DISTRIBUTION OF DIATOMS AMONG INTERMITTENT PONDS ON THE ATLANTIC COASTAL-PLAIN - DEVELOPMENT OF A MODEL TO PREDICT DROUGHT PERIODICITY FROMSURFACE-SEDIMENT ASSEMBLAGES

Inferences of past climate from the fossil record in lakes rely on the accurate quantification of a relationship of fossilizing organisms to their environment Whereas the relationship of diatoms to water chemis try parameters has been modeled in many systems, few studies adequatel y address the relationship of diatoms to physical properties, such as water depth or hydrology, that may be more directly tied to climate. W e examined the composition of modern diatoms in surface sediments of 7 5 isolated ponds (mostly Carolina bays) of the Atlantic Coastal Plain to: (1) assess the influence of physical and chemical variables on the distribution of diatoms among ponds of the region, and (2) develop a model that predicts hydroperiod (a measure of pond permanence) from di atom assemblages. We constructed two hydroperiod calibration models: t he first infers hydroperiod from the weighted-average optima and toler ances of taxa along the hydroperiod gradient, the second bases inferen ces on the hydroperiod estimates of compositionally similar samples. B oth approaches incorporate a-priori and post-hoc tests of assumptions often inherent in the construction of transfer functions. Diatom assem blage composition had strong, approximately linear relationships to hy droperiod, water depth, and calcium concentration in non-metric multid imensional ordination space; effects of other variables, including pH, were non-linear or ambiguous. Overall, the assemblages reflected the dilute, acidic chemical characteristics of bays. The assemblages conta ined differing abundances of euterrestrial, benthic and planktonic tax a, depending on a pond's susceptibility to drying. A weighted-averagin g regression model based on taxon-specific hydroperiod optima generate d adequate, unbiased hydroperiod inferences from diatom species compos ition (r(2) = 0.81). This model may be used to infer past drought epis odes from fossil diatom assemblages at appropriate sites on the Atlant ic Coastal Plain.