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Aquatic plant managers refer to these islands as tussocks, floating islands or floating forests. In this section, we will discuss the origins, impacts, utilization, and management of these phenomena which can be extremely expensive, costing the Bureau of Invasive Plant Management $8.6 million in the 2004-2005 fiscal year.
Tussocks and floating islands are a product of the aging process of water bodies and probably have always been a part of Florida’s shallow lakes. Historically, their numbers and magnitude were altered by periodic drought and fire that controlled them within lake margins, or occasional floods that deposited them high in the uplands or in downstream marshes.
Nowadays, water levels in most of Florida’s public lakes are manipulated by some sort of human-made structures, such as weirs, dams, or levees. This manipulation eliminates the extreme high and low water events that historically suppressed tussock and floating island formation. For example, the Kissimmee Chain of Lakes that once had a water level fluctuation range of more than 10 feet now is permitted to vary only a few feet each year.
Additionally, people who have moved near lakeshores don’t particularly cherish the arrival of tussocks and floating islands.
Tussocks and floating islands aren't all bad, though. They do provide habitat for animals like marsh rabbits, alligators and rookeries for birds. They are usually comprised of native plants so resource managers, who are charged with controlling invasive exotic plants, are reluctant to remove them unless they pose imminent economic or public safety hazards, or damage to beneficial plant and animal habitat.
So how do these seemingly benign rafts of plants and organic material affect people? Lake-side residents, fish camp owners and other water users have a few stories to tell!
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onto residential shoreline of Lake Pierce |
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water surface in Lake Davis |
Imagine the dismay after having just installed that new dock or just placed those emergent plants for fish and wildlife habitat, and a two-acre mass of 10-feet tall floating cattail washes up next to your property and re-attaches to the lake bottom. Or the shock of walking into your back yard after Hurricane Frances passed over Lake Pierce and seeing 18 acres of floating island "forest" with trees to 50 feet tall - a forest that used to bother no one when it was attached to the unpopulated eastern shore of the lake? Or the bewilderment of waiting four years for the drought to end and water levels to return to Davis Lake only to see nearly 40% of the 250-acre bottom pull loose and float to the surface in 12 feet of water?
Freely drifting tussocks and floating islands are not only aggravating to waterfront homeowners. What if you were one of six fish camp operators on Orange Lake who also waited for the water levels to recover from years of exceptional drought only to have your customers’ boat trail access to the main body of the lake blocked by some of the 3,500 acres of tussocks drifting in that lake? Or what if you were one of these customers whose safe return was cut off by a drifting tussock that blocked the boat trail while you were fishing in the lake? And what if you are the emergency preparedness manager responsible for flood control in the Kissimmee Chain of Lakes or Lake Hancock and hundreds of acres of tussocks are drifting toward the outfall structures that regulate water levels in these flood control reservoirs?
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flood control structure on Lake Kissimmee |
structure on Lake Hancock |
The occurrence of tussocks and floating islands may not be new, but the need for their large-scale management is. Ten years ago, the Florida Department of Environmental Protection’s Bureau of Invasive Plant Management (BIPM) spent an average of $300,000 annually to manage all native plants including tussocks and floating islands.In fiscal year 04-05, the BIPM spent more than $8 million strictly for controlling tussocks and floating islands.
This unprecedented level of problems has put a stress on available funding, technology, and equipment to control tussocks and especially floating islands, and draws into question, Which agency or agencies are responsible for managing them?
Historically, if tussocks drift and if they block access, flood control, or navigation in established boat trails, they are managed under contract through the BIPM under Priority Level 2 of the Cooperative Aquatic Plant Control Program. Previously, tussocks were controlled under priority Level 6 if they were simply drifting or floating in a lake. Funding for tussock control or any plant control in Priority Level 6 was usually insufficient. The Florida Fish and Wildlife Conservation Commission (FWCC) has restoration funds to remove organic deposit build-up, tussocks, or floating islands when they cover or degrade fish and wildlife habitat in high priority waters. The 2003-2004 outbreaks of tussocks and floating islands taxed both agencies’ budgets, causing them to redirect funds from other important projects. For example, any tussock control funded through the BIPM diverts funds from invasive plant control, especially hydrilla management. Tussock management funded through the FWCC diverts funds from de watering and muck scraping restoration efforts planned years in advance of the current tussock and floating island problems.
