Aquatic Plant Management on Lake Istokpoga

By Beacham Furse, Aquatic Resources Conservation Manager Lake Istokpoga/Southern Lake Wales Ridge Project

One of the most common questions we get about management of Lake Istokpoga is “Who treats the “weeds” on the lake?”. Often, people will just say “they” or “the state” treats plants on the lake. However, there are three agencies responsible for aquatic plant management on lakes in Highlands County. The Florida Department of Environmental Protection’s (FDEP) Bureau of Invasive Plant Management is responsible for the “control, eradication, and regulation of noxious aquatic weeds”, such as hydrilla, water hyacinth, and water lettuce, under the authority of the Florida Aquatic Weed Control Act (Florida Statute 369.20). The Highlands County Operations Department Aquatic Weed Program (HCAWP) works as a contractor for FDEP to manage and control water hyacinth, water lettuce, and other aquatic weeds which creates problems in Highlands County. The FDEP and HCAWP also work in coordination on the management and control of hydrilla in Lake Istokpoga and other Highlands County lakes. So, where does that leave the third agency, the Florida Fish and Wildlife Conservation Commission?

Management of cattail, pickerelweed, and other invasive aquatic plants not generally targeted for management by FDEP or HCAWP is part of the Florida Fish and Wildlife Conservation Commission’s (FFWCC) Aquatic Habitat Management Program for Lake Istokpoga. Since water level regulation was implemented on Lake Istokpoga in the early 1960’s, the lake’s littoral communities have undergone a dramatic change. The inability to flood the lake and its floodplain to historical levels, lack of sufficient seasonal and annual fluctuation, maintenance of a rigid water level regime (i.e., same water level during the same time of year on an annual basis), and inadequate management of invasive aquatic plants has encouraged expansion of cattail, pickerelweed, and other aquatic plants in Lake Istokpoga. Extensive bands of dense cattail allow the development and expansion of tussocks and tussock precursors (e.g., pickerelweed, water primrose, burhead sedge, water hyacinth, etc.) by reducing wind and water movement throughout littoral areas. This lack of water movement limits flushing of detritus on which tussocks may form. Dense monotypic stands of cattail, pickerelweed and other tussock-forming species not only displace more diverse aquatic vegetation communities, but also increase the deposition of organic detritus on the lake bottom. Although some animals exploit tussock and tussock precursors for nesting, foraging, and protective areas, the associated loss of diverse native littoral plant communities and sandy benthic substrates reduces the function of this shallow-water habitat.

FFWCC’s Aquatic Habitat Enhancement Program is broken into three components with the primary goal of the program being enhancement and management of Istokpoga’s aquatic habitat for the long-term benefit of fish and wildlife and the people who utilize those resources. The three program components include:

a) mechanical removal or consolidation of tussock and associated organic sediments;

b) establishment of native aquatic plant species through natural recolonization or revegetation with desirable native aquatic plant species;

c) management of future tussock formation through control of invasive aquatic plants with herbicides and aquatic harvesting.

Management plans are formulated each year by the FFWCC Aquatic Resource Manager for Lake Istokpoga (that’s me, Beacham Furse). These plans are reviewed by a multi-discipline team of FFWCC biologists (experts in wetland ecology, non-game wildlife [wading birds, ospreys, snail kites, etc.], game wildlife [alligators, waterfowl, etc.], and fisheries). Each plan is then presented to the Lake Istokpoga Management Committee, a citizens’ advisory group formed to advise the Highlands County Board of County Commissioners concerning management of Lake Istokpoga. Technical advisors invited to participate in this group include, but are not limited to, FDEP, Highlands County, South Florida Water Management District, U. S. Fish and Wildlife Service, U. S. Army Corps of Engineers, and Audubon of Florida.

In spring 2001, the FFWCC conducted an aquatic habitat enhancement drawdown on Istokpoga, in which 1,308 acres over 21 miles of shoreline were scraped of tussock and organic material. These enhanced areas provide sandy substrate on which diverse native plant communities are encouraged to grow. However, pickerelweed, as the primary aquatic plant invader in these sites, becomes dominant and creates monoculture stands, which begin to exclude other native plants. Also, since 1998, the FFWCC has maintained a seasonal aquatic harvester program around Big Island, Bumblebee Island, and Henderson’s Cove to control tussock expansion and reestablish productive littoral habitat. This program is limited, however, by water depths at the machinery can effectively operate and high program costs (>$5,000 per acre). An aquatic plant management program, which includes herbicide management of invasive plants and revegetation with desirable native species, is necessary to provide more diverse habitat, which promotes more diverse fish and wildlife utilization.

Invasive aquatic plants are controlled with aquatic herbicides to encourage recruitment of native plant communities and increase the diversity of aquatic plants throughout the lake. Invasive species targeted include, but are not limited to, pickerelweed (dense growth), cattail (dense growth), burhead sedge, water primrose, water hyacinth, and water-lettuce. A long-term maintenance program has been implemented in coordination with the HCAWP and FDEP to maintain diverse vegetation communities and prevent formation of tussocks. As part of FFWCC’s aquatic plant management program, dense stands of cattail, pickerelweed, and other tussock precursors are managed with herbicides by helicopter or airboat (only a quarter to one-half of the target plants within each treatment area are actually treated) to leave bands of cattail and pickerelweed, which allows other native plants to develop and are better utilized by fish and many wildlife species. In some areas, dense cattail and pickerelweed stands are left available for use by wildlife as nesting, roosting and protective habitat. Bulrush (buggy-whips), maidencane, knotgrass (Kissimmee grass), spikerush, and other “desirable” native vegetation are not sprayed. The active ingredients in the herbicides used in the aquatic plant management program are glyphosate and 2, 4-D Amine. These are the same active ingredients used in herbicides homeowners use to treat weeds in their yards; however, the herbicides used to manage aquatic plants are specifically tested and approved by the U. S. Environmental Protection Administration for use in the aquatic environment (In other words, the “Round-up” or “Weed-B-Gone” you use in your yard should not be used to treat aquatic plants behind your house.).

For more information about aquatic plant management or lake ecology, you may contact Beacham Furse at (863) 462-5190 or Gary Warren at (352) 392-9617.