| 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 to environmental forcing and biological stress. The shell calcification intensity (amount of calcite present in the adult shell) shows signs of a universal positive correlation with carbonate saturation of the sea water. When the carbonate saturation is kept nearly constant, however, it is evident that shell calcification intensity is also influenced by other factors like temperature and productivity. Those secondary influences act species-specific and are presumably able to mediate or modify the effects of carbonate saturation. It could further be shown that cryptic speciation is a severe problem for calcification studies, because shell calcification is already significantly different between pseudo-cryptic species that have been commonly pooled together in the past. Shell size was in no case related to species abundance, what would have been expected under the assumption that species are most abundant under optimal environmental conditions. Together with the fact that shell calcification intensity is also variably correlated to species abundance, this implies that either species abundance is no versatile proxy for optimal growth conditions, or that optimal conditions are not uniformly related to biometric traits. Other phenotypic traits were observed to show characteristic deviations in relationship to environmental stress. The observed trends all led to a clear change in population morphology over ecologically relevant timescales as result of natural selective patterns. In a community which is exposed to near-lethal stress levels, this can culminate in a unique morphology that is clearly different from that of a less stressed population. The obtained results imply that foraminiferal biometry, despite their unicellular level of organisation, reacts in complex ways toward changes in the environmental setting. Those reactions are complicated by the interplay of abiotic (environment) and biotic (stress) factors and the presence of hidden diversity. Further research is needed to minimize those problems.
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