Variation of geochemical environments associated with whale-fall biomass mineralization processes in the sediment during the mobile scavenger, enrichment opportunist, and sulfophilic stages

Abstract

The succession of faunal composition of a whale-fall community is believed to depend on the progress of decomposition of the whale carcass, which itself is dependent on microbial activity in the sediment around the carcass. This means that the faunal succession could be reflected in the variation of the geochemical environment. In this study, we examined sperm whale carcasses placed in two areas of Japanese waters. In Sagami Bay at a depth of ca. 500 m, dive surveys were performed 2 weeks, 2 months, and 9 months after implantation, when mobile scavengers and Osedax worms were observed. Off Cape Nomamisaki, 12 carcasses were implanted at a depth of 200–300 m, and dive surveys were performed there 17, 29, and 41 months after implantation, and abundant chemosynthesis-based mussels were observed at these times. The recovered sediment cores from beneath and around the carcasses in Sagami Bay showed increases in 15N-enriched ammonium and 34S-depleted sulfide and decreases in 34S-enriched sulfate during the 9 months following implantation, and their stoichiometric relationship suggested that proteinaceous matter from the carcass was decomposed preferentially followed by active bacterial sulfate reduction. In comparison, relatively low ammonium but high sulfide concentrations after 41 months off Cape Nomamisaki suggest that primarily lipids from the almost skeletonized carcasses were decomposed in the sediment during bacterial sulfate reduction. Such variation of decomposed organic substrates and geochemical environments result from interaction with the animals inhabiting in and on the sediments adjacent the whale carcasses. It is possible that evolution of the geochemical environment plays an important role in faunal succession, from the enrichment opportunist to sulfophilic stages.