Phytodetritus, chemosynthesis, and the dark biosphere: Does depth influence trophic relationships at deep-sea Barbados seeps

Abstract

This study explores the role of photosynthetically-derived carbon in seep communities at shallower, bathyal seeps and at deeper, abyssal seeps on the Barbados accretionary prism. Carbon sources and feeding guilds were assessed from five Barbados seeps and three putative non-seep sites through carbon and nitrogen stable isotope analyses on numerically-dominant and/or high-biomass species. Thiotrophy and methanotrophy were both observed in Barbados seep symbiotrophs, although no taxa appeared to contain both types of endosymbionts. There was little evidence of incorporation of photosynthetically-derived carbon in the seep food webs except among the deposit feeding ophiuroids and a single sipunculid at the bathyal seep. It is hypothesized that there may be a maximum depth after which chemosynthesis becomes the dominant primary carbon source at seeps. In addition, raphitomid snails and thyasirid clams at two of the three sites that were categorized as “non-seep” had carbon signatures indicative of thiotrophy. Thus, chemosynthesis may be more widespread within the Barbados accretionary prism seep field than currently thought, potentially through a greater sphere of influence surrounding the seeps or as a result of the ‘dark biosphere’ of infaunal chemosynthetic organisms. The trophic relationships described here provide useful baseline data for environmental management related to proposed oil and gas exploration and extraction.