Organic carbon and nitrogen source of sunken wood communities on continental shelves around Japan inferred from stable isotope ratios

The food source for animals associated with sunken wood on the deep-sea floor has not been clearly identified. In the present study, we analyzed carbon- and nitrogen-stable isotope ratios of these animals to clarify (a) whether or not species that are potentially able to ingest wood (direct user) really digest and use it as their food source in situ, or (b) whether or not sunken wood is the organic source for the species that cannot feed on it directly (indirect user). The average isotopic values of the direct user, wood-boring bivalves, were δ13C:−24.14±0.38‰ (n=13) and δ15N:4.05±0.27‰ (n=11). The value of δ15N was close to that of their host wood (1.97±1.87‰ (n=7)) and lower than that of suspended particles assumed from that of suspension feeding species, suggesting that direct users do not use either their host wood or suspended particles as their nitrogen source. Given ambient nitrogen gas value (δ15N: around 0.5‰), bacteria symbiotic with the direct user appear to autotrophically fix ambient nitrogen gas and the bacteria are probably the major nitrogen source for the direct user. The average δ13C of wood was −26.85±1.13‰ (n=7), suggesting that wood is the carbon source of the direct user. On the other hand, the average δ13C of the indirect user (−23.29±0.99‰ (n=27)) was distinctly higher than that of wood and they were considered to use organic matter derived from both phytoplankton and their host wood.