By: Dr. Benjamin Sperry, UF/IFAS CAIP Research Assistant Scientist
The outlet river connecting Lake Panasoffkee and the Withlacoochee River was infested with hydrilla earlier this year. The infestation resulted in significantly reduced water flow from the lake to the river. The reduction of water flow lead to flooding conditions for areas surrounding the lake and outlet river, including residential properties. The outlet river needs to discharge water at a rate of approximately 400 cubic feet per second (cfs) to operate properly.
After a brief investigation, managing authorities including Southwest Florida Water Management District (SWFMD) and Florida Fish and Wildlife (FWC) concluded large infestations of hydrilla spanning a significant portion of the outlet river were the cause of restricted water flow of near 200 cfs. With this information, the two state agencies developed a plan to remove the weeds and restore flow to its proper level.
The SWFMD contacted UF/IFAS Center for Aquatic and Invasive Plants (CAIP) biologist, Dean Jones and research assistant scientist, Dr. Benjamin Sperry requesting technical assistance with the treatment of hydrilla. While the SWFMD has the equipment and well-trained personnel who regularly treat hydrilla in lakes, they were not accustomed to treating in high water exchange systems. Due to the flowing nature of the outlet river system, the mechanics of treating submersed vegetation with herbicides was vastly different than treatments in systems with little to no water exchange.
Herbicides used to control hydrilla require the correct ratio of herbicide concentration and exposure time to provide control. Appropriately, this relationship is often referred to as concentration exposure time (CET) requirement. When herbicide is applied in lakes, the treated water generally does not leave the system. Therefore, if herbicides were applied in rivers the same way they are in lakes, the herbicide would not contact the hydrilla long enough to satisfy the CET requirements.
In order to provide sufficient exposure time in flowing systems, herbicides must be slowly applied to the water over time. This type of exposure creates a moving treated “section” of water to flow through the hydrilla. The application was accomplished in the Lake Panasoffkee outlet river by using four, low-output peristaltic pumps.
Each pump fed into tubes evenly distributed across the outlet river dripping into the water. To achieve the CET requirements for endothall, the operation had to run for 12 hours. Endothall was chosen for this treatment because of its relatively short CET requirement. The herbicide was also chosen due to its ability to control hydrilla without posing risk to desirable native vegetation or wildlife.
Upon completion of the treatment, managers reported increased water flow rate from 200 to 300 cfs a few days after. With little to no changes in lake and river levels, this increased flow rate was due to the dissipation of large mats of hydrilla from the herbicide treatment. Likewise, water levels on Lake Panasoffkee and the outlet river started to recede. The reduction in the water level allowed landowners to access their properties once again.
Due to the emergency nature of the situation, SWFMD and FWC contracted harvester boats to remove biomass upstream from the treated area as well as to harvest herbicide-treated populations to increase flow more rapidly and prevent treated plants from detaching from the soil and accumulating at the bridge. Since the completion of this treatment, the water level has dropped approximately 18 inches according to the SWFMD. This integrated management approach likely decreased the time to restore normal water flow levels.
To see and learn more about the project, watch this video.
For more information about the UF/IFAS Center for Aquatic and Invasive Plants please visit https://plants.ifas.ufl.edu. Be sure to follow us on social @UFIFASCAIP. UF/IFAS CAIP, Turning Science Into Solutions.