| Summary: | Understanding hydrothermal plumes in the Arctic and Nordic Seas is an important objective for ecosystem assessments of these habitats. Very little is known about what role the hydrothermal plumes play in the Nordic Seas. Recent visual observation and echo-sound imaging suggest that this habitat is densely inhabited by larger zooplankton compared to the surroundings. In this thesis I focused particularly on the diet of a pelagic predator that is commonly found in plumes of the Nordic Seas, the amphipod Themisto abyssorum (Paper III). The diet of T. abyssorum in the deep-sea and at hydrothermal vents was previously unknown and new tools were needed to access this information. Therefore, we developed a molecular assay to help us find and identify predator-prey interactions that are difficult to detect using classical microscopy methods (Paper I and II). We also conducted a genetic survey of the pelagic eukaryote microorganism community at the Jan Mayen Vent Field, Loki’s Castle and the Håkon Mosby Mud Volcano (Paper IV) to understand the vent impact on these communities. Molecular tools have become important in predator-prey interaction studies where dissection can be inadequate. The most common approach is to use specific primers that target known prey taxa. However, to study the complete range of prey items consumed we needed to work with universal primers. We therefore developed an application for the Denaturing High-Performance Liquid Chromatograph (DHPLC) for separating amplicons in a mixture, in order to study the total prey DNA from stomach contents. This technique was developed as part of a study of predator-prey interactions of the copepod Limnocalanus macrurus from the Bothnian Bay in the Northern part of the Baltic Sea (Paper I and II). The DHPLC assay was efficient, unbiased and could be used for any predator-prey interaction (Paper II). More than 30 taxa (at different taxonomic levels) were identified from the samples suggesting highly carnivorous feeding preferences. The assay development in Paper I ...
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