Description: Ecosystem stability depends to a great extent on food web structure. Recent research indicates that in particular the share of weak links in the web is a crucial feature. Link strength is to some extent related to feeding strategy, i.e. specialists should have stronger links to their prey than generalists. Therefore a closer look at the distribution of feeding strategies among the species representing a system may already provide some hints on link strength distribution.Information on diet and feeding strategy is easily obtained from species who offer a neat view on their stomach content, such as fish. Systems that comprise of animals which are small, ingest lots of ballast together with their food, are much more difficult to handle. Soft-bottom macrozoobenthic communities combine these unfavourable features, and therefore are widely neglected in detailed studies of trophic functioning. We used a stable isotope approach referring to the soft-bottom macrozoobenthic subsystem of the German Bight to tackle this problem. The relation between intra-population range in nitrogen isotopic ratios (δ15N) and feeding strategy was analysed. We hypothesize that the δ15N range is a proxy for trophic niche width and thus (i) is indicative of feeding strategy, i.e. omnivore/generalist, specialist, ontogenetic diet shifter; and (ii) will provide quantitative information on basic trophic differences.

Description: High trophic level consumers such as demersal fish influence the trophic structure of marine benthic communities. Owing to their high mobility, consumers could act as integrators by connecting food webs. But do fish species in the North Sea link food webs of benthic communities or do they feed habitat dependent on a local scale? We examined the diet of the dominant benthic fish species (dab, plaice, whiting and grey gurnard) at the North Frisian sublitoral in the German Bight. The study area comprises various habitats with their typical benthic associations. Sediments from fine to coarse sands, gravel and even rocky habitats are present. The diet of fish from two different habitats (fine and coarse sands) was compared by two methods: (a) stomach content analyses to identify distinct prey items as a kind of snapshot and (b) stable isotopes (δ15N) to investigate long-term differences of habitat-dependent diet, i.e. differences in trophic positions. Both methods enable us to estimate inter-specific differences in feeding, i.e. to determine trophic niches of species and to compare intra-specific differences of habitat-dependent diet. Knowledge on trophic interactions of fish based on habitat dependency might therefore provide a key to determine habitats essential for fish feeding, useful for the development of ecological management and strategies for conservation.

Description: In 1999, the AWI deep-sea research group established the first and only long-term deep-sea observatory beyond the polar circle, 'Hausgarten' (see Soltwedel et al.). Footage from underwater camera transects has shown that demersal fish largely belonging to the eelpout family (Zoarcidae) constitute an important fraction of the benthic fauna. Despite their numerical abundance, little is known about their biology and functional ecological role.Here, we used baited traps and trawls to sample the demersal ichthyofauna at six stations that constitute the shallower end of the Hausgarten depth transect (1200-3200m). Stomach contents analysis showed that trawled eelpout chiefly fed on small crustaceans (cumaceans, amphipods, isopods) and polychaetes whereas trap-caught fish fed largely on scavenging amphipods.Results from radio stable isotope analysis indicate that starfish (Hymenaster pellucidus, Poraniomorpha cf. tumida, Bathybiaster vexillifer) followed by scavenging amphipods (Eurythenes gryllus) occupied the highest trophic level at most stations (δ15N 11.9-24.6). Fish had an intermediate trophic position (Lycodes spp. δ15N 12.8-13.7), reflecting their preference for macrofaunal invertebrates. The lowest trophic level at different stations was occupied by various taxonomic groups due to differences in the composition of species sampled. Assuming a stepwise δ15N enrichment of 3.8 per trophic level, the benthic food web sampled at stations along the depth gradient ranged from four to six trophic levels. This large figure may be indicative of a complex food web structure caused by intense recycling of nutrients which is characteristic for food-limited deep-sea environments. Food limitation may be even stronger at high latitudes due to reduced primary productivity caused by seasonal ice coverage.