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David Eilers, James C. Griffin and Kimberly Koenig USF-FCCDR Tampa, FL Jason M. Mickel Hillsborough County Stormwater Section Tampa, FL

Purpose and Scope

This paper presents results of lake assessments conducted between May and August of 2006 on twenty four Hillsborough County (County) Lakes and comments on the findings for lakes that are presently listed by the Clean Water Act Section 303 (d) Impaired Water’s List (303(d).

Method and Approach

Both the University of Florida’s LAKEWATCH program (UF) and the University of South Florida’s Water Atlas Lake Assessment program (USF) conduct lake assessments that include lake morphology (bathymetry and shoreline mapping), biology (plant biology) and lake water chemistry (water column chemistry). Both programs have recently adopted procedures which allow a more rapid collection and analysis of these data. These new methods make it feasible to calculate the trophic state index (TSI) based on both lake water chemistry and lake water chemistry with the contribution of plant-bound nutrients. These calculations are based on earlier work by Canfield and Hoyer (2000). The USF program includes a summary of the results of the assessment in lake assessment reports which are published on the Hillsborough Water Atlas (www.hillsborough.wateratlas.org). The reports review trends in water quality with predictions of the trophic state index based on water column chemistry alone as well as with the potential contribution from existing plant biomass.

Between May 2006 and September 2006, USF conducted twenty-four assessments of lakes grouped by watersheds across the County. Our initial results demonstrate a large difference in lake water quality and plant diversity both between regions and within regions. These data also demonstrate that water column chemistry alone is not an adequate determinant of the health of a lake and that other factors that are allowed in the current Impaired Waters Rule (IWR) should be used for adding or removing a lake from the 303 (d) verified list.

Results and Discussion

Five lakes assessed in 2006 that were also listed in the 303 (d) verified list as impaired were selected for review. Table 1 lists these lakes. The questions asked in this review are: (1) would the overall assessment of the lake indicate impairment; (2) does the column water chemistry indicate impairment; (3) do submerged vegetation play a major role in maintaining nutrient balance and what is the estimated nutrient contribution and (4) given that the lake is impaired, what should be done. For question 1 we consider, based on our assessment, if the lake meets its intended use (Class III waters). Question 2 is solely based on water chemistry at the time of assessment; however, trend data is also included. Question 3 is based on the results of the vegetation assessment conducted as part of the lake assessment and includes the calculation

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