OF TUSSOCKS AND FLOATING ISLANDS
It is important to define them if we are to understand how and why tussocks and floating islands are managed.
Because the numbers and magnitude of these floating masses are unprecedented, at least during the past 30 years, the terminology to define them is still evolving. Managers have borrowed from terrestrial terminology and jargon. Until recently, any floating mass of vegetation that was not a true floating plant like water hyacinth and water lettuce was termed a tussock. Webster defines a tussock as a tuft of grass. So when mats of grass or other emergent plants were drifting in a lake, calling them tussocks seemed a natural fit, especially since they can be managed using traditional herbaceous plant control methods – most notably, aquatic-registered herbicides.
But how do you characterize a floating mass of mud, peat and trees, whose herbaceous plants are susceptible to herbicides, but which will nonetheless continue to float and colonize new herbaceous plants. How do you manage such a floating mass? Clearly new terminology was needed.
Tussocks
Tussocks are rafts of herbaceous plants that form in a number of ways. They can be controlled using herbicides that kill the plants, and the entire mass will soon after sink to the bottom.1) Floating invasive plants like water hyacinth and water lettuce and even native floating plants like frog’s bit, if not managed, can form large rafts, acting as substrate for emergent plants to colonize. The emergent plants like primrose willow tie the rafts together below the surface with their roots and above the surface with stems and branches.
2) Native emergent species such as pennywort and smartweed grow from the shoreline forming mats across the water surface. If the mats become large enough, wind and wave action can tear them loose generating floating vegetation tussocks. One of the most prolific plants at generating floating tussocks is another native species, Scirpus cubensis. This grass-like sedge sends long runners among and over other emergent plants and can eventually crowd them out forming a floating mat in their place consisting exclusively of Scirpus cubensis.
3) As water levels increase after draw downs or droughts, masses of spongy-tissue plants like cattail and pickerelweed that colonized shallow mud flats during low water occasionally pull loose if the sediments are soft and flocculent, or "woolly". In 2003-2004, thousands of acres of tussocks generated in this fashion were drifting in Orange Lake, and Lakes Hancock and Apopka.
Cattail tussock on shore of Lake Hancock Cattail tussock plugging entrance
to Apopka-Beauclair Canal
Hundreds of acres of tussocks that popped
to the surface of Todd and Dodd Lakes1) Floating islands can form in the same manner as tussocks that pull loose from sediments upon re-flooding of lakes. In some cases the sediments are compact or fibrous enough that the emergent plants, whose roots are interwoven into the sediments pull as many as a several feet of organics with them to the surface as lakes re-fill. This is the case with hundreds of acres of floating islands drifting in Hernando Pool of the Tsala Apopka lake / marsh system, as well as Lakes Okahumpka and Runnymeade.
2) Occasionally, wind from storms rips up vegetation; for example in Lake Istokpoga during the 1993 Storm of the Century, or rapid water level increases and wave action tear plants loose and shred them; for example in 1994 on Lake Okeechobee. Although the vegetation is dead, it is so compacted, often several feet thick and substantial enough to walk on, that it continues to float. As decomposition occurs at the surface, this soil supports a new crop of aquatic and wetland plants whose roots help to hold the mats together. In the case of Lake Okeechobee, a subsequent drought lowered the water level and the ~15-mile long x several hundred feet wide floating island settled to the bottom forming a berm that hinders water exchange between the main body of the lake and the adjacent marsh.
