Trophic relations between animal and non-animal matter in sedements investigated using stable isotopes

Three sets of sediment traps were placed in Otsuchi Bay, Northeast of Japan, one near the shore at 2m depth, two at the center of the bay at 2m and 13m depth each. Collections were made at least once a month, from 2003/12 to 2005/06. dS, dC and dN stable isotope values of the collected material were measured. Some samples contained visible animals, especially near the shore in spring, but animals were caught all year round in all sediment traps. There was a seasonal variation in the results, so the samples were divided into two kinds: samples from spring and summer, and samples from autumn and winter. Samples from the center of the bay at 2m and 13m in depth were very similar, so they were grouped together. In most of the cases, the samples containing animals had dC and dS similar to samples without animals, an indication that they have the same origin, except for samples from the center of the bay, especially in autumn and winter. The dN values of samples containing animals were always higher than those of samples without animals, except for near the shore in autumn and winter, with exceptionally low dN values for samples containing animals. It was assumed that the sedimentary organic matter reflected the isotopic composition of particulate organic matter (POM), and it was hypothesized that a great portion of the POM from near the shore might have been the main food source of the animals. If so, at the center of the bay, especially in autumn and winter, edible food was scarce, and the animals would be there for other reasons that not foraging. If the analyzed animals can be thought as representative of the primary consumers of the water column in the bay, then a part of POM would not enter the pelagic food chain.