Ecological succession of Florida phosphate mine lakes relative to age and design gradients

Background/Question/Methods

Lakes have been created on phosphate mine lands in Florida since the 1930’s. Lake design has constantly evolved as a result of mining technology advancements and Florida mine reclamation legislation initially introduced in 1975. These laws mandate creation of littoral zones and provide specific guidelines for their extent and slope. Consequently, lakes created after 1975 have reduced bank slopes and shoreline length, with the trend recently toward shallower depths. This study investigates ecological succession of phosphate lakes over age and design gradients based on water quality and sediment characteristics for 24 lakes uniformly distributed over four age/design categories. The year of lake creation spanned 1967 to 2006, and maximum water depth varied from 2m to 9m. Temperature, conductivity, pH, turbidity and dissolved oxygen were measured in situ, and chlorophyll a concentrations were determined spectrophotometrically. Petit ponar sediment samples were collected from profundal and littoral zones and analyzed for organic content by loss on ignition. Bank slope at the point of sediment collection was measured.

Results/Conclusions

Water conductivity, pH, turbidity, and surficial dissolved oxygen were similar among all age/design categories.  Chlorophyll a concentrations were characteristic of natural eutrophic lakes of the area and increased only slightly in older lake categories. Sediments of newly created lakes were dominated by either sand or clay. Profundal sediments of lakes over 5-10 years old displayed increased organic matter accumulation. Littoral sediment composition differed greatly among lakes and was determined by bank slope, density and composition of the vegetation community. Lake stratification was a function of maximum depth and not related to bottom configuration complexity. It appears that phosphate lakes do not undergo an initial trophic surge commonly experienced in new reservoirs.  Their unique geological location on top of phosphate deposits coupled with Cyanobacterial dominance ensures constant high productivity along the age gradient. Conservative water quality parameters, such as conductivity, seem to be independent of lake age or design. Turbidity also displays stability; however, a shift from clay suspension in young lakes to Cyanobacterial dominance as age progresses is indicated.  Thus, phosphate lakes appear to undergo no noticeable ecological succession during the first 30-40 years following creation.