ESTUARINE HISTORY AS INTERPRETED FROM A 900-YEAR OLD OYSTER REEF FROM THE MOUTH OF BROAD RIVER: IMPLICATIONS FOR WATER FLOW MANAGEMENT OF EVERGLADES NATIONAL PARK

The late Holocene development of oyster reefs can reveal the history of estuary formation through standard stratigraphic and paleontologic analyses. Reefs located at the terminus of the Florida Everglades' Shark River sloughway should record the effects of natural and anthropogenic variability in freshwater delivery. Using the 900-year history recorded in one reef from Broad River, this study determined relative trends in salinity and energy through time. The hypothesis tested was that the reef originated under higher energy and salinity conditions, becoming less energetic and progressively more brackish through history. A core was obtained from the mouth of Broad River according to standard vibracoring procedures. The 210 cm core was divided into 21 – 10 cm horizons and its stratigraphy was described. Oyster shells and fragments greater than 2 cm in length from each horizon were examined to evaluate taphonomic grade, percent encrustation, and Cliona sp. boring occurrence. Sedimentology was described using a Malvern laser particle grain size analyzer. T-tests were applied to the sedimentologic and taphonomic data. Cliona sp., a bioeroder requiring marine salinities, and mean grain size decrease up section; these trends were statistically significant. Although statistically insignificant, taphonomic grade showed a weak trend of improved preservation up section while encrustation exhibited a decrease. Results indicate salinity has decreased throughout history in Broad River. Commensurate with these changes is a transition from a subtidal origin of the reef 900 years ago to its current intertidal position. The subtidal interpretation is based upon the reef's deep position relative to the sea-level curve compiled for Southwest Florida. The increased volume of mud in the upper section could indicate a lowering of energy or reflect greater fluvial sedimentation associated with greater freshwater input. Alternatively, the finer sediments could be due to greater oyster productivity resulting in the trapping of more mud. These results have implications for the management and restoration of hydrologic flow through Everglades National Park. Because freshwater delivery is vital to oyster reef development, significant flow alteration could affect coastal geomorphic change and estuarine ecology.