EXPERIMENTAL ASSEMBLY OF SHALLOW-WATER FORAMINIFERAL ASSEMBLAGES: THE ROLE OF TEMPERATURE AND SALANITY

Our understanding of benthic foraminiferal environmental tolerances stems primarily from field-based studies in which environmental data are recorded either at the time of sample collection or, occasionally, over a longer interval of time. The use of experimental arrays in which “assemblages” are grown from the fine sediment fraction (<53 or 63 microns) under controlled conditions provides additional insight into the environmental tolerances of shallow-water taxa. We use only the fine sediment fraction because, (a) it is important to exclude adults which would reproduce in response to stress rather than the desired experimental conditions, and (b) as we demonstrated previously, the fine fraction contains numerous foraminiferal propagules (small juveniles) that undergo dispersal both within and beyond the distribution of the local assemblage. To assess dispersal and the environmental tolerances of species found in a selected mudflat assemblage of coastal Georgia (USA), surface sediment was collected in January, 2005, and sieved at 53 µm. The fine fraction was thoroughly mixed and divided (20-ml aliquots) into a series of small growth chambers. The treatments include three different salinity regimes (12, 22, and 36 psu; Instant Ocean) and a constant temperature of either 12 or 22 ºC. All growth chambers, including replicates, were sealed and illuminated on a 12 h cycle. Chambers were harvested (sieved, fixed, stained) concurrently, and different assemblages of ~400 – 600 individuals grew over a 6-week interval depending on the treatment. Haynesina germanica and Psammophaga cf. simplora were widely tolerant and grew abundantly in all chambers regardless of salinity or temperature. Two species, Ammonia tepida (warmer) and Elphidium excavatum (colder), were strongly affected by temperature, but not by salinity. Miliammina fusca and Ovammina opaca both grew at lower salinities (12 and 22), but differed with regard to temperature (former warmer). Several “exotic” taxa typical of the adjacent continental shelf also grew in selected chambers. This experimental approach illustrates the different environmental tolerances of some common shallow-water taxa and can thus improve our paleoenvironmental interpretations. It can also provide insight into the geographic extent of foraminiferal dispersal.