Invasive Species Management
Most nuisance aquatic plants in New England are invasive, meaning that they are non-native to the ecosystem and threaten the diversity or abundance of native species. The best (and cheapest) method for controlling invasive species is to be proactive and keep invasives from ever entering the waterbody in the first place. This can be done by washing all watercraft before entering the waterbody, having a boat ramp monitor check boats for plant fragments upon entering and exiting the waterbody, and educating the public on the harmful impacts of invasive species, as well as teaching invasive species identification.
There are several forms of aquatic plant management, falling into four main categories: mechanical removal, physical alteration, chemical management, and introduced biological predators.
Every lake is unique and therefore management plans must be tailored to meet the needs and constraints of each specific lake. Just because a management method was successful in one lake does not mean it will be successful in every lake.
How NOT to choose a management method (this applies to both invasive species and water quality management):
- The cheapest method
- The easiest method
- The method with the most websites or most aggressive sales propaganda
- Whatever seems the most practical at the time
- The method Du Jour
- Just do nothing
Diver Assisted Suction Harvesting
In this control method, divers hand pull aquatic plants, including the root system, then place the plants in a suction hose, which pulls the plants up to the boat above. Each plant is hand pulled individually so that there is less fragmentation. Additionally, because the plants are removed entirely, root system and all, they are less likely to regrow.
Mechanical Weed Harvesting
These machines cut the top 2 to 4 feet (or sometimes up to 6 feet) of plant material and pulls it up out of the water via a conveyor belt. The plant material is then deposited on shore and makes for excellent compost. Plants are cut at the stems, rather than completely removed at the roots, meaning that plants can begin regrowing immediately after the weed harvester cuts them. Additionally, some plant fragments may not be caught by the conveyor belt and can float to another part of the waterbody and reroot. Mechanical weed harvesters can not access areas that very shallow and/or too rocky.
These machines rake plant material from the sediment with a backhoe-type rake. The rake then dumps the plant material onto a floating barge or directly onto shore. These machines remove plants from the sediment, generally removing the root systems as well, and are effective at removing dense underground root and stem structures of water lilies, water chestnut, and emergent species. However, hydro-rakes generally leave a significant amount of fragmented plant material in the water, which has the potential to reroot.
Winter water level drawdowns seek to make aquatic habitat inhospitable for survival by drying and freezing the plant material. Drawdowns only affect plants that are left exposed by the drawdown and only affect perennial species. Plants that remain underwater often benefit from drawdowns. Drawdowns generally only reduce plant growth at the beginning of the growing season. By mid- to late-season, the plants usually return to pre-drawdown conditions.
Dredging is the process by which sediment is removed from the bottom of a waterbody via a suction pump to make the waterbody deeper. Excavators can also be used to scrape sediment from the bottom of the waterbody. However, this method is detrimental to the aquatic ecosystem as well as the shoreline. Dredging has the potential to control aquatic plants, but only if the waterbody is deepened to more than 12 feet (below the photic zone).
Benthic barriers are large mats that are secured to the lake bottom, covering nuisance plants. The barrier limits sunlight, preventing the plants underneath from photosynthesizing. These barriers work best on smooth, flat lake bottoms and in small areas. It will likely not be effective on rocky, uneven surfaces or on plants that can grow laterally until reaching the edge of the mat.
Aquatic herbicides are often used to combat invasive species. Each herbicide is only effective on specific species so it is important to choose an herbicide that will target the nuisance species. Herbicides must be administered by licensed applicators and permits must be issued before an herbicide treatment can conducted. Herbicides are generally very effective at weed control but many herbicides come with water-use restrictions such as irrigation, livestock watering, swimming, and/or drinking water use. These restrictions can last for anywhere from one day to multiple weeks. Aquatic herbicides generally fall into two main categories: contact herbicides and systemic herbicides.
When this category of herbicide comes in contact with plant tissue it disrupts the membrane integrity and inhibits protein synthesis. Contact herbicides only affect growing plant material and do not kill the roots, so regrowth within a year is expected.
Systemic herbicides are absorbed by the stem, leaves, or roots and move through the vascular system to kill the entire plant. These herbicides may require long contact time, meaning the herbicide may need to be applied more than once to maintain the required contact level. Some plants are not heavily affected by systemic herbicides but if the herbicide is effective and kills the plant roots then it could take years for regrowth to occur.
Introduced Biological Predators
Grass carp are vegetarian fish that feed on aquatic plants and are introduced to aquatic systems to manage watermilfoil. While these fish can eat a significant amount of plant material, they prefer almost all invasive species over milfoil, meaning that they will only begin to consume milfoil after all other plants in the lake are gone.
Weevils are herbivorous beetles that feed on watermilfoil. Unfortunately, weevils are very rarely effective at reducing milfoil abundance.