Water Regulation Schedules and the Use of Fluridone
Water regulation schedules for the Kissimmee Chain of Lakes (Lake Tohopekaliga, Lake Cypress, Lake Hatchineha, and Lake Kissimmee) and Lake Istokpoga received considerable attention at the Hydrilla Issues Workshop. This discussion was based on the direct linkage between water levels and release schedules, and the cost-effectiveness and overall efficacy of fluridone when managing FRH. It is important to note that the issue of regulation schedules and deviation requests is currently unique to fluridone applications, as other management practices are not typically impacted by regulation schedules. The participation of state and federal water managers with engineering backgrounds greatly facilitated the discussion addressing water regulation schedules and issues related to hydrilla and hydrilla control at the Workshop. The interaction of the engineers with field biologists, researchers, and resource managers focused mainly on the large flood control lakes noted above. In this section we provide an overview of the situation, describe issues associated with fluridone use and water schedules, discuss hydrilla in relation to flood control, and provide recommendations for future work in this area.
The Kissimmee Chain of Lakes (KCOL) often refers to the lakes regulated by the seven existing regulation schedules for lakes upstream of the S-65 structure on Lake Kissimmee. The following lakes are regulated under these seven regulation schedules in the Kissimmee River – Lake Istokpoga Basin Water Control Plan:
a. Lakes Kissimmee, Hatchineha, and Cypress
b. Lake Tohopekaliga
c. East Lake Tohopekaliga, Fell’s Cove, and Lake Ajay
d. Lakes Hart and Mary Jane
e. Lakes Joel, Myrtle, and Preston
f. Alligator Chain of Lakes (Alligator, Brick, Lizze, Coon, Center, and Trout)
g. Lake Gentry.
The vast majority of hydrilla problems, large-scale fluridone treatments, and issues with FRH have occurred on Lakes Tohopekaliga, Hatchineha, Cypress, and Kissimmee. Yet the proximity of lakes that contain FRH to upstream sites that are regulated by control structures is a cause for concern. In addition to the potential spread of FRH into these upstream lakes, water flows through these lakes can have a strong impact on downstream whole-lake herbicide applications. For brevity, subsequent reference in this section to the Kissimmee Chain of Lakes will be limited to the four large lakes noted above.
Large Lake Situations
Lakes Tohopekaliga, Cypress, Hatchineha, and Istokpoga have shown the capacity to support thousands of contiguous acres of hydrilla, with historic infestation levels from the late 1980s to the 2000s noted as high as 60 to 90 percent of the surface acreage on individual water bodies. In response to the potential for hydrilla to overwhelm these systems, management on these multi-purpose use lakes represents a large percentage (up to 65%) of the FDEP aquatic plant management budget. The increased reliance on fluridone for use on the KCOL and Istokpoga in the 1990s was a function of the magnitude of the hydrilla problem, increased funding availability to the FDEP, public pressure to greatly reduce the hydrilla, and early success with several large-scale applications. It is now documented that each of these lakes is dominated by FRH, and this has greatly increased the costs and technical challenges of managing hydrilla in these systems. If left unmanaged, there is a high likelihood that some or all of these lakes could support sustained and dense infestations of hydrilla that would have detrimental impacts on recreational use, access for navigation, tourism, real estate values, native plant communities, water quality, fisheries, and flood control. Given the history of dense hydrilla coverage on these lakes, some level of hydrilla management is necessary if these systems are to serve their multi-purpose functions.
The relationship between water regulation schedules and the management of FRH has become a complex topic that could, and probably will, justify its own workshop. As the authors of this document all have biology/limnology backgrounds, we will not make any recommendations regarding water schedules, as our suggested changes would likely have unforeseen impacts on other important uses of the water. We have therefore chosen to focus on the two major areas of debate that came out of the recent workshop. It is important that the readers of this document distinguish between 1) the issue of hydrilla control as it relates to regulation water schedules, and 2) the presence of dense hydrilla as a potential threat to flooding and flood control structures.