Peat mat that floated to
the surface in Croft Lake
Close-up of floating peat mat
showing thickness of sediments3)In lakes with high organic content in the sediments, gasses can build up to the extent that they lift small patches of decomposing fibrous material and peat to the surface. These patches coalesce along shorelines or emergent vegetation boundaries and are eventually colonized by wetland and even upland plant species.
4)When lake bottoms with high organic (especially peat) deposits are exposed for prolonged periods, the sediments thoroughly dry. If the water level increases rapidly, before the sediments can re-saturate, several feet of the sediment layer, especially highly buoyant peat, can rise to the surface bringing with it overlying aquatic, wetland, and upland plants.
Standing on floating forest drifting
in 8 feet of water on Lake Pierce5)In extreme cases, a rapid water level increase combined with severe wind and wave action can break loose wetland and upland vegetation that had been growing on floating mats for years attached to the shoreline. Examples are the floating forests with trees up to 50 feet tall drifting in Lakes Pierce, Marion, and Apopka resulting after the series of hurricanes that crossed central Florida in 2004.
Controlling tussocks and floating
islands
Tussock management using herbicides costs $100-200 per acre (2006).
- 2,4-D is applied to control Scirpus cubensis mats or rafts of
pennywort Much of the tussock controlled with herbicides decomposes at the surface.
Some of the material falls to the bottom to break down further. Although some
organic material is thus deposited on the lake bottom, the amount is less than
what would rain down through the years if the tussock were not controlled. By
removing long-existing tussocks, wave action, oxygen, and light, cut off by
the tussock, can once again penetrate to the sediments to further break down
accumulated organic deposits and stimulate regeneration of rooted submersed
and emersed plants. Preemptive control of tussocks as they begin to form
usually results in a one-time or short-term deposition of small amounts of
organics that do not significantly impact the lake bottom.
When harvesting is the method chosen for tussock and floating island
control, harvesting is just the beginning of the removal process. Harvesting,
especially floating islands, means hundreds of tons of material to haul and
dispose, either in the lake, which has further environmental implications, or
taken out of the lake. Removing harvested material from the lake requires
conveyors, elevators or loaders to transfer material to trucks. If trucks
drive on the road, they must be sealed. Disposal sites must be secured prior
to harvesting and may require tipping fees at dumps or additional costs to
purchase land or spread material out if dumped on private property or public
lands.
Lake Hancock in Polk County is the headwaters of the Peace River.
Water exits the southwest end of the lake through an outfall canal and a flood
A shredder provided immediate control to alleviate flooding while a
shoreline track hoe removed remaining live cattail tussock and shredded material.
The Southwest Florida Water Management District installed pilings and a cable
at the mouth of the outfall canal to intercept additional drifting tussocks.
Polk County, SWFWMD and private sector crews applied glyphosate herbicide by
helicopter and by airboats to control remaining tussocks drifting in Lake
Hancock. Shredders also consumed hundreds of acres of tussocks before they could break loose and drift to the outfall structure.
Tsala Apopka is a 19,000-acre marsh/lake system on the eastern side
of Citrus County. Much of the system is only a few feet deep and is about 80%
covered by emergent and submersed aquatic plants. Water control regulations
and structures, and encroaching development prevent periodic drying, fires,
and flooding that once naturally suppressed tussocks and floating islands. The
BIPM provides funds to maintain navigation trails that connect major open
water pools to each other and public boat ramps. Citrus County funds shoreline
boat trails that lead from private residences to the state-funded navigation
trails.
Water levels rose in just a few weeks time to near-record highs in 2003
after many of the shorelines and shallow marshes were dry during the late
1990s drought. Much of the bottom that had been covered for decades with
emergent vegetation and small trees popped up bringing with it 2-3 feet of
peat and mud. Problems were most notable in Todd and Dodd Lakes within the
northern-most Hernando Pool of the lake / marsh system. Nearly 300 acres of
floating islands, that arose from the eastern, mostly unpopulated side of the
lakes drifted toward the western, populated shore, blocking navigation and
boat trails and stranding people in the lake as winds shifted during the day.
