5
Bacteria and Marine Biogeochemistry
176
weak gradient, such as sulfate, the diffusive
boundary layer plays no role since the uptake of
sulfate is totally governed by diffusion-reaction
within the sediment. The function of the diffusive
boundary layer as a barrier for solute exchange is
also reflected in the mean diffusion time of
molecules through the layer, which is about 1 min
over a 0.5 mm distance (Table 5.1).
The existence of a diffusive boundary layer is
apparent from microsensor measurements of
oxygen and other solutes at the sediment-water
interface. In a Danish coastal sediment, the
concentration of oxygen dropped steeply over the
0.5 mm thick boundary layer (Fig. 5.4) which
consequently had a significant influence on the
regulation of oxygen uptake in this sediment of
high organic-matter turnover. Figure 5.5 shows, as
another example, oxygen penetration down to
13 mm below the surface in fine-grained sediment.
The profile was measured in situ in the seabed by
a free-falling benthic lander operating with a
100 µm depth resolution, which was just sufficient
to resolve the diffusive boundary layer. Based on
the boundary layer gradient, the vertical diffusion
flux of oxygen across the water-sediment interface
can be calculated (cf. Chap. 3).
5.3
Regulation and Limits
of Microbial Processes
Bacteria and other microorganisms are the great
biological catalysts of element cycling at the sea
floor. The degradation and remineralization of
organic matter and many redox processes among
inorganic species are dependent on bacterial
catalysis, which may accelerate such processes up
to 10
20
-fold relative to the non-biological reaction
rate. It is, however, important to keep in mind that
this biological catalysis is based on living
organisms, each of which has its special
Fig. 5.5 Oxygen gradient measured in situ by a benthic
lander in Skagerrak at the transition between the Baltic Sea
and the North Sea at 700 m water depth. Due to the high
depth resolution of the microelectrode measurements it
was possible to analyze the O 2 microgradient within the
0.5 mm thick diffusive boundary layer. The framed part in
the upper graph is blown up in the lower graph. (Data from
Gundersen et al. 1995).
Fig. 5.4 Oxygen microgradient (data points) at the sedimentwater interface compared to the ratio, E/D (logarithmic scale),
between the vertical eddy diffusion coefficient, E, and the
molecular diffusion coefficient, D. Oxygen concentration was
constant in the overflowing seawater. It decreased linearly
within the diffusive boundary layer (DBL), and penetrated only
0.7 mm into the sediment. The DBL had a thickness of
0.45 mm. Its effective thickness, δ e is defined by the intersection between the linear DBL gradient and the constant bulk
water concentration. The diffusive boundary layer occurs
where E becomes smaller than D, i.e. where E/D = 1 (arrow).
Data from Aarhus Bay, Denmark, at 15 m water depth during
fall 1990 (Gundersen et al. 1995).
Bacteria and Marine Biogeochemistry
176
weak gradient, such as sulfate, the diffusive
boundary layer plays no role since the uptake of
sulfate is totally governed by diffusion-reaction
within the sediment. The function of the diffusive
boundary layer as a barrier for solute exchange is
also reflected in the mean diffusion time of
molecules through the layer, which is about 1 min
over a 0.5 mm distance (Table 5.1).
The existence of a diffusive boundary layer is
apparent from microsensor measurements of
oxygen and other solutes at the sediment-water
interface. In a Danish coastal sediment, the
concentration of oxygen dropped steeply over the
0.5 mm thick boundary layer (Fig. 5.4) which
consequently had a significant influence on the
regulation of oxygen uptake in this sediment of
high organic-matter turnover. Figure 5.5 shows, as
another example, oxygen penetration down to
13 mm below the surface in fine-grained sediment.
The profile was measured in situ in the seabed by
a free-falling benthic lander operating with a
100 µm depth resolution, which was just sufficient
to resolve the diffusive boundary layer. Based on
the boundary layer gradient, the vertical diffusion
flux of oxygen across the water-sediment interface
can be calculated (cf. Chap. 3).
5.3
Regulation and Limits
of Microbial Processes
Bacteria and other microorganisms are the great
biological catalysts of element cycling at the sea
floor. The degradation and remineralization of
organic matter and many redox processes among
inorganic species are dependent on bacterial
catalysis, which may accelerate such processes up
to 10
20
-fold relative to the non-biological reaction
rate. It is, however, important to keep in mind that
this biological catalysis is based on living
organisms, each of which has its special
Fig. 5.5 Oxygen gradient measured in situ by a benthic
lander in Skagerrak at the transition between the Baltic Sea
and the North Sea at 700 m water depth. Due to the high
depth resolution of the microelectrode measurements it
was possible to analyze the O 2 microgradient within the
0.5 mm thick diffusive boundary layer. The framed part in
the upper graph is blown up in the lower graph. (Data from
Gundersen et al. 1995).
Fig. 5.4 Oxygen microgradient (data points) at the sedimentwater interface compared to the ratio, E/D (logarithmic scale),
between the vertical eddy diffusion coefficient, E, and the
molecular diffusion coefficient, D. Oxygen concentration was
constant in the overflowing seawater. It decreased linearly
within the diffusive boundary layer (DBL), and penetrated only
0.7 mm into the sediment. The DBL had a thickness of
0.45 mm. Its effective thickness, δ e is defined by the intersection between the linear DBL gradient and the constant bulk
water concentration. The diffusive boundary layer occurs
where E becomes smaller than D, i.e. where E/D = 1 (arrow).
Data from Aarhus Bay, Denmark, at 15 m water depth during
fall 1990 (Gundersen et al. 1995).