It is instructive to use the circumstances of 2004, to highlight how management philosophies impact different views of hydrilla control on the KCOL. After lowering the lake levels due to implementation of the Lake Tohopekaliga extreme drawdown,, a significant fluridone treatment (low water levels and low flow), and three successive hurricanes that impacted the KCOL in 2004, it is apparent that the current level of hydrilla biomass is much lower than recent historical levels. While hydrilla can be found throughout the lakes, it is sparse, and could hardly be considered problematic at the end of 2004. The length of time it will take to become problematic again cannot be predicted. There is one management viewpoint that says now is the time to take advantage of the low biomass and prevent the hydrilla from recovering. Another management viewpoint suggests that given the current level of infestation, large-scale management is not justified. These are honest differences of opinion; however, it is important that each group seeks to understand the other’s point of view. With or without management, it is apparent that hydrilla will continue to persist and cause problems in the KCOL for the foreseeable future. Therefore we are forced to debate the optimal amount of hydrilla and management effort that allows these lakes to serve their multi-function purpose.
Hydrilla Control in Relation to Regulation Schedules for Water
Based on comments of various agency personnel participating in this meeting, water regulation schedules are based on the multi-purpose function of these lakes and they encompass a wide range of uses including flood control, irrigation, downstream water delivery to the Kissimmee flood plain, habitat for fish and wildlife, and endangered species issues. One of the major concerns voiced by many participants was the increased frequency of deviation requests to support fluridone applications in the KCOL and Lake Istokpoga. Requests for deviations have become a yearly event, and they have coincided with the spread of FRH in these systems.
The current practice of FDEP requesting temporary water schedule deviations to reduce water levels and flow on the Kissimmee Chain or Istokpoga has strong merit when viewed solely in the context of improving the cost-effectiveness and efficacy of a fluridone treatment for hydrilla control (Table 1a and 1b). Due to the need to “maintain” much higher residues than in the past, this table reflects only the initial cost of treatment, and not the additional applications needed to sustain a target concentration.
It has been suggested that timing of the fluridone treatments be changed to increase compatibility of the treatments with regulation schedules. Regardless of the susceptibility of the hydrilla, it is well established that late winter or early spring is the optimal time to initiate fluridone applications. This treatment timing coincides with a period of low initial biomass and rapid growth as the plants start to emerge from the cool water temperatures and short days of the winter. Initiation of deviations will therefore typically be desired during the months of January through April, and limiting discharges from lakes may be desired as late as July.
Requests to the US Army Corps of Engineers for a deviation should be made from several months to a year prior to the desired deviation start date in order to allow time for the proposed deviation to be developed, coordinated, and evaluated. Based on experience with attempting to control dense stands of hydrilla on other systems, initiating large-scale fluridone treatments outside of the time-frames noted above are not recommended regardless of the water level and flow conditions. In situations where hydrilla biomass is low and active growth is occurring, treatment timing is not as critical.
Table 1a. The impact of initial water levels on the amount of fluridone required to achieve an initial target concentration on West Lake Tohopekaliga.
| West Lake Tohopekaliga | ~ Number of Acre- Feet |
~ Lbs of Fluridone to Achieve an initial 30 ppb |
~ Initial Product Cost to Achieve 30 ppb |
Full Volume |
151,000 |
12,292 |
$3.0 Million |
-1.0 Ft. Volume |
131,000 |
10,664 |
$2.6 Million |
-2.5 Ft. Volume |
102,000 |
8,303 |
$2.0 Million |
Table 1b. The impact of water discharge rates on the amount of fluridone lost from Lake Tohopekaliga on a daily basis. The cost and lbs of fluridone loss values are approximate.
| West Lake Tohopekaliga |
Discharge of 100 CFS |
Discharge of 250 CFS |
Discharge of 500 CFS |
Discharge of 1000 CFS |
Lbs of Fluridone Loss/ Day |
16 lbs / Day |
40 lbs / Day |
80 lbs / Day |
160 lbs / Day |
Cost / Day |
$4000 |
$10,000 |
$20,000 |
$40,000 |
Lbs of Fluridone Loss/ Month |
480 lbs/ Mo. |
1200 lbs/ Mo. |
2400 lbs/ Mo. |
$4800 lbs/Mo. |
Cost/ Month |
$120,000 |
300,000 |
$600,000 |
$1,200,000 |
Prior to the onset of FRH, management of susceptible strains of hydrilla, although complicated by high water levels and flow, was technically feasible due to the ability to maintain threshold concentrations over a long period of time. The following graph illustrates why FRH presents a significant technical and economic challenge in comparison to previously susceptible strains of hydrilla (Figure 1). The maintenance of threshold concentrations of fluridone for controlling FRH is greatly complicated by high initial water levels, and inflow and outflow through the lake. While improved control of FRH would result if water levels and flow were optimal, there is no indication that such treatments would provide long-term control (greater than 1 season) of hydrilla. The continued use of fluridone on these systems also increases the risk of selecting for hydrilla biotypes with an even greater level of resistance.
