for the multistage harvester can be greater than the single-stage harvester; the depth can extend as far as ten feet and the width can be up to twelve feet. With either harvesting method, the growth rates of some species of aquatic plants may require two or more harvests during the recreational season. This increases the costs and, especially when outside contractors are involved, can create scheduling challenges. • Target Plants and Non-Target Plants These techniques are generally non-selective since the mechanical harvesters cut most to all plants contacting the cutting bar. The machines cannot be easily maneuvered to selectively remove target plant species within diverse beds, particularly near the lake shoreline. Selectivity is limited to targeting only plant beds comprised of a single plant species. In recent years, most mechanical harvesting operations in New York State have targeted Eurasian watermilfoil. Historically a wide range of native plants, from submergent plant species such as Potamogeton amplifolius (large-leafed pondweed), and floating leaf plants such as water lilies, have been the target of harvesting efforts. • Advantages Simply stated, mechanical harvesting works to remove excess vegetation. Management of macrophytes can be limited to boat channels, launch sites, swimming areas, other high use areas or areas where weeds cause safety concerns. 35 Although mechanical harvesters are Case Study- Mechanical Harvesting slow-moving beasts, they provide Lake Setting: Saratoga Lake is a 4000 acre, heavily used immediate relief from surface canopies recreational lake in Saratoga County, at the foothills of the Adirondack Park. and dense underwater growth of nuisance plants. The tops of the aquatic The Problem: High development pressure and recreational use in the 1960s and 1970s resulted in degraded water quality and plants are cut, removing the growing impaired use of the lake for most recreational activities. At the leaves, nutlets and flowering parts of time, more than 50% of recreational users of the lake objected to the algae levels and water clarity (Koojoomjian and Clesari, strongly rooted plants. Weakly rooted 1973), and water clarity had dropped from about 5 meters in plants may be uprooted. For aquatic 1932 (with fully oxygenated conditions throughout the lake) to about 1.5 meters in 1967, with oxygen deficits beginning at a plants that propagate primarily from depth of about 6 meters. seed banks or nutlets, such as water In the 1970s, water quality improvements resulted from the chestnut, removing the top of the plant diversion of municipal wastewater out of the watershed (one of (which usually carries the seeds) prior to the inflows was locally called “Gas Brook” due to the persistent sewage smell), the implementation of non-point source control the maturation of the seeds can eliminate measures on agricultural lands, and nutrient inactivation- these the following year of growth. Multiple activities were funded in part by a federal Clean Lakes Project. However, in response to the increased water clarity, nuisance years of harvesting may serve will growth of Eurasian watermilfoil and curly-leafed pondweed gradually deplete the bank of seeds in dominated the littoral zone to a depth of about 4 meters. This resulted in a shift from an algae- to a macrophyte-dominated the sediments. Harvesting operations, as system, without significant improvement in recreational opposed to cutting, will remove the conditions (although walleye and bass fisheries may have improved). However, 75% of the lake residents indicated that nutrients stored within the plant the lake was “somewhat” to “much” clearer (Boylen et al., material. It has been estimated that this 1995). Water clarity improved from about 1.5 meters in 1967 to more than 3 meters by the mid-1990s (and higher in the late may comprise as much as 50% of the 1990s due to the introduction of zebra mussels). internal (sediment-bound) load of Response: The Saratoga Lake Protection and Improvement nutrients that might otherwise migrate District (SLPID), a local management and taxing authority into the overlying water and become authorized by the NYS Legislature in 1986, oversaw the use of two mechanical weed harvesters purchased in 1984 that cut from available for algae growth. 500-750 acres of nuisance vegetation per year, operating daily from May through September. The biomass of the major macrophyte species in the lake did not experience significant Harvesting will usually result in change between 1982 and 1994, when an aquatic plant survey continued blanketing of the lake floor by was conducted by Darrin Freshwater Institute: the lower portion of standing aquatic Species: Range of Range of plants. This will provide continued Biomass, 1982 Biomass, 1994 Eurasian watermilfoil 40-1000 g/m2 0-700 g/m2 cover and habitat for fish and other Curlyleaf pondweed 0-170 g/m2 0-250 g/m2 aquatic life at the same time that Southern naiad 10-400 g/m2 0-450 g/m2 recreational uses are supported by the Eelgrass 0-40 g/m2 0-600 g/m2 2 2 reduction or loss of the plant canopy. Water stargrass 0-140 g/m 0-30 g/m • Disadvantages The most significant side effect of mechanical harvesting is fragmentation. Fragments of cut plants that are not picked up and removed can move from the treatment area by wind or currents, spreading the plant to other portions of the lake or to downstream water bodies. This can result in enhanced propagation of those plants that spread primarily from fragmentation, such as milfoil. 36 Case Study- Mechanical Harvesting (cont) Plant communities may be altered by harvesting. If both native and fast-growing Some species were more abundant in 1982, while others were more abundant in 1994. Eurasian watermilfoil populations exotic plants are cut to the same degree, were substantially reduced in shallower water- up to depths of the exotic plants, often the original target about 1 meter- but this was probably due to the winter drawdown regularly conducted each year. By the early for harvesting, may grow faster and 1990s, in the midst of the harvesting program (and dominate the plant community. This is supplemental work in shallower areas with a suction harvester), more than 90% of the lake residents identified especially true for plants that propogate by rooted aquatic plants as at least a minor problem. This fragmentation. included impacts due to weed decomposition and floating weeds cut by boats or harvesters. 40% identified this problem as significant. However, about 60% viewed the harvesting An improperly designed or executed program as successful (versus about 70% for the sewering and drawdown conducted through the Clean Lakes Program). harvest can have other unnecessary side efforts. Small, slowmoving fish may be The harvesters were replaced by larger, more efficient machines in the late 1990s, and the SLPID has been trapped in the cutting blades or removed investigating an integrated approach to aquatic plant by the conveyer. If all cut vegetation is management, conducting small-scale experiments since 2000 on the use of aquatic herbicides and herbivorous insects not removed, oxygen levels may (while continuing the use of the mechanical harvesters). temporarily fall and nutrient levels, such Lessons Learned: Mechanical harvesting may not result in a as phosphorus, may rise. Turbidity significant reduction in aquatic plant density or coverage, but resulting from the harvesting process is it may be viewed favorably by many lake residents, particularly in light of (what may be perceived as) less also usually short-term. palatable alternatives. For a lake this size, however, it is an expensive operation. The logistics involved with harvesting Sources: Boylen, C.W., L.W. Eichler, and T.B. Clear. 1995. result in some disadvantages to the use of An aquatic plant assessment of Saratoga Lake. RPI publication. Troy, NY. this technique. Many lakefront property Hardt, F.W., G. Hodgson, and G.F. Mikol. 1983. owners are frustrated with the inability of Saratoga Lake Phase I Diagnostic-Feasibility Study and Management Plan. USEPA Clean Lakes Program. 236pp the harvesting equipment to operate in Kooyoomjian, K.J. and N.L Clesari. 1973. shallow areas near docks and shorelines. Perception of water quality by selected groupings in inland water-based recreational environments.