Transport and deposition of macrophytes to the dysphotic bottom of coastal waters

Some macrophytes are transported to the deep-sea bottom and are utilized by heterotrophs in the deep-sea as a food source. We inferred the transport route of macrophytes toward the deep-sea based on similarity in the species compositions of macrophyte pieces collected from the dysphotic bottom off the Izu Peninsula and the drifting macroalgae reported for the study area. We also examined whether or not the macrophytes are buried in the sediment, based on stable isotope distributions of organisms. Macrophytes collected by dredging at a depth of 100–300 m included 93 species, whereas 43 species were found by trawling at depths from 200 to 400 m. Only 15 of 76 dredged species (19.7%) that were identified to the species level were identical to the drifting macroalgal species reported for this area, whereas 15 of the 29 trawled species (51.7%) that were identified to the species level were identical to the reported drifting species. It was thus inferred that macrophytes were mainly transported through sliding along the sea bottom for the macrophytes collected by dredging and through sinking from the surface water for the macrophytes collected by trawling. The δ13C of sedimentary organic matter (SOM) from the 200–300 m zone was similar to the δ13C distribution of particulate organic matter in the surface water reported for the study area. The SOM in the zone likely originated from almost exclusively phytoplankton. In contrast, the 13C of SOM was significantly more enriched in shallow areas ≤100 m deep. We infer that not only phytoplankton but also macrophytes could supply organic matter to heterotrophs on the shallow bottom.