Epiphytic foraminifera along a climatic gradient, western Australia

The presence of Posidonia australis seagrass habitat along 1000 km of the W estern Australian coast provides an opportunity to study variation in epiph ytic foraminiferal populations along a climatic gradient. Epiphytic foramin ifera were collected in summer and winter from three nearshore shallow-wate r sites, which span a north-to-south gradient in climate from subtropical s emi-arid to subtropical humid. Living epiphytic assemblages at each geograp hic site are distinct. Warmer water assemblages are characterized by Penero plis spp. Vertebralina, Amphisorus and Marginopora, whereas Laniellodiscorb is and Rosalina spl) characterize cooler water assemblages. The differences in assemblage composition partly reflect a north-to-south change in abunda nce of species related to climate, but also reflect local factors. Trends r elated to the climatic gradient include a decrease in millolid and bulimini d specimens and taxa, as well as a decrease in species with tropical affini ty from north-to-south. Seasonal sampling at each geographic site also show s a decrease in miliolid and buliminid specimens and taxa, as well as a dec rease In species with tropical affinity between summer and winter. This sug gests that both regional and seasonal differences in epiphytic foraminifera l populations reflect a spatial vs. temporal response to change in sea temp erature. Porcellaneous taxa are particularly sensitive to both regional and seasonal changes in sea temperature and appear to be good indicators of cl imatic setting. Using selected and critical species of foraminifera, it is possible to divide the subtropical region of Western Australia, traditional ly viewed as a single biogeographic province, into at least two sub-climati c regions. Such refinement would be useful for interpreting paleoclimatic a nd paleoenvironmental settings of fossil assemblages.