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Biotic communities in highly variable, frequently perturbed habitats are usually expected to be structured mainly by abiotic factors. In the highly variable deep-sea hydrothermal vent environment, physical and chemical factors are known to play an important role in limiting the distribution and abundance of species, but the importance of biotic interactions remains largely unresolved. The high density and biomass attained by the vent macrofauna suggest that resource partitioning and competition may be significant in these communities. This study of food resource utilisation at northeast Pacific deep-sea hydrothermal vents uses an approach based on stable carbon and nitrogen isotope analyses to characterise trophic interactions between the 3 dominant alvinellid polychaetes, Paralvinella palmiformis, P. sulfincola, and P. pandorae. We also examine size structure in sympatric and allopatric populations of P. palmiformis and P. pandorae. Results indicate that food resource partitioning occurs both intra-and interspecifically in P. palmiformis and P. sulfincola, and we advocate that this process contributes to explaining their co-existence at very high densities. In contrast, P. pandorae has a much more restricted trophic niche, overlaps P. palmiformis in diet and is much smaller in size when found in sympatry with P. palmiformis. P. pandorae is the most likely of the 3 species to be affected by intra-and/or interspecific competition for food, and this may explain the drastic change in the population size structure observed between successive years. Our work indicates that within the limits imposed by environmental conditions, biotic interactions such as food resource partitioning and competition can be significant factors structuring deep-sea hydrothermal vent communities.

Deep-sea hydrothermal vent communities are supported by local microbial chemolithoautotrophic production. While nutritional symbiotic associations between microbial primary producers and their metazoan hosts are well characterised, food sources used by the diverse and abundant non symbiont-containing vent species remain poorly known. Vent suspension-and deposit-feeders are usually considered as primary consumers directly relying on free-living microbial primary production, but other sources of particulate organic matter (POM) may also be part of their diet. We investigated the origin, composition and nutritional quality of POM at Axial Volcano (NE Pacific) vents, using microscopic observations, stable isotopic and biochemical analyses. A positive correlation between the stable carbon isotopic composition of POM and that of vent fluid dissolved inorganic carbon (DIC) indicates that the bulk of vent particulate organic carbon is derived from the local chemolithoautotrophic fixation of vent DIC. Low estimates of bacterial and total microbial carbon might reflect a rapid turnover of free-living bacterial biomass in the vent ecosystem. The low microbial fraction, together with the presence of abundant debris, point to the existence of a large detrital fraction in the POM pool. This implies that the previously largely overlooked detrital compartment may be a significant part of the diet of consumers, and that organic matter recycling may be a major process in these ecosystems.

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