Natural variability or anthropogenically-induced variation? Insights from 15 years of multidisciplinary observations at the arctic marine LTER site HAUSGARTEN

Time-series studies of arctic marine ecosystems are rare. This is not surprising since polar regions arelargely only accessible by means of expensive modern infrastructure and instrumentation. In 1999, theAlfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research (AWI) established the LTER(Long-Term Ecological Research) observatory HAUSGARTEN crossing the Fram Strait at about 79◦N.Multidisciplinary investigations covering all parts of the open-ocean ecosystem are carried out at a totalof 21 permanent sampling sites in water depths ranging between 250 and 5500 m. From the outset,repeated sampling in the water column and at the deep seafloor during regular expeditions in summermonths was complemented by continuous year-round sampling and sensing using autonomous instru-ments in anchored devices (i.e., moorings and free-falling systems). The central HAUSGARTEN stationat 2500 m water depth in the eastern Fram Strait serves as an experimental area for unique biologicalin situ experiments at the seafloor, simulating various scenarios in changing environmental settings.Long-term ecological research at the HAUSGARTEN observatory revealed a number of interesting tem-poral trends in numerous biological variables from the pelagic system to the deep seafloor. Contrary tocommon intuition, the entire ecosystem responded exceptionally fast to environmental changes in theupper water column. Major variations were associated with a Warm-Water-Anomaly evident in sur-face waters in eastern parts of the Fram Strait between 2005 and 2008. However, even after 15 years ofintense time-series work at HAUSGARTEN, we cannot yet predict with complete certainty whether thesetrends indicate lasting alterations due to anthropologically-induced global environmental changes of thesystem, or whether they reflect natural variability on multiyear time-scales, for example, in relation todecadal oscillatory atmospheric processes.