standardization of environmental data and information - International ...

Sediment-trap levels
The Workshop discussed a suggestion to add a sediment trap below
the two proposed by LTC, the lower of which would be at 500 m above the
bottom. One proponent of this idea noted that the depth of sediment and
nodule separation was still unknown; while another urged deep placement
– perhaps 10 m above the seabed -- in order to collect baseline data
needed to assess mining impacts. Smith observed that a trap could not be
used to track primary flux if it were put in the benthic boundary layer, 100-
200 m above the seafloor; 500 m was a standard deployment to look at
deep flux that was probably getting to the seafloor, without having the trap
impacted by the benthic boundary layer. Another participant agreed that
there must be a trap above this layer but argued that an additional trap
near the bottom could provide information about resuspension material.
Smith countered that the transmissometer (nephelometer)
suggested for physical oceanographic measurements in the benthic
boundary layer might provide even more data about resuspension than a
sediment trap. However, it was pointed out that a transmissometer
recorded data on small particles whereas a sediment trap collected large
particles. The French had combined nephelometry and the sediment trap
for more than ten years, employing a trap at 10 m together with a
transmissometer and another trap at 200 m from the bottom.
Bioturbation
Smith remarked that bioturbation was an easily measured index of
one form of community activity that was useful in indicating the depths at
which redeposited sediment would be mixed during community recovery
from a redeposition event. The LTC recommendations called for contractors
to gather data on the mixing of sediment by organisms. The Commission’s
explanatory commentary suggested that rates of bioturbation be assessed
with profiles of Pb-210, a standard isotope. It also suggested depth
intervals going from 0-1 down to 14-15 cm, a range that was probably not
relevant in the CCFZ, according to studies showing that bioturbation
penetrated only about 2 cm. He therefore recommended measurement
over a depth of 0-5 cm, which he expected to be uncontroversial. He
further recommended that five replicate profiles be taken at each station to
give a statistically valid sample size.
INTERNATIONALSEABED AUTHORITY 301