Diversity of Arctic deep-sea nematodes from the LTER observatory HAUSGARTEN

The LTER (Long-Term Ecological Research) observatory HAUSGARTEN in the Fram Strait between NE Greenland and Svalbard is influenced by processes in the Marginal Ice Zone (MIZ). Since 1999, an annual multidisciplinary sampling program is carried out at 21 stations, following a depth transect from 250m off the Kongsfjorden to over 5500m in the Molloy Deep, the deepest depression in the Arctic Ocean. As part of the time series, virtually undisturbed sediment samples are taken with a multiple corer for the analysis of various biogenic compounds and the inhabiting meiofauna. Nematodes are typically the most abundant metazoan meiofaunal group in deep-sea sediments. This is also reflected in the samples from the HAUSGARTEN, where usually over 90% of all metazoan meiofauna consists of nematodes, which promotes them as an ideal taxon to study the deep-sea response to the rapidly changing Arctic Ocean. Via density gradient centrifugation and the preparation of permanent specimen slides, a morphological light microscopic determination, at least to genus level is carried out, while also measuring the organism sizes to determine the biomass of the nematodes. Based on the buccal morphology, nematode feeding types are determined, allowing for a general assumption of their ecological function and role. At the deep-sea stations, the mean nematode density and biomass is highest in the upper bathyal between 1200m and 2000m water depth and generally decreases with increasing depth. A significant increase in density compared to the other deep stations is found at the deepest station in 5500m depth. In total 124 genera from 30 families have been identified, also decreasing in diversity with increasing water depth. Nematode biomass and diversity strongly correlate with the availability of food sources (e.g., settled phytodetritial matter, indicated by sediment-bound chloroplastic pigments) and the exo-enzymatic activity of sediment-inhabiting bacteria. The warming Arctic Ocean alters the extend of the MIZ and thus also the extend and timing of the primary production in the area. The ongoing time series at the LTER-HAUSGARTEN allows to monitor the impact of altered food input on deep-sea meiobenthic communities and contributes to a better understanding of climate change effects in the deep-sea