Arctic pelagic amphipods - community patterns and life-cycle history in a warming Arctic Ocean

Indications accumulate that the Arctic Ocean is in transition towards a new, warmer state. With this development, we expect shifts in the composition of Arctic zooplankton communities, as well as adaptations in the life-cycle history of established species. In the Arctic marine food web, carnivorous pelagic amphipods are an important link between herbivorous mesozooplankton and higher trophic levels such as planktivorous fish and seabirds. The present study closely investigates the community patterns of three important epipelagic amphipods Themisto abyssorum, T. libellula and T. compressa, and the deep-water amphipod Cyclocaris guilelmi. The objectives of this work focus on the year-round life-cycle characteristics of pelagic amphipods and on their function in the food web structure at study sites between Greenland and the Svalbard archipelago. In this thesis, three publications address the year-round life-cycle characteristics of Themisto spp. and C. guilelmi sampled by moored sediment traps in the northeastern Fram Strait. The sediment traps are deployed in a long-term observatory, the HAUSGARTEN, which is maintained by the Alfred Wegener Institute for Polar and Marine Research at the boundary between the central Arctic Ocean and the Greenland Sea since 2000. In the epipelagic trap samples hyperiids of the genus Themisto represent the dominant taxa among the collected amphipod community. Length-frequency analyses of year-round samples from 2000 to 2009 indicate a life span of two years for the sub-arctic species T. abyssorum and at least three years for the typical Arctic amphipod T. libellula. From 2004 to 2008, the meso- and bathypelagic amphipod communities in the sediment traps from 800 to 2700 m depth show the prevalent appearance of the deep-water amphipod C. guilelmi. Two other studies focus on the winter feeding strategies of Themisto spp. and C. guilelmi. The results from the gut content analyses indicate for the first time that T. abyssorum and T. libellula are active predators during the Arctic winter, with calanoid copepods as their major prey. In addition, a comparison of summer and winter fatty acid profiles shows a dominance of copepod biomarkers of the genus Calanus in the four investigated amphipod species, indicating that T. abyssorum, T. libellula, T. compressa and C. guilelmi are all part of Calanus-based food web and perform a business-as-usual feeding-mode during the polar night. To conclude, I show that long-term observations and a detailed knowledge of species specific ecology are of high importance for the identification of possible shifts within zooplankton community patterns and to predict their consequences in the food web structure of Arctic ecosystems.