Introduction

 Lake Rousseau, also known as the Backwaters, is a one hundred year old, 4,000-acre impoundment. This man-made reservoir is bordered on the north by Levy and Marion Counties and on the south by Citrus County.  The dividing line is the old Withlacoochee River channel, which generally meanders through the middle of the lake.  When the reservoir was impounded in 1909, trees were not cut, leaving numerous stumps and logs, which remain today.  The reservoir is supplied by water from two main sources, the Withlacoochee River and Rainbow River.

Input from Stakeholders

 On January 22, 2014 a public meeting was held at the Lake Rousseau Campground and RV Resort.  Input was received from stakeholders regarding aquatic plant management activities on Lake Rousseau.  An online survey was provided again this year so that stakeholders could more easily provide recommendations and to assess stakeholder satisfaction with Lake Rousseau’s aquatic plant management program.

People described their relationship with the lake as concerned citizens, fishermen, boaters, wildlife viewers and paddlers.  Most responded that they used the lake more than 20 days per year.  The main concerns about aquatic plant management relate to ecological effects of chemical control, effects on duck hunting, fishing quality, and boating access.  Regarding the amount of hydrilla in the lake last year, thirty-four per cent of the forty-eight responders indicated that the control was about right or too little.  Thirty-five per cent responded that there had been too much hydrilla.  When asked about the amount of water hyacinth and water lettuce on the lake this past year, thirteen per cent of responders indicated that it was too little, thirty-three per cent thought it was about right, and twenty-one per cent indicated that there had been too much. Sixty-one per cent thought that the number of boat trails and channels on the lake were about right or too few.

Primary Lake Uses

 FWC’s Hydrilla Management Position Statement directs FWC biologist to determine the primary uses of the water body using a two tiered approach.

The FWC tier one considerations for Lake Rousseau are navigation, irrigation, flood control and hydropower.  Navigation is extremely important on a reservoir filled with stumps.  The main channel and associated boat trails must be continuously maintained free of emergent, floating and submersed aquatic vegetation to maintain access and navigation on the lake.  The lake has two water control structures, which play an important role in flood control.  During lake meetings, it has been pointed out, that if these structures get dammed up by aquatic plants, flooding may occur.  The bypass structure which releases water to the lower Withlacoochee River at a maximum rate of 1,540 cubic feet per second cannot release adequate amounts of water during high rainfall conditions. When flows exceed 1,540 cubic feet per second the excess water has to be released through the main dam into the Cross Florida Barge Canal.  From a presentation at a Citrus County Task Force meeting, a hydropower plant is proposed at the site of the bypass spillway.  The presentation indicated that this plant may come online around 2015, but no work has begun at this time.  No large agricultural related surface water withdrawals occur from Lake Rousseau; however some homeowners irrigate their lawns using lake water.

The FWC tier two considerations for Lake Rousseau are angling, recreation, fish and wildlife habitat, waterfowl and economic considerations.  Angling and boating were primary considerations of those responding to the FWC survey.  Most anglers and duck hunters that responded to the survey want hydrilla coverage well beyond the areas selected for hydrilla to remain.  Access for recreational activities such as boating, fishing, duck hunting and wildlife viewing was important to most people.  The removal of stumps to accommodate recreational activities such as water skiing or personal water craft has not been looked on favorably by lake residents in the past.  Most of the stakeholders responding to the FWC survey live, fish, hunt or view wildlife on this lake.  There are no less than ten bait stores and fish camps that derive a large part of their income from Lake Rousseau.

 Aquatic Plant Management Priorities

The Florida Aquatic Weed Control Act-369.20 (2) Florida Statutes states, “The Fish and Wildlife Conservation Commission shall direct the control, eradication, and regulation of noxious aquatic weeds and direct the research and planning related to these activities, as provided in this section, so as to protect human health, safety, and recreation and, to the greatest degree practicable, prevent injury to plant and animal life and property.”

In order to follow this statutory mandate on Lake Rousseau, the first priority will be to keep the invasive non-native floating plants (water hyacinth/water lettuce) under maintenance control.  The next priority will be to keep established navigation channels and boat trails open and to control any plants blocking access and navigation from public boat ramps.  The third priority will be to keep open areas for fishing in dense hydrilla mats, as technology, current conditions, and funding will allow.  There was little support at the meeting or in the survey from lake users for hydrilla control beyond keeping the boat trails open.

Survey and Monitoring

Aquatic plant control conducted on Lake Rousseau is accomplished using herbicides that are registered with both the U.S. Environmental Protection Agency and Florida Department of Agriculture and Consumer Services for use specifically in water.  Prior to all herbicide applications, the lake is surveyed by the FWC aquatic plant biologist.  These inspections allows the biologist to determine if floating plant levels require treatment or whether boat trails are too narrow for safe navigation, blocked by tussocks or hydrilla maintenance is necessary to sustain navigation.   After the work schedule is completed, the FWC aquatic plant biologist inspects the lake to determine if the work has been completed satisfactorily.  If any problems are noted then these are discussed with the contractor and corrected immediately.