Todd and Dodd Lakes required floating island control and
several approaches were tried in order to resolve problems in a fiscally as
well as environmentally compatible manner. At first, floating islands were
harvested and removed from the lake or piled onto small existing disposal
sites within the lakes. As soon as one floating island or portion of an island
was removed from navigation and boat trails, another would break off and fill
the void. It quickly became apparent that all of the floating islands would
need to be controlled to resolve the problem. It was also evident that
costs to remove floating islands from the lakes could exceed $2 million. The
in-lake disposal sites permitted by the USACE would quickly exceed capacity if
all of the floating islands were harvested and dumped on them.
The DEP-BIPM consulted with the FWCC and other agencies and developed a
multi-faceted plan for cost-effective floating island control compatible with
the uses and functions within Todd and Dodd Lakes. Tussocks were targeted for
control with herbicides to prevent them from forming more floating islands.
About 150 acres of floating islands that had not yet drifted from their
point of origin on the east side of the lakes were targeted for shredding
provided that there were still several feet of organic deposits below them.
The BIPM did not want to drop shredded material onto sandy bottom or onto
residential shorelines and navigation trails. After consulting with FWCC
fishery managers, the DEP-BIPM was convinced that harvesting floating islands
that still had 4-5 feet of organic material would not benefit fisheries. On
the contrary, shredding floating islands would expose lake bottom that had
been covered for years to oxygen and light, and allow fish into previously
Shredding was conducted leaving at least a 50-feet wide floating island
barrier to contain shred material from escaping to the rest of the lake.
After about four weeks, floating shredded material was scheduled for re-shredding
and any remaining material, including the barrier, harvested, consistent with
techniques learned in other lakes infested with floating islands. The
shredding process took about two months.
As many as three high capacity harvesters removed floating islands that
drifted into navigation and boat trails and access points. Repeated shredding
in these areas would result in organic accumulation in areas in which it did
not originate. Harvested material was stacked up to 12 feet high onto in-lake
disposal sites. Small harvesters work along shorelines to pick up small
floating islands and drifting material from shredders and take it to conveyors
for disposal outside of the lake.
Lake Runnymeade is a 300-acre lake connected to East Lake Toho in
Osceola County. Therefore, it is part of the water management regimen that
lowers central Florida lakes during the summer months to provide water storage
in the event of tropical storm-related flooding. Low water and exposed
sediments in the summertime provides opportunities for cattail expansion as
occurred along the perimeter of Lake Runnymeade. Cattail and other emergent
species encroached far into the lake basin during the late 1990s drought. More
than 200 acres of tussocks rose from the bottom following rapid re-flooding in
2002. Floating islands were a concern from habitat and water management
perspectives – blocking water inflow as well as discharge.
An integrated management strategy was applied that included treating some
of the vegetation with herbicides, and controlling tussocks and floating islands with shredders and
harvesters. Tussocks were first shredded leaving a containment barrier of
uncontrolled plants. Sediments returned to the bottom and vegetation floated
at the surface where it was allowed to oxidize and harden.
Removal of the tussocks allowed light to penetrate into the water column
for the first time in several years. Agitation from the shredder and wave
action from opening the water surface to wind helped to consolidate and
further decompose flocculent organic material that had filled the water column
like paste prior to shredding – resulting in 6-9 feet of open water.
This page was authored by Jeff Schardt, with assistance from Alison
Moss. Karen Brown is the editor.
DEP review is by Jeff Schardt.
A collaboration of
the Center for Aquatic and Invasive Plants, University of Florida,
and the Invasive Plant Management Section of the Florida Fish and Wildlife Conservation Commission.
As mentioned earlier, the decision to manage tussocks and
floating islands is often difficult because they are usually made up of native
plants that provide habitat for fish and wildlife. Once a decision is made to
proceed with control, the most cost-effective methods that are compatible with
the uses and functions of the water body are selected and management is
coordinated among agencies and stakeholders with interests in that water body.
