Chemosynthetic trophic support for the benthic community at an intertidal cold seep site at Mocha Island off central Chile

Abstract

We analyzed C and N stable isotope ratios of benthic fauna and their potential food sources at an intertidal methane seep site and a control site without emanation at Mocha Island (central Chile). The objective was to trace the origin of the main food sources used by the local heterotrophic fauna, based on the hypothesis that chemosynthetic production could be partially fueling the local food web at the seep site. Food sources sampled at both sites included macroalgae, particulate organic matter and bacteria-like filaments found growing over the red algae Gelidium lingulatum within the areas of active methane release. At the control site, located 11 km away from the gas emanation, fauna exhibited moderate δ 13C values ranging from −16.2‰ (in a nereid polychaete) to −14.8‰ (in a cirolanid isopod), which were consistent with those of the potential photosynthetic food sources sampled at this site (−20.2 to −16.5‰). δ 13C values of the photosynthetic food sources at the seep site similarly ranged between −25.4 and −17.9‰. However, a portion of the animals at this site were consistently more 13C-depleted, with δ 13C values close to that of the seeping methane (−43.8‰) and the bacteria-like filaments (−39.2 ± 2.5‰) also collected at this site. Specific examples were the Marphysa sp. polychaetes (δ 13C = −44.7 ± 0.6‰), the Schistomeringos sp. dorvilleid polychaetes (δ 13C = −42.9‰), and the tanaid crustacean Zeuxo marmoratus (δ 13C = −37.3 ± 0.2‰). The significantly higher δ 13C values of the herbivorous gastropod Tegula atra at the seep site (−29.3 ± 3.1‰) than at the control site (−12.6 ± 0.3‰) also indicated differences among sites of the preferred carbon sources of this species. Mixing model estimates indicate that at the seep site bacteria-like filaments could be contributing up to ∼60% of the assimilated diet of selected invertebrates. Furthermore, several indicators of trophic structure, based in isotopic niche metrics, indicate a considerably higher variety of basal resources used at the seep site and a consequent enhanced trophic diversity, compared with the non-seep site. The isotopic data gathered at the seep site clearly and unequivocally reflects the ingestion of a 13C-depleted primary food source, which could be the bacteria-like filaments observed at this site.