Description: Greenland fjords receive considerable amounts of meltwater discharge from the Greenland Ice Sheet, influencing the physical and biogeochemical conditions within the fjords. Because ice melt will increase with ongoing climate change, research on present-day conditions is urgently needed to make better projections for the future. In the present study, a comprehensive analysis of the carbon cycle in Scoresby Sund, the world’s largest fjord system situated at the southeastern coast of Greenland, was conducted. In summer 2016, Scoresby Sund and its northernmost branch, Nordvestfjord, were visited. While the narrow Nordvestfjord is influenced by numerous marine-terminating glaciers and surface meltwater discharge, the wide Outer Scoresby Sund is much less affected by meltwater. Surface partial pressure of CO2, primary production, particulate organic carbon (POC) flux, and remineralisation within the water column are reported. The data reveal that meltwater significantly influenced the carbon dynamics within the fjord. First, meltwater itself increased the uptake of carbon dioxide from the atmosphere. Second, meltwater limited net community production in Nordvestfjord to 31 - 35 mmol C m-2 d-1 compared to the Outer Scoresby Sund and the shelf (43 - 67 mmol C m-2 d-1) by inhibiting the resupply of nutrients to the surface and by shadowing of silts contained in the meltwater. Finally, the POC flux close to glacier fronts was enhanced due to ballasting by silts, which diminished the remineralisation within the water column and increased the share of organic carbon that reached the sea floor. In Outer Scoresby Sund, by contrast, most remineralisation took place in the upper water column and particle concentrations below were mainly dependent on the present water mass. This study presents the first findings ever about biogeochemical cycling in Scoresby Sund. The results imply that Greenland fjords should be examined on a regional scale to highlight significant differences in carbon dynamics depending on the degree of meltwater discharge within a single fjord system.

Description: Pteropods are marine pelagic snails that form shells out of aragonite, a form of calcium carbonate (CaCO3). When CaCO3 precipitates, CO2 is set free and changes the chemistry of the surrounding water. As soon as pteropods die, the shells sink into deeper water layers and contribute to the marine carbonate flux. During the way through the water column, dissolution of the shells can occur which leads to a chemical bond of CO2. Thus, pteropods are, in addition to other calcifiers, an important part of the carbon storage system. Because aragonite is more soluble than calcite (which is formed by foraminifera and coccolithophorids) under the same physical and chemical conditions, the contribution of aragonite to the total carbonate flux is of special interest. The easiest way to determine this contribution is to multiply the amount of shells with an assumed shell weight. In this thesis, pteropods collected with sediment traps were investigated. The sediment traps are moored at the long-term deep-sea observatory HAUSGARTEN (operated by the Alfred Wegener Institute) in eastern Fram Strait. They sample the ambient particle flux in defined time periods during one year. Additionally, zooplankton can enter the traps. The samples examined in this thesis are from 2011/2012 and 2012/2013 from ~ 200 m and ~ 2500 m depth. In the Fram Strait two pteropod species, Limacina helicina and Limacina retroversa, are regularly recorded. To date, an average shell weight of 0.145 mg/shell was assumed to calculate the contribution of aragonite to the total carbonate flux. The aim of this thesis is to refine this guide value by investigating whether there is a difference in the shell weights of both species at both depths. Besides, it was tested if it is possible to consider the shell size in the calculation of the aragonite contribution. L. retroversa is the dominant pteropod species in the HAUSGARTEN area with a proportion of 90 % of the pteropod individuals. There is a significant difference in the shell weights of both species as well as of the shells at different depths and defined size classes from L. retroversa. The median shell weight of L. retroversa at ~ 200 m depth is 0.118 mg/shell and at ~ 2500 m depth 0.081 mg/shell. Because of the small amount of shells, a statistically relevant refinement of L. helicina shell weights was not possible. Therefore, the median shell weight of L. helicina at both depths is 0.043 mg/shell. With the obtained refined shell weights of both species considering the parts of the defined size groups of L. retroversa, it was calculated that aragonite contributes with 13 – 15 % (200 m) and 0,5 – 5 % (2500 m), respectively, to the total carbonate flux. It is recommended to use the refined and weighted guide values in future determinations of the carbonate flux at HAUSGARTEN.