Tussock and floating island control usually requires integrating several methods
to alleviate the problem.
Tussock
control
Examples of tussock
and floating island control projects
In the case of tussocks, management is fairly
straightforward. Most tussock control is achieved using herbicides registered
with the USEPA and the Florida Department of Agriculture and Consumer
Services.
The most appropriate herbicide is chosen to control the target
tussock plants as well as provide the greatest measure of protection for
adjacent beneficial rooted plants. For examples:
- glyphosate is used to control rafts of para grass
-
mixtures of 2,4-D and glyphosate are applied to wild taro or frog’s bit
mats
- imazapyr or triclopyr are used to control tussocks with mixed
vegetation including woody species like water primrose and willow.
(For
more information about herbicidal control of aquatic plants, go to this page of
this web site.)
to
clear bridge after Hurricane Charlie
Occasionally, tussocks must be immediately controlled,
for example when they are lodge against bridge pilings or flood control
structures, such as can occur during a hurricane. In these cases, much more
labor intensive and expensive methods like shredders, harvesters,
barge-mounted track hoes, or draglines are employed. Since these methods are
usually applied for floating island control, they will be discussed in more
detail in that section below.
Shredders
on Tsala
Apopka
Shredders are machines with rotating
blades that shred and grind suspended organic sediment material, herbaceous
and woody plants, as well as trees up to 12 inches in diameter. Several
variations and prototypes have been tested. All units operating in Florida
lakes function with two bow-mounted counter-rotating blades that are thrust
into the floating island. Shredders need about two or more feet of water depth
to be most effective. Depending on vegetation and sediment density and composition,
shredders can dismantle up to 10 acres of floating islands each day.
vegetation island out of a lake
Harvesters
from
Caloosahatchee River in 1939
Harvesters are machines that can
lift floating island material, including sediments and small trees, out of the
water via a conveyor belt that extends from below the water surface to a
storage area on the barge. Material is then transferred to a transport barge
or hauled within the shredder to an in-lake or shoreline disposal site. Harvesters have operated
in Florida since at least the 1930s and have evolved into specialized machines
that range in width from 5 to 10 feet and in carrying capacity from 4-15 tons.
High capacity harvesters can remove up to about an acre of dense floating
islands each day.
(For more information about mechanical controls,
go to this page
of this web site.)
into
trucks at Tsala Apopka
Following are examples of
integrating several methods for controlling tussocks and floating islands in
Florida public lakes.
control structure that regulates the water
level and flow through Lake Hancock. Much of the bottom of Lake Hancock is
covered with several feet of flocculent ooze that was colonized with as much
as 2,000 acres of cattail during extreme low water in the late 1990s drought.
When the water level quickly rose about six feet in 2003, hundreds of acres of
spongy-tissue, buoyant cattail pulled loose from the sediments, drifted down
the outfall canal, and clogged the flood control structure.
to
intercept cattail tussocks
in-lake disposal site in Dodd Lake
inaccessible maidencane grass marshes.
Experience from other similar sites also showed that shredding would draw some
of the sediments from the bottom and further oxygenate them, resulting in
about four feet of open water above the sediments after the floating islands
were shred as opposed to 2-3 feet of open water if floating islands were only
harvested.
of
floating islands in Tsala Apopka
that
emerged in Lake Runnymeade in 2002
Within about 3-4
weeks after shredding, germinating plants covered the mat reducing drying from
sunlight and oxidation. At that time, the tussock remnants were harvested,
much like cutting brownies from a pan, and hauled to shoreline conveyors and
awaiting dump trucks. Material was hauled across the street to a donated
disposal site, allowed to dry further, and then spread out across the field.
Layout was by Alison Moss and
Jeff Schardt.
CAIP-WEBSITE@ufl.edu
Copyright 2006 University of Florida
