Benthic food web structure in the Comau fjord, Chile (∼42°S): Preliminary assessment including a site with chemosynthetic activity

Abstract Using C and N stable isotopes we analyzed different trophic aspects of the benthic fauna at two sites in the Comau fjord: one with presence of venting of chemically reducing fluids and extensive patches of bacterial mats (XH: X-Huinay), and one control site (PG: Punta Gruesa) with a typical fjord benthic habitat. Due to the widespread presence of such microbial patches in the fjord and their recognized trophic role in reducing environments, we hypothesize that these microbial communities could be contributing to the assimilated food of consumers and transferring carbon into high trophic levels in the food web. Food sources in the area included macroalgae with a wide range of δ 13 C values (−34.7 to −11.9‰), particulate organic matter (POM, δ 13 C = −20.1‰), terrestrial organic matter (TOM, δ 13 C = −32.3‰ to −27.9‰) and chemosynthetic filamentous bacteria (δ 13 C = ∼−33‰). At both sites, fauna depicted typical values indicating photosynthetic production as a main food source (>−20‰). However, at XH selected taxa reported lower δ 13 C values (e.g. −26.5‰ in Nacella deaurata ), suggesting a partial use of chemosynthetic production. Furthermore, enhanced variability at this site in δ 13 C values of the polyplacophoran Chiton magnificus , the limpet Fissurella picta and the tanaid Zeuxoides sp. may also be responding to the use of a wider scope of primary food sources. Trophic position estimates suggest three trophic levels of consumers at both sites. However, low δ 15 N values in some grazer and suspension-feeder species suggest that these taxa could be using other sources still to be identified (e.g. bacterial films, microalgae and organic particles of small size-fractions). Furthermore, between-site comparisons of isotopic niche width measurements in some trophic guilds indicate that grazers from XH have more heterogenic trophic niches than at PG (measured as mean distance to centroid and standard deviation of nearest neighbor distance). This last could be ascribed to the utilization of a mixture of photosynthetic and chemosynthetic carbon sources. In addition, corrected standard ellipses area (SEAc) values in suspension-feeders and carnivores at both sites suggest a similar magnitude of exploitation of food sources. However, grazers from XH have a greater expansion of their isotopic niche (SEAc), probably explained by the presence of species with low δ 13 C and δ 15 N values, and directly associated to chemosynthetic carbon incorporation.