How picky can you be? Temporal variations in trophic niches of co-occurring suspension-feeding species

• We investigated trophic niches of three common suspension-feeding species.
• We examine changes in individual and population isotopic compositions with time.
• Metrics derived from isotopic- and proportion-spaces can reveal trophic plasticity.
• Species able to retain smaller particles showed higher trophic diversity.
• Species with higher selection capacities showed less temporal variation in niches.

Suspension-feeders largely dominate faunal communities on rocky shores and compete for food using different feeding strategies. We used stable isotopes to assess the individual specialization within common suspension-feeder populations and to evaluate both inter-specific and intra-specific differences in food source exploitation. Trophic niches were characterized by metrics calculated in a space formed by mixing model outputs. Honeycomb worms (Sabellaria alveolata), blue mussels (Mytilus edulis) and barnacles (Chthamalus montagui), as well as three organic matter sources (benthic microalgae, phytoplankton and green macroalgae) were surveyed over a year using stable isotopic compositions (δ13C, δ15N). Adult specimens from each species were analyzed separately. Results showed persistent differences in δ13C and δ15N signatures between consumers, suggesting a possible limitation in inter-specific competition among co-occurring suspension-feeders, yet receiving the same food mixture during high tide. The width of the trophic niche was estimated for each species by transforming the isotopic space defined in a δ-space (δ13C vs. δ15N) into a p-space defined by the proportions of food sources in a ternary plot. For each species, several metrics were calculated to estimate the diversity of individual diets of species. Species able to retain small particles and selectively sort particle based on their quality showed higher trophic diversity within the population but a smaller temporal variation in trophic niche.

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