Measurement
(^) Sediment community oxygen consumption (SCOC) has been extensively studied as a
measure of the metabolism of everything living in the seafloor. That is of importance
for evaluating the role of the seafloor in the overall economy of organic matter in the
sea. Ronnie Glud (2008) has provided a landmark review of the status of this work,
including evaluation of technical issues not considered here.
(^) If you wanted to measure the oxygen consumption of all infaunal residents (from
bacteria to clams) of an intertidal mudflat, you could take a can open on one end,
install a valve on the other and a stirrer inside, wade out when the tide was at mid-
thigh height, fill the can with bay water, and insert the open end slowly into the
sediment with the valve open to let water out as the can slid into the mud. You would
be careful to insert it to a known depth so that the volume inside would also be
known. Then you would attach a syringe to the valve, take a water sample, and close
the valve. That sample would be analyzed for oxygen content ([O 2 ]) before an
incubation period. A series of samples taken over time would show a decline in [O 2 ].
The rate of oxygen use, or total oxygen utilization rate (TOU) would be an estimate of
the benthic metabolism for the area under the can over the study interval.
(^) Modern technology allows this to be done with automatic sampling at any depth in
the sea, by using canisters attached to benthic landers. Landers, essentially big tripods,
descend to the seafloor, set down gently, wait for the dust to clear, and then slowly
sink one or several canisters into the sediment below them. Oxygen concentration is
determined in a time-series of syringe samples or by a recording electrode.
Respiration measures have also been tried by inserting canisters into retrieved box
cores. However, return to the surface involves decompression and warming, both of
which alter oxygen-uptake rates, generally increasing them.
(^) Consumption of oxygen by the biota of sediments would render the sediments
anoxic if oxygen did not diffuse in from the water column above. Oxygen
concentration can be profiled in sediments with exquisite vertical precision, using
microelectrodes – usually platinum wires embedded in glass micropipettes and
compared to silver–silver chloride reference electrodes above the sediment surface.
Microelectrodes are also mounted on motorized microprofiling instruments carried by
landers (e.g. Reimers 1984). After the dust clears, the electrodes are driven slowly
into the sediment, producing profiles (Fig. 14.20). Electrode output can be calibrated
with the oxygen concentration (perhaps determined by Winkler titration) of a bottom-
water sample and the value several centimeters in the sediment (usually zero). More
recently glass optical-fibers have been doped at the tip to produce fluorescent signals
convertible to oxygen concentration, creating oxygen “optodes”. These, too, have
been adapted to penetrate the sediment from landers (Wenzhöfer et al. 2001).