111
ses will be discussed more thoroughly in the following Section 3.6 concerned with bioirrigation.
The total oxygen consumption at the transition
between sediment and bottom water is measured
in so-called incubation experiments which are best
to be carried out under in situ conditions. To this
end, a distinct area of the sediment surface area
(usually several dm
2
) is covered with a closed box
in such a manner that a part of the bottom water is
entrapped. Inside the closed box, the concentrations of oxygen and/or nutrients are repeatedly
measured over a sufficient length of time. The
total exchange of substances is deduced from the
measured concentration changes in relation to the
volume of entrapped bottom water and the area
under study.
Figure 3.24 shows the result of such an in situ
experiment taken from a study conducted by
Jahnke and Christiansen (1989). The sediment’s
uptake of the electron acceptors oxygen and
nitrate is easy to quantify, as well as the concomitant release of phosphate, silica, and carbonate
into the bottom water.
Glud et al. (1994) also carried out in situ incubation experiments in the upwelling area off the
coast of Namibia and Angola, in a parallel procedure to their recording diffusion controlled
oxygen profiles by means of microelectrodes.
Thus, they were able to obtain values for the
sediment’s total oxygen uptake and, at about the
same site, simultaneously measured the exclusively diffusion-controlled oxygen uptake. The ratio
of total uptake and diffusive uptake yielded
values between 1.1 and 4. The higher ratios were
coupled to the altogether higher reaction rates
observable in organic-rich sediments at shallower
depths. Similarly a good correlation between the
difference of total oxygen uptake minus diffusive
oxygen uptake on the one hand, and the abundance of macroscopic fauna on the other hand
was observed.
Peepers, Dialysis, and Thin Film Techniques
At first glance, dialysis would appear to be a very
useful technique for obtaining pore water samples
under in situ conditions which can be analyzed
for nearly all constituents. Different types of
‘peepers’ are presently used which can be pressed
into the sediment from the sediment surface. Such
a peeper usually consists of a PE-column 0.5 - 2 m
long with ‘peepholes’ on its outside. Cells filled
with de-ionized water inside the peeper are
allowed to equilibrate via dialysis through a filter
membrane with the pore water outside the peeper.
However, an equilibration time of many days or
even a few weeks may be necessary if a volume of
fluid greater than 5 - 10 cm
3
is necessary for
analysis and if a small peephole is required in
order to achieve a certain depth resolution. This
Fig. 3.25 Concentration-depth profiles for Mn and Fe through the sediment/water interface. For these
measurements, a DET gel was deployed for 24 hours in situ. (after Davison et al. 1994)
3.5
In situ Measurements
ses will be discussed more thoroughly in the following Section 3.6 concerned with bioirrigation.
The total oxygen consumption at the transition
between sediment and bottom water is measured
in so-called incubation experiments which are best
to be carried out under in situ conditions. To this
end, a distinct area of the sediment surface area
(usually several dm
2
) is covered with a closed box
in such a manner that a part of the bottom water is
entrapped. Inside the closed box, the concentrations of oxygen and/or nutrients are repeatedly
measured over a sufficient length of time. The
total exchange of substances is deduced from the
measured concentration changes in relation to the
volume of entrapped bottom water and the area
under study.
Figure 3.24 shows the result of such an in situ
experiment taken from a study conducted by
Jahnke and Christiansen (1989). The sediment’s
uptake of the electron acceptors oxygen and
nitrate is easy to quantify, as well as the concomitant release of phosphate, silica, and carbonate
into the bottom water.
Glud et al. (1994) also carried out in situ incubation experiments in the upwelling area off the
coast of Namibia and Angola, in a parallel procedure to their recording diffusion controlled
oxygen profiles by means of microelectrodes.
Thus, they were able to obtain values for the
sediment’s total oxygen uptake and, at about the
same site, simultaneously measured the exclusively diffusion-controlled oxygen uptake. The ratio
of total uptake and diffusive uptake yielded
values between 1.1 and 4. The higher ratios were
coupled to the altogether higher reaction rates
observable in organic-rich sediments at shallower
depths. Similarly a good correlation between the
difference of total oxygen uptake minus diffusive
oxygen uptake on the one hand, and the abundance of macroscopic fauna on the other hand
was observed.
Peepers, Dialysis, and Thin Film Techniques
At first glance, dialysis would appear to be a very
useful technique for obtaining pore water samples
under in situ conditions which can be analyzed
for nearly all constituents. Different types of
‘peepers’ are presently used which can be pressed
into the sediment from the sediment surface. Such
a peeper usually consists of a PE-column 0.5 - 2 m
long with ‘peepholes’ on its outside. Cells filled
with de-ionized water inside the peeper are
allowed to equilibrate via dialysis through a filter
membrane with the pore water outside the peeper.
However, an equilibration time of many days or
even a few weeks may be necessary if a volume of
fluid greater than 5 - 10 cm
3
is necessary for
analysis and if a small peephole is required in
order to achieve a certain depth resolution. This
Fig. 3.25 Concentration-depth profiles for Mn and Fe through the sediment/water interface. For these
measurements, a DET gel was deployed for 24 hours in situ. (after Davison et al. 1994)
3.5
In situ Measurements