Submerged Aquatic Vegetation Control

Origin

Eurasian Watermilfoil

Eurasian Watermilfoill was introduced to the United states unintentionally in the 1950s, and has since spread throughout the nation. It grows in lakes and rivers forming dense mats on the surface to severely restrict the use of the waters. It is fragile, and stems broken by boat propellers, passage of personal watercraft and mechanical harvesters, sprout roots to promote new growth from each small fragment.

Eurasian Watermilfoil is now prohibited by law from being owned or transported in the United States.

Hydrilla

Hydrilla was brought to the United States for sale as an aquarium decoration in the 1950s from Asia. It was planted in lakes and canals for harvest and sale through pet shops. It grows from submersed corms and when it reaches the surface hydrilla grows across the surface to form dense tangled mats, impeding boat traffic and causing flooding by jamming against bridges and blocking drainage works.

New plants sprout from broken stems or root buds (tubers). It is now prohibited by law from being owned or transported in the United States.

How they spread from place to place

Each is spread from one water body to another as fragments attached to boats, trailers or vehicles or attached to wildlife, and transported by water currents. Now, almost every state suffers from infestations, and yet these invasive speciesA taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety, composed of individuals possessing common characters distinguishing them from other categories of individuals of the same taxonomic level. In taxonomic nomenclature, species are designated by the genus name followed by a Latin or Latinized adjective or noun. continue to be sold through aquarium supply dealers and on the Internet.

In a water body such as Crystal River/Kings Bay, they are particularly spread by being chopped by boat propellers.

The Damage They Do

These invasive plant species cause major detrimental impacts upon water use. In drainage canals they greatly reduces flow, which can result in flooding and damage to canal banks and structures. In irrigation canals they impede flow and clog pump intakes used for conveying irrigation water. In utility cooling reservoirsA natural or artificial pond or lake used for the storage and regulation of water. they disrupt flow patterns that are necessary for adequate cooling of water. They can severely interfere with navigation of both recreational and commercial craft. In addition to interfering with boating by fisherman and water skiers in recreational waters, they interfere with swimming, shade out and displace native vegetation communities, and can adversely impact sport fish populations. Decaying plant material reduces dissolved oxygenMeasures the amount of gaseous oxygen dissolved in an aqueous solution. Oxygen gets into water by diffusion from the surrounding air, by aeration (rapid movement), and as a waste product of photosynthesis. levels. Birds and fish are adversely affected as their feedstock becomes less accessible.

The economic impacts of such damage upon real estate values, tourism, and user groups can be staggering. For example, an economic study on Orange Lake in North Central Florida indicated that the economic activity attributed to the lake was almost $11.0 million and during years that hydrilla completely covers the lake these benefits can be virtually lost (Milon et al. 1986). Cost of invasive plant management is also extremely high, especially when funding is insufficient for adequate management. An estimated $10.0 million is necessary to manage hydrilla alone in Florida public waters in 1994-95 and $14.5 million was necessary in 1995-96, as infestations continued to spread.

The Good They Do

Before migrating to richer pasture, it has been reported anecdotally, that manatees feed on the milfoil to last them on their journey. In the absence of natural feedstock, for example eelgrassA marine plant with long ribbon-like leaves. [Zostera marina.], or, a submerged aquatic plant with narrow, grass-like leaves. [Genus Valisneria.] and nyad, the manatees will feed on hydrilla.

Some sport fishermen consider hydrilla to benefit largemouth bass habitat (Tucker 1987). While the opinion that hydrilla is beneficial for sport fish production is supported by certain research (Estes et al. 1990; Porak et al. 1990), other research suggests that largemouth bass are adversely affected when hydrilla coverage exceeds 30% (Colle and Shireman 1980). Canfield and Hoyer (1992) found no relation between standing crop of harvestable largemouth bass and percent area covered with aquatic macrophytes in 60 Florida lakes (many dominated by hydrilla).

Clear water is valued by the public and large populations of submersed aquatic macrophytes, such as hydrilla, will tend to increase water clarity (Canfield et al. 1984). The exact reasons for this increase in water clarity are not completely understood but it probably results from a combination of factors which include reducing sediment resuspensionThe suspension of particles in the water column after having once been settled and disturbed. and reduction of phytoplanktonSmall photosynthetic organisms, mostly algae and bacteria, found inhabiting aquatic ecosystems. Also see plankton and zooplankton. populations by compartmentalizing nutrientsAny food, chemical element or compound an organism requires to live, grow, or reproduce.. Regardless, very large amounts of aquatic macrophytes would be necessary to cause substantial increases in water clarity (Canfield et al. 1984; Canfield and Hoyer 1992).

Hydrilla is eaten by waterfowl, and maintaining hydrilla populations is sometimes advocated by waterfowl scientists because it increases the feeding habitatThe place or set of environmental conditions in which a particular organism lives. for ducks (Johnson and Montalbano 1984, Esler 1989). Attempts to benefit sport fish or waterfowl habitat, or produce clearer water has led to deliberate dispersal of hydrilla by individuals unwary of the severe detrimental impacts that can be caused by these plants.

In Conclusion

Obviously, the use of mechanical harvestersFloating machinery driven by paddlewheels for lifting algae and plants from a water body and transporting them for disposal risks leaving chopped residuesWhat is left over or remains; the part of a molecule that remains after portion of its constituents are removed. Residues of some contaminants may remain after behind to grow again more densely than before. The literature describes draw down of the water in lakes and ponds to access and remove the plants, but this is of little use in a river run. Herbicides are effective but result in ecologic shock when applied over larger areas, and harmful residues of muck accumulate in sedimentsSolid material that has been or is being eroded, transported, and deposited. Transport can be due to fluvial, marine, glacial or aeolian agents., allowing persistent bioaccumulative toxins to occur.

Detriments caused by water-milfoil and hydrilla far outweigh benefits and they are usually more difficult to manage than native plant populations which they displace. It seems clear that before water qualityA term used to describe the chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose. restoration can be undertaken effectively, some method of isolation and removal of these plants should be determined, so that progressive replacement by native plants can take place.