only was +20), clearly enriched by bomb carbon (DeMaster et al. 2002).
(^) Particularly during the open-water season of diatom blooms, the coastal Antarctic is
subject to substantial upwelling, so bomb carbon is a much smaller component in
surface plankton, with ^14 C negative relative to standard at −135 ± 10 ppm . There
remains, however, a strong difference between that level and carbon in surface
sediment at −234 ± 13 ppm. A variety of trawl-collected surface-deposit feeders,
mostly echinoderms (sea cucumbers and urchins), had tissue ^14 C levels of −125 ± 13
ppm, comparable to surface plankton not to bulk sedimentary organic matter
(Purinton et al. 2008). Clearly, the benthonts are constituted primarily of carbon from
recent, near-surface production. They must achieve that by selective feeding on more-
nutritious particles and by digestion and assimilation primarily of newly produced
organic matter. The latter is partly insured by the refractory character of molecules of
greater age; they are resistant to enzymatic attack. For example, Ahrens et al. (2001)
have shown, with isotope-labeled food, that the polychaete Nereis (again, experiments
favor readily accessible nearshore forms) absorbs fresh algae with 55–95% efficiency,
compared to 5–18% for carbon in bulk sediment. Larger worms showed the greater
efficiencies, possibly due to longer gut passage times. Selective ingestion has been
observed in experimental work with a wide variety of benthic animals (e.g. Taghon
1982). In Taghon’s study with miniscule glass beads, smaller beads were selected over
larger, and protein-coated beads over bare ones, by deposit-feeding worms and clams.
Either or both “preferences” could result from more favored particles bonding more
effectively to mucus on feeding surfaces. To some extent, also, “selection” may
simply reflect accelerated feeding when fresh organic matter arrives at the seafloor
(Taghon & Jumars 1984).
Bulk Benthic Processes
(^) As for pelagic ecosystems, processes in benthic habitats can be bundled into bulk
effects, and their overall rates measured. Studies in this mode allow placement of the
seafloor into general biogeochemical models of the global ecosystem. Many such bulk
measures are of sediment mixing and oxygen-utilization rates.
Bioturbation
(^) One bulk effect of benthic biota is sediment mixing, referred to as bioturbation. This
mixing is most readily demonstrated by profiles of radionuclides in the upper layers
of sediment. For example, radioactive carbon (^14 C) is generated in the atmosphere by
cosmic rays (also for a while by bomb tests) and, like other carbon isotopes, it
becomes incorporated in organic matter and reaches the sediment in descending
organic particles, including marine snow, fecal pellets, and whale carcasses. Then it is