@PhdThesis{diss_mods_00013971, author = {Berger, Christine}, title = {Coccolithophore response to modern and past ocean acidification events}, year = {2014}, publisher = {Christian-Albrechts-Universit{\"a}t zu Kiel}, address = {Kiel}, keywords = {Coccolithophores; ocean acidification}, abstract = {The absorption of the recent rising atmospheric CO2 alters the oceans carbonate system and affects the living conditions for marine calcifiers. Coccolithophores as major calcifying phytoplankton largely contribute to the modern carbonate production and play an important role in the global carbon cycle. Thus, changes in the calcite production of the coccolithophores have an impact on the carbon cycle. The effect of ocean acidification on coccolithophore calcification has been observed in several types of experimental, field, and sediment studies, even in combination with other environmental factors, but most studies base on short-term laboratory experiments with single species or strains. Despite various results with species and strain specific response, the most frequent finding in recently conducted experiments is a decrease in coccolithophore calcification under future CO2 levels. In contrast to monospecific laboratory experiments, natural coccolithophore assemblages consist of a heterogenous composition with diverse species and morphotypes, adapted to various environmental conditions. In a natural assemblage changing seawater conditions can lead to a dominance shift to better adapted species or morphotypes with different coccolith weight or size. To gain insights into the possible impact on the carbon cycle due to changing coccolithophore calcification, the response of the entire assemblage to changing environmental conditions in recent and past oceans should be taken into account. The main objective of this thesis was to investigate coccolithophore calcification of the dominant coccolithophore family Noelaerhabdaceae in different past atmospheric CO2 scenarios to obtain the influence of the changing seawater carbonate chemistry and to untangle the response from other environmental factors. Coccolith weight estimates are suggested as a possible indicator to reconstruct calcification rates and were obtained with the automatic recognition system SYRACO. To examine the variability of Noelaerhabdaceae mean coccolith weights during times of predominantly stable atmospheric CO2 conditions, coccolith weights from the pre-industrial Holocene were measured on 3 sediment cores from the North Atlantic. The results show opposing trends in different regions in an amplitude of weight variability which is similar to the previously reported weight change of the last glacial/interglacial change that was associated to ocean acidification. The changes in the Noelaerhabdaceae mean coccolith weight from the Holocene are referable to variations in the coccolithophore assemblage (shifts in species and morphotypes) but also to changing calcification. Apparently, these changes are induced by differences in nutrient or productivity settings between the studied sites. To assess the response of Noelaerhabdaceae mean coccolith weights to a natural increase in atmospheric CO2, two sediment cores from different locations in the North Atlantic were selected which cover the atmospheric CO2 increase of the penultimate deglaciation (Termination II). At the temperate Rockall Plateau with its changing environmental conditions due to a shift in the oceanic frontal system, mean coccolith weight shows positive variances around the Heinrich event 11 which is in close connection to a shift within the assemblage. In the Florida Strait, which is far from influences of frontal zones, mean coccolith weight doubles during Termination II, primarily due to more heavily calcified coccoliths. This increase in calcification at the Florida Strait is simultaneous to the rise in atmospheric CO2 and an increase in seawater HCO3- concentration accompanied by a high total alkalinity, DIC, CO32- and calcite saturation state, which indicates favourable conditions for calcification in the ocean. A comparison of the weight record from the Florida Strait with earlier studies from the CO2 increase of Termination I in tropical regions shows opposing weight trends under similar changes of the seawater carbonate system. In the tropics, different carbonate systems in both Terminations indicate better ion concentrations conditions for calcification during Termination II. The results illustrate that the total CaCO3 production of a coccolithophore assemblage under increasing CO2 depends on regional seawater characteristics and the local assemblage composition. But despite rising atmospheric CO2 the conditions of the seawater carbonate system can be favourable for coccolithophore calcification. Noelaerhabdaceae mean coccolith weight from a sediment core in the Gulf of Taranto in the central Mediterranean Sea was studied to investigate the response to the rising ocean acidification of the past 200 years. The study area is under influence of enhanced anthropogenic nutrient load from the Po River which is known to affect the coastal ecosystems. So far, the results reveal no negative influence of the ocean acidification on coccolith weight or the assemblage composition. Noelaerhabdaceae mean coccolith weight is positively influenced during times of negative North Atlantic Oscillation which strengthens the Po River discharge and leads to enhanced nutrient transport to the coring site via Adriatic Surface Water. The higher nutrient concentration extends the coccolithophore productivity season from winter until late spring and raises the mean Noelaerhabdaceae coccolith weight. The results suggest that a possible negative effect of the rising acidification of the ocean on coccolith calcification can be outcompeted by enhanced nutrient content. The results of this thesis point out the importance of understanding the response of natural coccolithophore assemblages to changing seawater carbonate chemistry and other environmental conditions. The high variability of the Noelaerhabdaceae mean coccolith weight under stable and rising atmospheric CO2 conditions indicates an assemblage specific response, which is further able to mask possible negative effects of rising atmospheric CO2 conditions when other environmental factors, i.e. productivity are favourable.}, url = {https://macau.uni-kiel.de/receive/diss_mods_00013971}, file = {:https://macau.uni-kiel.de/servlets/MCRFileNodeServlet/dissertation_derivate_00005277/diss_c_berger.pdf:PDF}, language = {en} }