Deep-sea nematode community changes over two decades at HAUSGARTEN observatory (Fram Strait, Arctic Ocean)

Over the past 2 decades, deep-sea nematode communities in the Arctic Ocean have undergone significant changes in structure and diversity, likely linked to shifting organic matter input and environmental conditions. Free-living nematodes were collected in 2000, 2004, 2009, 2014 and 2019 at 3 stations along a bathymetric transect (1300, 2500, 4000 m) at the Long-Term Ecological Research (LTER) observatory HAUSGARTEN, a region of the Arctic Ocean undergoing rapid environmental change. Nematodes were identified to genus level and their biomass size distribution was calculated. Sedimentary food indicators, i.e. chloroplastic pigments (phytodetritus) and bacterial abundance/biomass, were analysed as explanatory variables. Food availability changed over time, with initial chlorophyll a decline at shallower depths, followed by increasing total pigment concentrations and bacterial biomass at greater depths, especially at 4000 m. Nematode abundances declined significantly across all depths, most notably by ~75% at 1300 m. Multivariate analyses revealed progressive and significant shifts in community composition, influenced primarily by depth and with clear separation between early (2000) and late (2019) samples. Alpha diversity (EG(50), J ‘, H ‘(log2)) declined over time, remaining highest at 1300 m. Beta diversity based on genus exchange ratios showed high genus turnover (29-77%) and changes in dominance (12-55%), suggesting a combination of immigration and replacement of rare genera. Our findings indicate that long-term warming in surface waters and an accompanying shift in productivity are potentially reshaping deep-sea nematode communities, particularly at bathyal depths. This study highlights the value of sustained long-term time-series for understanding deep-sea benthic responses to climate change.