Maintenance Control of Floating Plants

 Invasive exotic plants grow rapidly and like a lawn, cannot be managed just once a year.  Maintenance control is a management strategy in which plants are controlled on a routine basis in order to prevent small populations of invasive aquatic plants from expanding into large problems.  Keeping floating plants under maintenance control has numerous benefits.  First, it reduces the environmental impacts from these invasive exotic weeds.  Floating mats of water hyacinth and water lettuce can contribute to lower dissolved-oxygen levels, deposit tons of senescing plant material on the bottom, provide ideal breeding environments for mosquitoes, and dam up structures creating flooding problems.  Maintenance control also prevents aquatic weeds from taking over large areas of the lake, which maintains more of the lake in a condition that will support native plants for fish and wildlife habitat, afford the public better access for recreation, and protects the flood control functions of the system.  Frequent small-scale herbicide applications also reduce management costs and herbicide use.

Water lettuce filled the barge canal and lake in 2005 when plant control had been suspended for several months during the summer growing season after a prolonged period with low dissolved oxygen levels in the lake

Applicators target only the floating plants, but sometimes these are mixed in with water lilies and other floating mats of vegetation.  After each treatment, the work is inspected by FWC staff to ensure that as little non-target impact as possible occurs.  Low rates of the herbicides, diquat or flumioxazin are used to control floating plants on Lake Rousseau.  At these rates, spotting or browning of some non-target plants may occur, but these plants generally recover quickly and flourish without the competition from the non-native floating plants.  A newly registered herbicide, flumioxazin (Clipper) has been applied and evaluated to control water lettuce on Lake Rousseau.  This herbicide has shown good control of water lettuce at low rates and will be used as another method of control for water lettuce to lessen the chance of herbicide resistant plants developing from the long-term use of diquat.

Boat Trails

 Keeping established boat trails open for access and navigation requires the removal of floating, submersed, and sometimes emergent vegetation.  Floating vegetation in boat trails or hanging on channel markers is maintained with the herbicide glyphosate.  Submersed vegetation is maintained using the herbicides, Aquathol K, Aquathol Super K and diquat.

Boat Trails can fill in quickly without continual monitoring and maintenance

Hydrilla Control

FWC‘s management strategy (FWC Hydrilla Management Position Statement) regarding the invasive submersed aquatic plant hydrilla is attached.  The implementation guidelines of the Hydrilla Management Position Statement include the following steps: 1) obtain external stakeholder input; 2) determine the primary uses of the water body using a two tiered approach; 3) draft a management plan using the input from external stakeholders; 4) obtain internal stakeholder input on the draft plan; 5) refine the plan based on internal stakeholder input if necessary.

Generally speaking, hydrilla can provide ecological benefits at low levels, but must be routinely managed or it will quickly expand and impair the multiple uses and functions of a lake.  Since FWC began monitoring hydrilla levels on Lake Rousseau in 1980, hydrilla has covered over 1,000 acres of lake surface during 16 different years; the most recent events were recorded in 2007 at 2,300 acres and 2013 at 1,335 acres.

Duck hunters and bass fishermen want hydrilla at various levels, densities, and locations.  Lake residents want unimpeded navigation throughout the lake.  When areas of the lake become overgrown, boat trails along the shoreline are maintained just beyond the docks, providing a trail through the hydrilla to create access for as many lake users as possible.  These access trails are maintained using the herbicides, Aquathol K, Aquathol Super K and diquat. 

The optimum time to control hydrilla is in the late winter or early spring before it becomes a problem. Hydrilla is actively growing while most native plants are

Hydrilla mats on Lake Rousseau in 2001

dormant, improving control selectivity. Less herbicide is required to control young hydrilla that has not built up carbohydrate reserves, resulting in more thorough control and a slower hydrilla recovery. Dissolved oxygen levels are generally at their highest levels to improve decomposition of controlled plants.

Since hydrilla can grow quite rapidly, as much as four inches per day during the peak growing season, waiting for summer to control hydrilla is extremely expensive and much less effective, and provides only short-term control.  Controlling large mats of hydrilla in the summer involves substantial risk in that already low oxygen levels may be further suppressed below levels to sustain fisheries. Since hydrilla consumes oxygen at night and on cloudy days, even a live mat of hydrilla is a concern in this system that has such low oxygen levels when summer rains flush organic material from the Withlacoochee River and its adjoining swamps.

