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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.

1. Coral-reef managers must detect and reverse collapses in habitat and evaluate the success of such interventions. Since these responsibilities must be met with limited time and resources, methods used should balance statistical power with practical and logistical constraints.2. Photoquadrat analysis is a commonly used method to survey coral habitats. This method, which involves photographing substratum along transects and digitally analysing habitat at points on the 'photoquadrats', affords efficiency in the field but is costly and requires extensive desk-based analysis. It remains unclear what is the optimal combination of sampling units (points, photoquadrats and transects) needed to detect important trends in coral habitat.3. Here, a dataset on Philippine coral-reef habitats, collected using intensive photoquadrat surveys, was used to explore the reliability of using different numbers of points per photoquadrat, photoquadrats per transect and transects per site to detect spatial differences in habitat.4. Results of leave-some-out analyses were compared with analysis of the complete dataset. Using fewer points per photoquadrat and fewer photoquadrats per transect caused little decline in ability to detect key trends, and lessened desk-based time; reducing the number of photoquadrats also lessened field time. Using fewer transects reduced time requirements but at the expense of statistical reliability. 5. Prospective power analyses revealed that common rates of coral recovery could not be detected using even the most intensive photoquadrat protocols. This result implies that coral recoveries within protected areas might go undetected using standard surveying techniques. 6. Using fixed rather than randomly placed photoquadrats, or more sensitive indicators of habitat recovery than coral cover (e.g. coral surface area) may improve power to detect coral recoveries. Finally, protocols that minimize desk time rarely also minimize field time and vice versa, which highlights the need to prioritize different logistical constraints when designing methods.

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