Changing Land Use and Trends of Water Quality for Lake Manatee, Florida.

The watershed for Lake Manatee, a primary drinking water source for Manatee and Sarasota Counties in Florida, has experienced mixed and changing land use (residential, row crops, citrus, cattle, phosphate mining and woodlands) for the past few decades.  A study was conducted to determine trends of water quality parameters in the Lake Manatee using 25-years of physical and chemical properties data (1983-2007) and whether these changes could be attributed to land use changes. Results showed a distinct seasonal pattern for most water quality parameters such as temperature, electric conductivity (EC), turbidity, color, dissolved oxygen (DO), phosphorus (P), nitrate (NO₃), chlorophyll a (Chl-a), and pheophytin a. The annual mean temperature during the study period was 24.6 ^oC; EC varied from 41 to 710 µS/cm; DO varied from 2.9 to 14.8 mg L^-1.The mean concentration of total phosphorus (TP) was much greater than the USEPA ambient quality criteria recommendation (17.5 µg L^-1) for the study area and provide evidence that Lake Manatee is a N-limited lake. Nitrate seasonal trends were opposite from the changes in Chl-a concentrations. In the wet season, chl a concentrations increased but NO₃ concentrations decreased. According to the trophic state index (TSI) values, Lake Manatee is mid-eutrophic to eutrophic.  A correlation analysis demonstrated that Chl-a had a direct relationship with temperature, and EC and NO₃ had inverse correlations with DO. A Chl-a regression model using stepwise multiple linear regression with various water quality parameters was used to determine whether algal blooms (as indicated by Chl-a) could be attributed to changes in specific parameters of water quality.  Over the 25-years, water quality did not deteriorate but an obvious increase in chl-a over the last 10 year and a slow increase in TP concentration during all the study period were detected.