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Understanding hydrothermal plumes in the Arctic and Nordic Seas is an importantobjective for ecosystem assessments of these habitats. Very little is known aboutwhat role the hydrothermal plumes play in the Nordic Seas. Recent visual observationand echo-sound imaging suggest that this habitat is densely inhabited by largerzooplankton compared to the surroundings. In this thesis I focused particularly on thediet of a pelagic predator that is commonly found in plumes of the Nordic Seas, theamphipod Themisto abyssorum (Paper III). The diet of T. abyssorum in the deep-seaand at hydrothermal vents was previously unknown and new tools were needed toaccess this information. Therefore, we developed a molecular assay to help us findand identify predator-prey interactions that are difficult to detect using classicalmicroscopy methods (Paper I and II). We also conducted a genetic survey of thepelagic eukaryote microorganism community at the Jan Mayen Vent Field, Loki’sCastle and the Håkon Mosby Mud Volcano (Paper IV) to understand the vent impacton these communities.Molecular tools have become important in predator-prey interaction studies wheredissection can be inadequate. The most common approach is to use specific primersthat target known prey taxa. However, to study the complete range of prey itemsconsumed we needed to work with universal primers. We therefore developed anapplication for the Denaturing High-Performance Liquid Chromatograph (DHPLC)for separating amplicons in a mixture, in order to study the total prey DNA fromstomach contents. This technique was developed as part of a study of predator-preyinteractions of the copepod Limnocalanus macrurus from the Bothnian Bay in theNorthern 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 than30 taxa (at different taxonomic levels) were identified from the samples suggestinghighly carnivorous feeding preferences. The assay development in Paper I and IIconfirmed that it was possible to explore stomach DNA using universal primerswithout significant bias from predator DNA.Until now, dissection and microscopy have been the most common tools used tostudy trophic interactions of T. abyssorum. However, sole use of such tools may havecaused an underestimation of prey diversity, particularly among those prey items notvisible in the microscope. Themisto spp. are very useful as target species in deep-seastudies with long haul times because they have long gut passage time, so prey itemsare not digested as quickly as in some other species. The novel technique forexcluding predator amplicons developed in Paper II was used to analyse the gutcontent of T. abyssorum. Specimens from three localities; The Jan Mayen VentFields, The Loki’s Castle Vent Field and The Håkon Mosby Mud Volcano, wereanalysed using a 3-500 bp long 18S rRNA sequence. The number of OperationalTaxonomical Units (OTUs) reported in Paper III was highest in T. abyssorum gutsfrom Loki’s Castle while the Jan Mayen samples had the lowest numbers of prey.The most abundant prey was calanoid copepods, supporting previous findings.However, the molecular approach has also revealed soft-bodied prey and possiblydetritus. In summary, our study showed much higher diversity of prey than previousstudies and the wide range of taxa found also indicates that T. abyssorum is likelyhighly omnivorous.In Paper III, our data indicates that the localities were quite different from each otherin terms of prey diversity and we therefore hypothesised that biodiversity ofhydrothermal vents might be more closely linked to the general water masses of theNordic Seas than to the hydrothermal activity alone. A molecular diversity assay wasused to describe the eukaryote community of the hydrothermal vents in theNorwegian Sea (Paper IV). Water samples from five localities and from threedifferent depths were collected. The five localities were Jan Mayen Vent Fields witha reference station, Loki’s Castle with a reference station and the Håkon Mosby MudVolcano. The results obtained from Paper IV support the hypothesis from Paper III,where water masses gave the strongest effect followed by depth, and thirdly adifferentiation between Loki’s Castle and the reference station indicated an effect ofthe hydrothermal vent.Papers I, II and III illustrate the power of molecular markers in dietary studies,generating a more in-depth understanding of predator-prey interactions. In addition,both the stomach content analysis in Paper III and the genetic survey of the watercolumn in Paper IV suggests that the plankton taxa composition was not ventspecific. The results illustrate the need to go deeper and further into a completeinventory of the plankton community surrounding vents and seeps. Furthermore,several technical challenges regarding sampling for stomach content analysis are yetto be solved and an even better understanding of predator-prey interactions dependson that development.</p

Seasonal variation in wax ester concentration and gut content in a Baltic Sea copepod [Limnocalanus macrurus (Sars 1863)]