As noted above, the onset of FRH has resulted in a significant decrease in the longevity of hydrilla control following fluridone applications. For example, while control efforts on Lake Istokpoga used to be conducted every other year, fluridone treatments have been initiated for the past 3 consecutive years due to the quicker recovery by the FRH. The resistance issue has created a cycle that has resulted in reduced long-term efficacy, and yet the need for more frequent treatments at higher rates is necessary to achieve FDEP program goals of managing hydrilla to the lowest feasible level. While this presents a conundrum, it is obvious from the Workshop that current alternative tools are either limited in their ability to provide cost-effective control on a large scale, or they are not proven to provide selective control on a large-scale basis.
The issue of what new regulation schedules may be implemented in the future to facilitate hydrilla control is beyond the scope of this document. Currently it is expected that revised regulation schedules for some or all of the lakes of the Kissimmee Chain will be developed through the KCOL Long-Term Management Plan, and that these revised schedules may be implemented in late 2007 or later. The need for control of hydrilla should be considered in developing some or all of the revised schedules. Before new regulation schedules for Kissimmee Basin lakes are implemented which adequately facilitate hydrilla treatments, the choice facing plant managers would seem to be one of changing management tools and learning
Figure 1. Hypothetical Fluridone treatment scenarios for control of a resistant and susceptible population of hydrilla. Yellow shaded areas represent concentration thresholds below which fluridone is no longer phototoxic to hydrilla. The large X’s represent additional bump treatments required to maintain residues above thresholds.
to live with more hydrilla, requesting temporary deviations to facilitate hydrilla treatments, increasing funds available to compensate for high water and flow, or living with the control that is achievable with a fixed budget and existing regulation schedules. Stakeholder comments will be an important part of this process.
It is important to note, that as future new chemistries are developed, there is a good possibility that some of these products will require a prolonged exposure period to provide optimal control of hydrilla. Therefore, while the current issue of water regulation schedules is unique to fluridone, water regulation management in relation to new products may be a topic for future discussion. It is therefore important that upcoming discussions and long-term management plans regarding regulation schedules be broadened beyond the issue of fluridone.
Lake Specific Issues
Topics that deserve special note include the impacts of hydrilla management on endangered species (specifically snail kites), the use of water for irrigation from Lake Istokpoga, and the potential for enhanced degradation of fluridone. The snail kite and irrigation issues are tied to water regulation schedules and deviation requests to support fluridone treatments. Enhanced degradation has not been a common observation, but it is a new phenomenon that has been observed to negatively impact some fluridone applications.
Large-scale hydrilla control efforts can include the potential for indirect adverse effects to the endangered snail kite (Rosthamus sociabilis plumbeus). These impacts are largely related to water deviation requests that can impact nesting success of the snail kite. Lowering water levels at a different rate at certain times of the year can cause nests to collapse. In the Kissimmee Chain of Lakes nests are more frequently constructed in herbaceous vegetation that is more susceptible to collapse. Snail kites must nest over water to help support the vegetative structure below the nest and to reduce predation of the nests by terrestrial predators. Another potential indirect impact results from the actual herbicide application activities. Snail kite biologists and aquatic plant managers have recently begun coordinating to exchange information on the location of active nests in order to create buffer zones that prevent disturbance of the nest. From a longer-term perspective, the use of higher rates of fluridone could impact the composition of emergent vegetation in the littoral zone of lakes. However, it is not known if these impacts would have a negative or positive effect in terms of snail kite habitat.
Although non-target plant injury has become a greater long-term concern, there is no evidence of emergent plants rapidly succumbing to fluridone treatments early in the course of an application when snail kites are beginning to establish nests. While improved data regarding non-target species susceptibility, rapid collapse of emergent plants is inconsistent with the use of fluridone. The issue of the impact of sustained dense infestations of hydrilla on organisms such as the snail kite, apple snail, and native vegetation, while important, did not generate much discussion in the Workshop.
