QUANTITATIVELY IDENTIFYING TEMPORAL ASSEMBLAGE TURNOVERS AND SPECIES CONTRIBUTIONS

Foraminiferans are unicellular marine organisms that can be used as proxies to identify significant environmental change caused by anthropogenic activity or climate change. This is achieved by assessing community fluctuations in live species (stained with Rose Bengal) abundances and diversity that extend well outside the established limits of natural variability. As such, natural variability (for example seasonality) of foraminiferal population dynamics must be differentiated from abrupt and persistent changes in the structure and function of the community. The assemblage turnover index (ATI) is a quantitative measure of differentiation diversity, which examines the change in diversity between samples (β diversity) collected over time. Peak sample-wise ATI (ATI_s) values are used to delineate intervals of time in which relatively little change in foraminiferal assemblage is recorded. This is assumed to coincide with minimal flux in ecological factors influencing temporal turnover, whereas a peak in ATI_s value indicates a marked change in foraminiferal assemblage has occurred, presumably due to a change in ecological factors. The conditioned on boundary index (CoBI) identifies the proportional contribution of each species to the peak values of ATI_s. Time series of live foraminiferal densities collected at Caroni Swamp, Claxton Bay (Trinidad, SE Caribbean) and Cowpen Marsh (NE England, UK) are used to illustrate the use of ATI and CoBI in identifying major assemblage turnovers related to seasonal variations, and those species which had significant contributions. This is a novel approach to foraminiferal ecology and has potential to substantiate the use of foraminiferans as effective bioindicators in assessing the health of ecosystems. The use of ATI and CoBI indices, as demonstrated here to identify temporal boundaries, may also be applied to core samples to identify flooding services and major changes in depositional environment for sequence stratigraphy.