Effects of sediment disturbance on deep-sea nematode communities: Results from an in-situ experiment at the arctic LTER observatory HAUSGARTEN

The present study examines the effects of experimentally generated disturbance on bathyal nematode communities at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN, situated in the Fram Strait, between Greenland and Svalbard. In order to understand the complex interactions between the biota and environmental perturbations we deployed a free-falling device (bottom lander) equipped with three rotating fork-like disturber units, able to perturbate the upper sediment layers with different disturbance frequencies at chosen time intervals. During a one-year deployment at 2493 m water depth, disturber unit DI was programmed to rotate every 14 days, DII every 28 days, and DIII every 72 days, resulting in 28, 14, and 7 perturbations, respectively. Sediment sampling following this experimental period was conducted with push-coring devices deployed by the Remotely Operated Vehicle “QUEST 4000” (MARUM, Bremen). These sediment cores were sub-sampled to determine the effect of the sediment perturbations on various sediment parameters (i.e., grain size distribution, chloroplastic pigment concentrations) as well as on benthic nematode communities. A total of 4773 nematodes from 27 families and 81 genera were identified. Nematode densities in the disturbed areas ranged from 617 ind. / 10 cm2 to 1566 ind. / 10 cm2, with a mean density of 1193 ind. / 10 cm2 observed overall in the disturbed sediments. Control sediments contained on average 20% more nematode specimens than were found within the disturbed sediments, with an average density of 1477 ind. / 10 cm2 observed. Nematode evenness (J'), genera richness (EG(51)) and heterogeneity (H′) were not significantly different between the treatments and controls (undisturbed vs disturbance). We found a significant effect of the interaction of disturbance frequency and sediment depth (interaction term Fr x De) on heterogeneity and genera richness, while evenness significantly differed between different sediment depths (De) and within the disturbed sediments between different disturbance frequencies (Fr). Although surface sediments in the three disturbed areas were effectively perturbated, sediment depth still has the most pronounced influence on nematode community attributes, while the experimentally induced disturbance had only a limited impact on nematode diversity and community structure. Although we found the density of some nematode genera was negatively affected by the disturbances, the deep-sea nematode community at HAUSGARTEN was generally characterized by a relatively high diversity and seemed to be largely resilient to the experimental disturbances.