Lake Istokpoga Water Quality Evaluation and Nutrient Budget Development

Water quality in Lake Istokpoga (Iss-tuh-poh-guh) has been impacted by numerous activities in the watershed of the lake as well as by lake level regulation and aquatic plant management activities that have occurred in the lake.  Our office began taking lake samples at 12 sites in the lake and from water flowing out of the S-68 structure at the south end of the lake in January 2001, prior to the tussock removal project. We are also using Lakewatch data and data from the South Florida Water Management District to determine where nutrients are coming from, how much they impact the lake, how much of the nutrients entering the lake are retained in lake sediments and vegetation and how much phosphorus (the primary nutrient of concern) leaves the lake through the S-68 structure, heading for Lake Okeechobee. 

Lake Istokpoga is a 28,000 acre lake with two primary tributaries, Josephine and Arbuckle Creeks at the north end of the lake, and two outflows, the S-68 structure at the southeast end of the lake and the Istokpoga canal.  Historically, the Istokpoga canal was actually a small creek that provided the primary outflow of water from the lake when it wasn't flooding the prairie to the south. The lake level fluctuated about 4 to 6 feet every year, allowing aquatic plants to grow when water levels were high and to die back when water levels dropped, preventing large masses of plants from ever forming around the shoreline of the lake.  Water levels used to drop so dramatically in fact that cattle were able to walk to the islands in the lake to graze. 

S-68 Structure:                                                   

Water management began in 1963 when the S-68 structure was installed as part of the Central and South Florida Flood Control Project.  The structure allowed water to be rapidly released from the lake in the event of a storm to prevent flooding of homes and cropland around the lake by shuttling the water down the newly dug C-41a canal toward the Kissimmee River and Lake Okeechobee.   The structure is also used to hold water in the lake until the dry season to be used as a water supply for agriculture along the C-41a canal.  Unfortunately for the health of the lake, the water level now only fluctuates less than 2 feet per year, helping to create dense mats of vegetation near the shore. There was approximately 2000 acres of these vegetation mats, called tussock, in the lake until March of 2001.

In March of 2001 the Florida Fish and Wildlife Conservation Commission was able to take advantage of the severe drought occurring in Florida by moving up the date of their habitat enhancement project by several years.  Numerous agencies worked together to put together the funding and permits necessary to conduct this project in record time and the $3 million project then went forward to remove over 1300 acres of this tussock.  About one-third of the removed material was transported for upland storage, while the rest was formed into wildlife islands in the lake. 

Another significant aquatic plant problem in Istokpoga is the presence of hydrilla - an invasive plant that restricts boat access, among other things, by rapidly taking over surface water bodies.  Hydrilla in Lake Istokpoga is treated primarily by aquatic herbicides, which essentially gives the plant a terminal case of sunburn.  Hydrilla has been sprayed several times since 1988, with whole lake treatments occurring in 1989, 1992, 1997, 2001 and 2003.  The 2001 hydrilla treatment occurred at the same time as the tussock removal and covered the 20,000 infested acres.  Currently, state and local agencies are practicing adaptive managment techniques in controlling small patches of hydrilla from spreading over the whole lake.

There are water quality concerns related to both of these issues.  Lakewatch and South Florida Water Management District (SFWMD) data indicate that nutrient levels are low in the southern/outlet end of the lake compared to the northern/inlet end of the lake, where a large quantity of nutrients enter the lake each year, especially through Arbuckle Creek.  This suggests that the aquatic plants and sediments of Lake Istokpoga are removing phosphorus from the water as it flows through the lake, essentially cleaning the water.  This is of even greater importance because water leaving the lake flows to Lake Okeechobee, which is heavily polluted by phosphorus.   Also, data indicates that nutrient levels in the water increase following whole lake hydrilla treatments, possibly due to the release of nutrients from the decomposing hydrilla plants.  Water clarity also declines following whole lake treatments, especially in the south end of the lake where hydrilla growth is the thickest. 

An intensive water quality investigation of Istokpoga ran from April of 2005 to February of 2006, funded by SFWMD.  The Natural Resources Office also sampled 3 sites on Arbuckle Creek through the Lakewatch program to determine how nutrient concentrations increase as the creek flows through the county.  Currently available data indicate that nutrient concentrations increase significantly as the creek flows south to Istokpoga and phosphorus levels are at times very high by the time Arbuckle Creek enters the lake.  Sources of nutrients along the creek must be identified and addressed through future work. 

For more information concerning this investigation, please contact our office at 402-6545.