Individual deposit-feeder, community and ecosystem level controls on organic matter diagenesis in marine benthic environments

Three studies explored the mechanisms that affect the fate of organic carbon at the ecosystem, community and individual organism levels. Carbon budgets and C isotope mass balances were used to identify long-term controls (1000's of years) on the fate of organic material deposited at four stations on the U.S. continental slope off Cape Hatteras, N.C. from 36° 20' to 35o 23'N. The controls on subsurface remineralization appear to be a complex function of the relative amount of metabolizable carbon delivered to the seabed both now and in the distant past (≥500ybp) and the extent of seabed irrigation. The age of DIC and CH4 produced within the seabed indicates that young, reactive carbon is advected below the sediment surface and fuels subsurface remineralization. The isotopic composition of DIC produced within the seabed indicates the selective degradation of specific fractions of the organic matter.;Additional experiments were used to identify processes affecting detritus recently deposited on the seafloor. In the first, 13C-labeled diatoms were added to coastal and slope sediments contained in microcosms. The labeled carbon was followed as it was mixed into the sediment and converted into deposit feeder biomass, fecal material and dissolved inorganic carbon (DIC). Movement of the tracer into the sediment was non-local in most experiments, regardless of the environment and its associated assemblage of animals. Irrigation was a more important factor in the distribution of tracer-derived DIC in the sandier coastal cores. Although the species of animals advecting organic matter into the seabed in both the coastal and slope sediments were different, it is hypothesized that animals in both locations will perform similar functions and fill the same niches.;Finally, the amino acid composition of diatoms was examined before and after digestion by deposit feeders responsible for subduction and ingestion of the tracer in the microcosm experiments. The polychaetes converted labeled diatomaceous amino acids into biomass and CO2 and then deposited the remainder in fecal material. The amino acid composition of the diatoms was altered during gut passage. The patterns of alteration suggested that the animals stripped organics off the surface of the diatom cell and did not easily access intracellular constituents.