Concerns with snail kite nesting on Lake Istokpoga are minimal, yet there is fairly extensive use of Istokpoga water for irrigation of crops. With presence of FRH in some sections of Istokpoga requiring higher fluridone use rates for control, it has become increasingly important for management plans with fluridone in Istokpoga to address potential exposure of certain sensitive crops to herbicide residues in irrigation water above the 1 ppb limit described on labels for fluridone herbicides. There have been no irrigation problems reported with the past use of fluridone in Lake Istokpoga, yet the increasing use rates due to the presence of FRH can cause a
potential conflict between fluridone residues necessary to control hydrilla, and residue levels that could result in phytotoxicity to sensitive crops.
Treatments conducted in the spring of 2005 resulted in fluridone residues in the outlet canal water that feeds an extensive irrigation network. In response, the FLDEP used large quantities of powder activated carbon applied from a boat and drip system to reduce these residues in the irrigation canal water. The rapid degradation of fluridone following the spring 2005 treatments in Lake Istokpoga and decisions to halt further treatments, prevented any further potential issues with fluridone residues in irrigation source water. This situation did point out that Lake Istokpoga is an important source of agricultural water, and any future treatment recommendations need to consider this critical use of the water.
In the last three years, there have been sporadic events of rapid loss of fluridone in an isolated number of Florida lakes treated for control of FRH. While fluridone loss is often influenced by typical environmental variables such as flow and water quality, studies have determined a factor in this phenomenon is enhanced degradation of fluridone by a microbial agent(s). Since this discovery, investigations have been ongoing to determine the causative agent(s) responsible for enhanced degradation and the environmental factors that trigger such events. While research efforts by industry and government scientists have provided some understanding of this phenomenon and informed managers on potential improved strategies for fluridone use, results to date have demonstrated that the processes involved are complex and difficult to study under laboratory and field conditions. Ongoing research continues to examine processes affecting fluridone dissipation and potential mitigation techniques to address confirmed events of enhanced microbial degradation.
Fluridone treatments conducted in the spring of 2005 resulted in a rapid loss of residue in both the Kissimmee Chain of Lakes and Lake Istokpoga. Operational plans called for several split applications to maintain a target threshold of fluridone on the lakes; however, residues dropped rapidly and the ability to maintain these thresholds was compromised by the rapid half-lives observed early in the treatment cycle. Due to the rapid loss of fluridone in these systems, additional treatments were halted. There are numerous variables that contribute to fluridone dissipation/degradation, yet the enhanced loss of fluridone residues in the Spring of 2005 is still under investigation.
Hydrilla as a Threat to Flood Control
There was considerable debate at the Hydrilla Workshop regarding the potential for hydrilla to threaten the flood control function on the KCOL and Lake Istokpoga. While there was a strong divergence of opinion regarding the nature of this threat, it was apparent that there are more unknowns than knowns. When the structures were placed on the KCOL and Lake Istokpoga, these lakes did not support hydrilla, and it was likely unforeseen by resource managers that hydrilla could occupy thousands of contiguous acres on these lakes. The potential threat, if any, that hydrilla poses to flood control problems is a key question, as one of the main justifications for the intense management of hydrilla on the KCOL and Lake Istokpoga is largely predicated on the potential of dense hydrilla to pose an increased threat of flooding. The need to manage hydrilla on a large scale obviously ties back in to the requests for water schedule deviations on an annual basis.
In regards to hydrilla and flood control, there were two major questions posed to the engineers that attended the workshop. First, at what level of infestation would hydrilla potentially pose a threat to the ability to move water downstream, and hence the flood control function on the KCOL or Lake Istokpoga? The second question involved the dense growth of hydrilla near the structures, and the threat of a large mass of plants jamming against structure. There were no definitive answers to either of these questions, although there were several lines of interesting discussion both within and outside of the formal session. The following issues were discussed:
1. Within the KCOL, Lake Tohopekaliga and Kissimmee contain water control structures while Lakes Cypress and Hatchineha do not have water control structures associated with them. Should we view these lakes differently in terms management practices and the threat of hydrilla to impact flood control?