Stakeholders have requested that some areas of hydrilla remain. Hydrilla areas have been selected with input received from fishermen and duck hunters as well as FWC alligator, fishery and waterfowl biologist.  These areas need to be away from residential properties and flood control structures.    One person’s comments were “Fine line between recreation and preservation. The 1960s and 1970s ducks were thick but fish die offs were severe. Need a balance between fish and birds. Cold water holds more oxygen so the winter grass should not have such a negative effect.”  As noted, too much hydrilla can led to biological problems within the system.  Generally speaking, leaving large areas of unmanaged hydrilla, results in quicker recovery and spread to additional locations within the lake.  Managing for higher levels of hydrilla in the system will at some time likely increase the overall cost of hydrilla management and the amount of herbicide applied to the lake.  With stakeholders requesting a reduction in hydrilla control on the lake, an increase in hydrilla could create conditions during the warmer months that in the future may lead to fish kills.  Low oxygen water combined with a large amount of hydrilla on Lake Rousseau favor conditions which may lead to a fish kill. 

Rousseau Hydrilla Areas

The areas shaded in red below are the areas that have been selected after stakeholder input to leave hydrilla unmanaged.  Boat trails located within these areas will be maintained to provide access and navigation.

Other Plant Control Options

Some stakeholders have suggested other plant control options such as biological control, drawdown, grass carp and mechanical harvesting. 

Biological Controls

Biological control insects have been released over the years on Lake Rousseau.  While there have been successes related to selective biocontrol of invasive species like alligatorweed and melaleuca, insects have proven ineffective at suppressing hydrilla, water hyacinth and water lettuce growth on the reservoir.

Drawdown

Lake managers know that stabilized water levels are not desirable for most water bodies.  Stabilized water levels allow year-round recreational use but also allow organic sediments to accumulate on the lake bottom. Without periodic flooding and drying, emergent and submersed plants can form dense masses without some sort of management intervention.  Drawdowns mimic the normal cycle of high and low water on Florida waterbodies.

Under present conditions a drawdown on Lake Rousseau would do little for weed control and possibly create some weed problems in the form of tussocks.  Drawdowns consolidate organic sediments when dried out and this creates benefits for fisheries.  Without replacing the current bypass water control structure on Lake Rousseau, the current structure does not allow lowering the lake level enough to expose adequate lake bottom and keep the required flow to the lower Withlacoochee River.  The many environmental factors presently affecting the lower Withlacoochee River would also have to be better understood and reviewed.  Most people on the lake and some from Inglis and Yankeetown have not been in favor of a drawdown in the past.  Previously, when a drawdown was discussed, lower river residents had concerns about the decreased potentiometric surface levels and the effects it might have on salt water intrusion and their wells.  A Lake Rousseau drawdown would also affect environmental conditions on Rainbow River and the upper Withlacoochee River.

Some lakes have had drawdowns that worked well and improved fisheries, but many residents and local businesses have been inconvenienced for months.  Public support has to be strong for a drawdown and in the past this has not been the case.  Some refer to Rodman Reservoir drawdowns as an example of how Lake Rousseau should be managed, however, this reservoir is different.  Water levels on Rodman reservoir can be raised to flush floating plants into surrounding wetlands.  Then water levels are quickly dropped to strand and dry the plants to kill them.  When   

the reservoir is refilled it is also surcharged to high levels for a short period of time before bringing the reservoir back down to normal levels.  These surcharge events have shown to be a large component of the success of the Rodman drawdowns.  Additional, the water level in Rodman Reservoir is fluctuated two to three feet during non-drawdown years.

Unlike the mostly unpopulated Rodman Reservoir, Lake Rousseau has numerous residences and businesses around the lake.  The lake level cannot be raised to float plants into the shallow areas along the shore without flooding lake residents or lowered enough to strand and dry plants enough to kill them.  Drawdown concerns raised in the past include: impacts to homeowners and local businesses, tussock creation and possible increases in salt water intrusion down river.  Before any drawdown is undertaken, there would have to be public meetings, considerable interagency coordination, changes to the current water control structures and funding to cover cost.  The Office of Greenways and Trails, Southwest Florida Water Management District, and the U. S. Army Corps of Engineers are agencies that will have a major involvement in addressing concerns as to whether a drawdown occurs on Lake Rousseau.

Grass Carp

The use of grass carp in Lake Rousseau has been considered several times during the last 20 years and continues to be suggested by some stakeholders.  Lake Rousseau is an open system, which means that the fish cannot be contained within the lake.  The carp could go downstream to the lower river or upstream to the Rainbow and upper Withlacoochee Rivers.  Not only would the investment in

fish be lost if this happens, but there could be large environmental impacts from the grass carp consuming not only target plant species such as hydrilla, but also native plants from areas outside of the reservoir.  For additional information, you may wish to visit the web-sites listed at the bottom of this plan.

