Time-series studies in a gateway to the Arctic Ocean: Impact of Climate Change vs. natural variability at the deep-sea observatory HAUSGARTEN

The Long-Term Ecological Research (LTER) site HAUSGARTEN is located in Fram Strait, representing one of the most sensitive regions with regard to Climate Change in the North. The unique long-term record assessed at the observatory is one of the best data sets available to study Arctic marine ecosystem dynamics. HAUSGARTEN represents a network of 20 sampling sites arranged along a bathymetric transect (1000-5500 m) and along a latitudinal transect following the 2500 m isobath. Annual water and sediment sampling as well as the year-round deployment of moorings and free-falling systems, which act as local observation platforms, has taken place since 1999. Multidisciplinary research activities at HAUSGARTEN cover almost all compartments of the marine ecosystem from the pelagic zone to the benthic realm. Time-series studies at HAUSGARTEN already exhibited trends from which, at the moment, we do not know whether these already indicate Climate Change induced lasting alterations of the system or simply reflect natural variability on multi-year time scales, e.g. in relation to variations in the Arctic Oscillation. Water temperatures in the Fram Strait generally increased over the last years. For the period 1997-2012, there was a positive linear trend in Atlantic Water mean temperature of 0.1°C per year. A massive temperature increase was observed within the upper 500-1000 m of the water column between the summers of 2005 and 2008. Records from moored instruments at the central HAUSGARTEN site (2500 m) between 1999 and 2013 showed a similar overall increase in water temperature at 2 m above the seafloor. Water temperature as well as sea ice transport and melting showed to be the main factors influencing the amount and occurrence of the phytoplankton in Fram Strait. During the last decade, chlorophyll a biomass has slightly increased in the eastern part of the passage, whereas it was relatively constant in the colder western parts. Microscopic and molecular studies revealed a shift in micro- and nanoplankon populations from diatoms to Phaeocystis pouchetii and smaller flagellates. For the zooplankton, we observed a decrease in polar species and an increase in boreal species. Alterations in planktonic assemblages affect the flux of organic matter (food/energy) to the seafloor. Analyses of various biogenic sediment compounds between 2000 and 2005 revealed a generally decreasing transfer of detritus to the seafloor and subsequently decreasing microbial biomass and bacterial richness in the sediments. A comparison of footage between 2002 and 2007 demonstrated a parallel decrease in megafaunal densities at 2500 m water depth. Since then, we observed a reverse trend with increasing food availability at the seafloor and again increasing microbial biomass in the sediments. First results from photo/video surveys in 2011 showed again rising megafauna densities, probably reflecting a direct effect of (again) enhanced organic matter availability for the benthos. The remarkably rapid response of the benthic community to variations in food/energy supply is in contrast to earlier assumptions on the sensitivity of deep-sea ecosystems and obviously involves shifts in the species composition, thereby affecting benthic biodiversity as well as the functional diversity at the deep seafloor