Constraining the ecological niche of planktonic foraminifera in the Arctic

• Main Author: Greco, Mattia
• Format: Thesis
• Language: English
• Published: Universität Bremen 2020
• Subjects: foraminifera; ecology; plankton; Arctic; 570; Arctic Ocean; Baffin Bay; Baffin; Climate change; Foraminifera*; Fram Strait; Global warming; Neogloboquadrina pachyderma; Nordic Seas; Planktonic foraminifera; Sea ice
• Online Access: https://dx.doi.org/10.26092/elib/344; https://media.suub.uni-bremen.de/handle/elib/4547

The effects of global warming are especially pronounced in the Arctic: temperatures have increased at a rate twice as fast as in other regions of the world during the past century. This trend implies that the Arctic Ocean will likely become entirely ice-free during the summer before the end of this century. Paleoclimatic studies have shown that abrupt large-volume meltwater discharges into the Arctic Ocean and its surrounding seas, were capable of disturbing the global ocean circulation and triggering further climatic transformations. Hence, a better understanding of the past natural variability of the Arctic Ocean is needed for more accurate model predictions of future climate change. Planktonic foraminifera represent a powerful tool for palaeoceanographic reconstructions. Their fossil assemblages and the chemical composition of their calcite shells allow reconstructing the physical state of the ocean in the past. The correct interpretation of these paleo-reconstructions highly relies on a thorough understanding of species-specific ecology of living planktonic foraminifera in the water column as, for example, preferred depth habitat, calcification conditions, and biotic interactions. In the Arctic Ocean, due to the fragmented observations on this marine group, no consensus exists on the ecological preferences of the different species, hampering the correct interpretation of the paleosignal present in their shells. This thesis aims to extend the understanding of the ecology of Arctic planktonic foraminifera species by focusing on various levels of organismal biology and physiology. To constrain the environmental and biological factors controlling the vertical distribution of the species Neogloboquadrina pachyderma, a compilation of 104 vertical density profiles from the Arctic Ocean and its marginal seas was investigated using a statistical approach (Chapter 2). Contrary to what has been previously assumed, no significant relationship between N. pachyderma depth habitat and depth of chlorophyll maximum was observed. The depth habitat of the species could instead be predicted with a model including sea-ice concentration, surface chlorophyll concentration, and days since ice-break-up as predictors explaining 33% of the observed variability. The biotic interactions of N. pachyderma with the eukaryotic pelagic community were assessed using a single-cell metabarcoding approach (Chapter 3). The eukaryotic DNA present in 39 specimens and contextual seawater from the Baffin Bay was extracted, amplified, and sequenced. The analyses revealed that N. pachyderma is omnivorous as it lives and opportunistically feeds on diatom-fuelled aggregates. The data also showed a particularly high occurrence of reads belonging to Syndiniales in the foraminifera samples, suggesting that this widely distributed parasite could infect N. pachyderma and possibly influence its population dynamics. To test the assumption that planktonic foraminifera can tolerate low salinity and record the chemical signature of past meltwater discharge events in their shells, specimens of Neogloboquadrina incompta were exposed to a gradient of salinities between 35 and 25 PSU (Chapter 4) as part of a culturing study. Survival was monitored over 26 days by measuring the extent of the rhizopodial network. The highest rhizopodial activity occurred at salinity levels between 35 and 31 PSU. The results indicated that the species can survive long-term exposure to salinities as low as 28, but no rhizopodial activity and signs of cytoplasm degradation were observed in all specimens exposed to 25 PSU. The responsiveness of Arctic planktonic foraminifera to current climate change was investigated by analysing a compilation of 51 species-resolved stratified population profiles collected in the Fram Strait between 1985 and 2015 (Chapter 5). The data revealed an ongoing Atlantification of the community not mirrored by changes in local environmental conditions. The abundance of Atlantic expatriates is instead rising because of processes favouring their growth in the Nordic Seas, the “source” area. On the contrary, the resident species Turborotalita quinqueloba showed declining density and habitat shoaling due to the ongoing extensive sea-ice export from the Arctic and associated cooling in the Fram Strait. These conditions favour the other resident species, the polar N. pachyderma being better adapted to the cold conditions of the area. These results advance our understanding of the abiotic and biotic processes regulating the ecology of planktonic foraminifera in the Arctic Ocean and can be used to refine palaeoceanographic reconstructions in the polar regions and to improve predictions of future climate change.