EXPLORING HISTORIC ANTHROPOGENIC DISTURBANCE IN FLORIDA ESTUARIES WITH MULTIPLE RADIOISOTOPIC TRACERS

Florida has experienced a tremendous growth in population over the past 50 years, especially in coastal regions. This rapid growth has placed considerable demands on water usage, reducing freshwater input to estuaries. In conjunction, dredging and maintenance of inlets and estuaries has greatly modified salinity and sediment dynamics. Agricultural and urban development within estuarine watersheds has significantly increased sediment loading due to poor soil management processes and increased nutrient loading. The sedimentary record of these various processes was explored in two estuaries along Florida’s east coast: the small (<500 sq. km) Loxahatchee River estuary (LRE) in Palm Beach county and the large (3500 sq. km) Lower St. Johns River estuary (LSJR) in northeastern Florida, which includes the city of Jacksonville. Seasonal sediment dynamics were explored using the short-lived radioisotopes Th-234 and Be-7 coupled with sedimentary pigment (Chl-a) analyses and revealed significant physical mixing (10-20 cm) and seasonal transport of sediment in the LSJR estuary, in addition to non-tidal pulses of salinity up-estuary. Sediment loading due to urban development was established for both estuaries using Pb-210 and Cs-137 derived sediment budgets, and increased Ra-226 derived from the radiogenic Hawthorn Group reveals the dredging of large canal system in the headwaters of the LSJR. The long-lived K-40 was a useful radioisotope in detecting provenance and grain size changes due to its presence within mica-rich clays. Pb-210 and Cs-137 derived sediment chronologies were used to constrain lithologic (bulk density, magnetic susceptibility, grain size) and geochemical (carbon, nitrogen stable isotopes; C:N:P ratios, and biogenic silica) tracers of the impact of nutrient loading within the LSJR estuary.