| Summary: | Understanding the effect of environmental stress on the morphology of a population can be developed into a versatile tool to reconstruct stress levels. Such knowledge could help to reconstruct past environments and to predict the state of a population, including future extinction. Especially for the latter aspect, morphometrics could be a valuable alternative for population-dynamics approaches, which suffer from the naturally high variability of population sizes. Calcitic marine microplankton, such as planktonic Foraminifera, offers an excellent model system for such studies. Planktonic Foraminifera occur in high abundances in the fossil record and their chambered shells allow the reconstruction of individual morphologies during their entire ontogeny. Their excellent fossilisation potential further allows to study natural experiments, which occurred over ecologically effective timescales that would have been impossible to simulate during laboratory experiments. Planktonic Foraminifera have already been broadly applied for geochemical and population studies to reconstruct past environments. Their morphology and shell calcification have in contrast been subject to comparably few studies so far. This is unfortunate, since both parameters could be useful for past environmental reconstructions, recent environmental monitoring, and phylogenetic research. Since planktonic Foraminifera have a large share on the worldwide marine calcite deposition, environmentally induced changes in their shell calcification could furthermore significantly influence the oceanic carbon pump. This study therefore aims at a better understanding of the influence of changing environments, including results of environmental stress, on the biometry of planktonic Foraminifera. For this purpose, several foraminiferal species were investigated within three selected environmental settings: two Pleistocene sediment cores and one sediment trap series. The shell calcification intensity and morphology have been investigated in light of their relation ...
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