Christiane Hasemann

Small-Scale Heterogeneity in Deep-Sea Nematode Communities around Biogenic Structures

The unexpected high species richness of deep-sea sediments gives rise to the questions, which processes produce and maintain diversity in the deep sea, and at what spatial scales do these processes operate? The idea of a small-scale habitat structure at the deep-sea floor provides the background for this study. At small scales biogenic structures create a heterogeneous environment that influences the structure of the surrounding communities and the dynamics of the meiobenthic populations. As an example for biogenic structures, small deep-sea sponges (Tentorium semisuberites Schmidt 1870) and their sedimentary environment were investigated for small-scale distribution patterns of benthic deep-sea nematodes. Sampling was carried out with the remotely operated vehicle Victor 6000 at the Arctic deep-sea observatory HAUSGARTEN. In order to investigate nematode community patterns sediment cores around three small sponges and corresponding control cores were analysed. A total of approx. 5800 nematodes were identified. The comparison of the nematode communities from sponge and control samples indicated an influence of the biogenic structure “sponge” on diversity patterns and habitat heterogeneity. The increased number of nematode species and functional groups found in the sediments around the sponges suggest that on a small scale the sponge acts as a gradient and creates a more divers habitat structure. The nematode community from the sponge sediments shows a greater taxonomic variance and species richness together with lower relative abundances of the species compared to those from control sediments. Obviously, the more homogeneous habitat conditions of the control sediments offer less micro-habitats than the sediments around the sponges. This seems to reduce the number of functional groups and species coexisting in the control sediments.

Effects of dropstone-induced habitat heterogeneity on Arctic deep-sea benthos with special reference to nematode communities

Abstract During an expedition to the deep-sea long-term observatory HAUSGARTEN in the eastern Fram Strait in summer 2003, the availability of a Remotely Operated Vehicle allowed a targeted sampling of surface sediments around a relatively large dropstone (0.9 m2) to determine suspected differences in community structure and dynamics of nematode assemblages in relation to the confined flow regime and patchy food availability in the immediate vicinity of the stone. The almost rectangular dropstone was about 150 cm in length, 60 cm in width, and up to 15 cm in height. Small-scale current measurements around the dropstone using a MAVS-3 acoustic current meter exhibited a rather complex pattern. A computational fluid dynamics simulation revealed areas of constantly flowing near-bottom currents as well as the generation of vortices in certain areas around the dropstone. Concentrations of biogenic compounds in the sediments surrounding the dropstone generally followed the complex flow pattern. The differences in physical and biochemical conditions around the dropstone were reflected in species composition and diversity, trophic structure and life-history traits of the nematode communities, and to a lesser extent in their total abundance and biomass.