2. The potential for placing a structure between Lake Hatchineha and Lake Kissimmee was discussed. This structure would allow increased latitude to manipulate water levels on individual lakes and could facilitate and isolate the impacts of future fluridone treatments as well as drawdowns.
3. There are contingency plans in place to remove debris that may become lodged against structures on Lake Tohopekaliga and Kissimmee. There was discussion as to whether these plans are adequate in terms of a severe hydrilla infestation.
4. While it is well documented that submersed plant growth in canals create resistance to water flow and thereby increase the chance of flooding, it was unclear if this principle applies to larger lakes. Given the large size of Lake Tohopekaliga, Kissimmee, and Istokpoga in relation to the small structures, it was asked if plants could create enough resistance to flow to prevent downstream water movement?
5. It was noted during the Workshop that there is not an extensive history of submersed plants impacting structures (flood control, bridges) in the State of Florida. While floating plants have impacted structures, there is not sufficient experience with submersed plants to make a definitive statement regarding the threat they pose.
The only engineering opinion that presently exists regarding the potential impact that rooted aquatic plants could have on flood control was conducted for Lake Istokpoga and was authored by Howard L. Searcy consulting engineers in July 1993 (Contract # C-3019 for the South Florida Water Management District). The authors ran simulation models (FEMA/SURGE model) that included the presence of two different levels of hydrilla infestation. The model represented rooted macrophytes for potential friction losses, modified bathymetry, partial blockages, or total blockage. The worst-case plant scenario was a 50% lake infestation of 13,000 acres, and the other scenario included hydrilla occupying up to 2000 acres of Lake Istokpoga. The model results indicated that the higher levels of hydrilla could nearly double the flooding threat in comparison to the lower density hydrilla from 3.4 to 7.1 feet above regulation following a 100-year flood event. This differential was reduced from 3.2 to 5.3 feet for a 50-year event, and further reduced from 1.9 to 2.3 feet for a 10-year event. The model would suggest that the increased flooding threat posed by a 100-year storm is significant in comparison to the relatively minor threat for a 5 or 10-year flood. The validity of this model is not well known, and it will be the topic of future discussions. Future modeling will need to take into account a much greater range of hydrilla acreage (2000 to 25,000 acres), as well as location of the infestations within the lake. The conclusions from this model would tend to support both intense management based on the 100-year flood risk, as well as reduced management based on the 10-year flood risk.
Recommendations
Recommendation 8: A formal request will be made to appropriate Water Management Districts for a detailed response as to the threat hydrilla causes to flood control. This inquiry should include all water bodies where FLDEP Aquatic Plant funds are likely to be spent to reduce hydrilla. The response should include an engineering assessment of the amount and locations of hydrilla that could create an increased risk of flooding. Once such a response is formulated, aquatic plant managers can develop plans to insure that hydrilla is managed in critical areas that represent an increased risk of flooding.
Justification: It was apparent from the workshop that the threat hydrilla poses to the flood control function of these lakes is not well understood. For FDEP to consider changing management practices on these lakes, there needs to be a clear understanding of the implications of leaving high levels of hydrilla in the system. While it was noted that mechanical measures are in place to deal with plants becoming lodged in the structure (track hoes or draglines), it was unclear if these plans take into account a large infestation.
Recommendation 9: As it is likely that new herbicides may require an extended exposure period, it is recommended that an assessment of regulation schedules take into account the improved economics and efficacy that reduced water levels and flow can afford. In lieu of deviation requests on a yearly basis, the impact of deviation requests every two or three years should be studied, including the impacts to fish and wildlife. The seasonality of treatments may be adjusted based on the ability to manipulate water levels/flow during various times of the year.
Justification: Resistance management plans will likely prevent sequential or back-to-back use of new products within these lakes. Therefore, when treatments are initiated, it is likely that we will be dealing with a significant hydrilla infestation, and it is important to provide optimal conditions to allow extended control of the hydrilla.
Recommendation 10: With the long-range viability of fluridone in large lakes with FRH in doubt, the FDEP, FWC and South Florida Water management District (SFWMD) need to develop long-term aquatic plant management plans for how, when, and where to manage hydrilla on the large flood control lake systems.
Justification: If the hydrilla infestations become more severe on these systems, increasing fluridone rates may not be a feasible option. It is important that priority zones for access, navigation, and habitat improvement are included in a lake management plan that does not include the use of fluridone.