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Most of the seabed macrofauna in the CCFZ were deposit feeders --
animals that fed on organic matter sinking to the seafloor, ingesting it along
with sediment particles. The vast bulk of animals in the CCFZ in the
macrofaunal size class were surface-deposit feeders, focusing their foraging
at the sediment/water interface on material recently settled to the seafloor.
Another subset of the macrofauna, subsurface-deposit feeders that
ingested sediments below the sediment/water interface, were relatively
rare compared to other sedimentary environments.
Deposit feeders in the deep sea were particle selective, as
demonstrated by a variety of particle-associated tracers, for example
chlorophyll a or radionucleides such as Th-234. They fed on recently
deposited particles – presumably organically rich particles such as
phytoplankton detritus -- that had settled to the seafloor within the previous
100 days or so. Because they needed to feed on recently arrived material,
any dilution of such food, for example by sediments resuspended from the
seafloor during mining, was likely to have a major deleterious impact on
their ability to feed and to grow.
Another consequence of lower POC flux, in addition to low
respiration rates and biomass, was the fact that bioturbation occurred at a
low rate. Radiotracer profiles to examine the rates at which sediments were
stirred by animal activity indicated that at 9? N, at the northern end of the
EqPac transect, bioturbation rates for Pb-210 were roughly one order of
magnitude lower than at 5? N, a short distance to the south, where
productivity and flux rates were substantially higher. The low rates at which
sediments were mixed had consequences for the rate at which redeposited
material might be integrated into the sediment column.
In addition to the basic mixing rates, the penetration depths of
particle-associated radiotracers were also low. One important parameter
for modelling chemical distribution in sediments and the fate of redeposited
material on the seafloor was the depth at which animals were mixing with
sediment. Once again, as in abundance and biomass, mixed layer depth
was strongly correlated with POC flux. Data correlating POC flux to the Pb-
210 depth in sediment showed that, in the CCFZ, mixed layer depths were
shallow -- only about 2 centimetres -- consistent with a low energy and
biomass regime.
Growth and recolonisation rates of animals in the deep sea in
general, and by inference in the CCFZ, were also low. One could only
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