Mechanical Harvesting

A few stakeholders suggest that FWC use mechanical harvesters for aquatic plant management on Lake Rousseau.  Due to the cost, distance to unloading sites, stumps and the non-selective nature of the harvesters, this idea is not feasible. Applying herbicides is the most cost-effective, efficient, and environmentally sound method of control for rapidly growing floating plants and hydrilla on Lake Rousseau. Costs for herbicide control are in the hundreds rather than the thousands of dollars per acre required for mechanical control.  Floating plants can be reduced from any site in a timely manner, and invertebrates, fish and wildlife are not negatively impacted when herbicides are applied properly.

Aquatic Plant Management Concerns and Challenges

Two problems confronting resource managers on Lake Rousseau are the periodic, naturally occurring high flows and low oxygen levels. Waters of the Withlacoochee River that flow from surrounding swamps and wetlands during the summer rainy season are dark tannin stained, and sometimes quite turbid.  Dark turbid water helps reduce submersed plant growth by limiting light penetration in the water column.  During prolonged high flow events, growth of submersed plants is greatly reduced. 

Flow rates are determined by natural events over which plant managers have no control.  Yet, such events dictate the type of aquatic plant control conducted and the

success or failure of such treatments.  During periods of high rainfall, organic material is flushed out of adjacent wetlands and swamps into the Withlacoochee River.  This decomposing material can substantially reduce the oxygen level in the river.  Such oxygen-depleted water can severely impact the Lake Rousseau fishery, especially in summer months when dissolved oxygen has naturally fallen below 2

parts per million (ppm) due to organic leaf litter breaking down and higher water temperatures.  When this occurs, fishing success drops off dramatically.  During the summer of 2004, after three hurricanes passed through the Withlacoochee watershed, oxygen levels were so low that large numbers of fish died in Lake Rousseau.  These natural low oxygen levels may preclude submersed plant control treatments due to the increased risk of fish kills.

The Rainbow River is a clear, nutrient rich, spring fed river, which has dissolved oxygen levels in the range of 6-9 parts per million, helping buffer the low oxygen effects of the Withlacoochee River.  However, when flows on the Withlacoochee River are reduced and clear Rainbow River water is dominant in Lake Rousseau, light penetrates through the water column stimulating submersed plant growth.  Under these conditions, hydrilla grows quickly and can reach and cover thousands of contiguous acres of the water surface in as little as one season. 

With low flows, the western part of the reservoir experienced a planktonic algae bloom in 2012.  This helped to prevent hydrilla from rapidly coming into this part of the system.  The brown/black filamentous algae (Lyngbya) has increased in the western section of the lake.  It is unknown at this time what is causing the increase in Lyngbya, but increased nitrogen levels in the system may play a role.  The recent droughts or increased nutrients from Rainbow Springs are several possible causes.  

During the summer of 2013 hydrilla became established in Smith’s Pasture, Old Mill, Peaceful Acres and the western section of the lake.  All open water areas of Lake Rousseau now have hydrilla.

Brown/Black Filamentous Algae (Lyngbya)

 Lake Rousseau has a well established bird population and maintaining a healthy environment for birds on the lake is an important part of FWC’s invasive plant management program.  For the past 30 plus years, Lake Rousseau’s bird population and FWC’s Invasive Plant Management program that is designed to conserve or enhance native plant diversity and habitat have coexisted without problem.    Species of concern observed on Lake Rousseau include: brown pelicans, Florida sandhill cranes, limpkins, little blue herons, snowy egrets, tricolored herons, white

ibis, and wood storks.  Marion County Audubon has documented 152 different species of birds around the lake.

An additional challenge for plant managers is that bird nesting and the invasive plant growing season overlap.  Nesting birds are found all around Lake Rousseau and in order to maintain invasive plants at a reasonable level some herbicide control will be required near nesting birds.  As in the past, herbicide applicators will continue to use caution and good judgment while operating near these areas.  Lake Rousseau’s herbicide applicators are instructed to: 1) reduce noise by idling boats as much as possible and trying to avoid revving them up to a high rpm around rookery areas, 2) carefully observe bird behavior while applying herbicides, and 3) exit areas adjacent to rookeries immediately if birds leave their nests or an upflight (large numbers of birds exiting a rookery) occurs. If such disturbance occurs, applicators are instructed to return later in the day or during another application time period when nesting is completed if possible.

Plant management around rookeries and nesting sites will be conducted in a manner to best allow native aquatic plants to remain for the protection and foraging of birds and other wildlife.

In closing, the complexities and challenges of invasive aquatic plant management require a good management plan to be adaptive with its management strategies, as lake and environmental conditions change with time.

For additional information on invasive plant management, please visit the following websites:

http://www.myfwc.com/wildlifehabitats/invasive-plants/

http://plants.ifas.ufl.edu/manage/