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Florida Lake Management Society Annual Conference, Naples, Florida, June 4 – 7, 2007

ratio median of 12.4 (107 verified period (2001). Environmental Protection

values) in the planning period (1998) and 17.6 (164 values) during the Annual average TSI, as measured by the Florida Department of

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    Southwest District was greater than 60 in 1998 and 2001.

Following the installation of a stormwater pipe (2003) that drains a nearby intersection into a stormwater pond and ultimately, into Lake Valrico, the residents became extremely vocal and were successful in getting the County involved with addressing the hydrilla problem. In 2004, the County developed, fully funded, and implemented a plan to treat the lake with SONAR. The SONAR was effective and reduced the biomass of hydrilla nearly 100% but the hydrilla rapidly grew back the following spring. Therefore, a new approach was developed and implemented in 2006, which is the scope of this paper.


In early 2006, the County’s Stormwater Management Section along with Mosquito and Aquatic Weed Control Unit developed a new plan to address the hydrilla population, which spread back to 100% PAC. This plan, which spanned one year, consisted of conducting two Lake Assessments (with USF), treating 6 acres with Aquathol Super K (endothall), releasing 875 triploid grass carp into the lake (~ 7/acre given a 20% mortality rate), conducting monthly inspections and collecting water quality data (Lakewatch and Stormwater), and obtaining vegetation removal permits, right-of-way permits, and triploid grass carp permits from the respective agencies.

The method for the Lake Assessments consisted of collecting water quality samples, and conducting lake bathymetric and vegetative analyses. A Lowrance WAAS GPS and “Fish- finder” fathometer system was used for the Lake Assessments. University of South Florida faculty and students worked in conjunction with the County Stormwater Management Section to complete fieldwork for the lake. All data from the field has been processed and bathymetric maps from this effort have been uploaded to the watershed atlas for display.


Data from the 2006 Lake Assessment (before new management activities) indicate a lake totally inundated with vegetation with a large vegetative biomass load. The primary submerged vegetation included Hydrilla verticillata (Hydrilla) with a small population of Ceratophyllum demersum (Coontail). Observational inspections throughout the late fall and winter, indicated that the vegetation biomass was reduced but regenerated quickly in the early spring. In fact,

according to the May 2007 Lake Assessment,


Water quality data for this period

the vegetative biomass has only been reduced remained relatively stable and consistent with

seasonal variations.


The triploid grass carp need more time to control the hydrilla. A second treatment of Aquathol Super K will be considered to give the carp an opportunity to keep up with the growth rate.

Session 6A – Page 7

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