finally sequestered below the bioturbation limit near the continental margin. This
“burial efficiency” is much less, probably less than 2%, in bottoms beneath
oligotrophic, oceanic waters. For Station M, at the outer fringe of the California
Current upwelling zone, 9% is a reasonable number.
(^) Smith et al. (2008) used time-series of seafloor photographs to evaluate the
importance of large organic aggregates arriving at the seafloor – massive clots of
diatoms and radiolaria. These masses were not collected by the traps – the baffles
apparently hold them until they are swept off. Aggregates did provide additional
pulses of potential food to the benthos, mostly simultaneous with the seasonal peaks
of POC flux measured by traps. They were generally not enough to match the rates of
sediment metabolism (SCOC). Glud (2008) states that all comparisons on shorter time
scales show the same discrepancy: more metabolism implied by SCOC than can be
supported by trap-measured POC flux.
(^) Smith’s measurement of the standing stock of organic matter at Station M is 150 g
C m−2 in the top 3 cm of sediment. While the seasonal cycle of respiration implies
that this may be somewhat degraded stuff, it could sustain metabolism of 3–4 mg C m
−2 yr−1 for a considerable time, requiring only very occasional replenishment beyond
the usual flux of POC. Traps do not measure the transfer via large food falls (whale
bodies, etc.), a food source redistributed over the bottom by scavengers at a rate that
cannot at present be estimated. Lateral supply in near-bottom currents is also possible,
particularly adjacent to the continental slope where Station M is located. It might be
thought, since most of the actual benthic metabolism is attributable to bacteria and
other microbes which could assimilate DOC from interstitial water, that this would be
an additional source. However, that DOC is dilute (standing stock ∼0.16 g C m−2 to 3
cm), and it is unlikely to be replenished from DOC in the overlying water where it is
even less concentrated with older ^14 C ages, implying less food value. It must come
from sediment POC, so metabolism of DOC does not help to explain the apparent
imbalance of oxygen uptake and POC sedimentation. Andersson et al. (2004) take the
view that SCOC must represent the real rate of organic-matter flux, estimating a
relatively large global rate of transfer to the deep sea. However, a map of DOU and
TOU measurement sites (Seiter et al. 2005) shows extreme bias toward coastal zones,
leaving vast oceanic areas with no measurements and bottom oxygen concentration
varying 10-fold.
Community Response
(^) Another question of interest is how do macro- and megafaunal communities respond
to the variations in food input. Ruhl (2008) has examined the Station M megafauna
from the time-series of camera sled photographs, showing both substantial stability of