6
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
226
most areas of the global ocean. With regard to
oxygen a possible intensification of organic matter
decay, triggered by the temperature increase or
the lysis of cells, might result in a reduction of the
oxygen penetration depth due to higher oxygen
consumption. This effect has intensely been
studied by Glud et al. (1994) and Wenzhöfer and
Glud (2002) who could show that there might be
significant discrepancies between in-situ and exsitu measured fluxes and oxygen penetration
depths. This effect is illustrated in Figure 6.16 and
shows that the difference obviously increases
with increasing water depth. At or above 1,000 m
the sampling effect seems to be more or less
negligible.
In analogy to oxygen, such artifacts are
believed to exist also for nitrate profiles determined ex-situ. The occurrence of increased
(compared to Redfield stoichiometry in Eq. 6.1)
subsurface nitrate concentrations has been
described in a number of studies (e.g. Hammond et
al. 1996; Martin and Sayles 1996), but is mostly
attributed to the centrifugation method in pore
water extraction (see Section 3.3.2). Artificially
increased subsurface nitrate concentration would
consequently lead to an overestimation of benthic
fluxes of nitrate. Berelson et al. (1990) and
Hammond et al. (1996) found evidence for increased nitrate fluxes after pore water centrifugation
compared to lander measurements, which were up
to a factor of 3, but also agreement between both
methods for quite a number of stations was found.
However, the possibility of low C/N organic matter
or oxidation of reduced nitrogen species
(diffusing upwards from deeper layers) can be
important natural factors increasing nitrate
concentrations deviating from the general
expected stoichiometry. Following results of
Luther et al. (1997) the amount of subsurface
nitrate production due to nitrification can also be
regulated by the manganese oxide content of the
sediment. As discussed in Section 6.3.2.3 high
MnO 2 concentrations favor a catalytic reduction
of nitrate to N 2 already in the oxic zone of the
sediment so that nitrate peaks only occur when
the solid-phase manganese content is low.
A further aspect of decompression that should
be kept in mind is the degassing of CO 2 and the
resulting precipitation of CaCO 3 , which might
affect pore water concentrations of phosphate by
adsorption or co-precipitation (Jahnke et al. 1982).
This effect can even imply negative fluxes as discussed below (see Section 6.5.1).
In general, fluxes, which are calculated on the
basis of ex-situ data should be interpreted with
caution; the overall result, however, in most cases
reveals a reasonable approximation to real conditions (cf. Section 12.2).
6.4.3
Determination of Denitrification
Rates
The downward flux of nitrate (e.g Fig. 6.5)
indicates the depth of active denitrification, but is
no measure for the total rate of denitrification. As
mentioned above, denitrifying bacteria are facultative anaerobic and denitrification is generally
located directly below the oxic zone (Christensen
et al. 1989). The shape of a nitrate profile depends
on the total rate of nitrification and the denitrification rate. Considering the example shown in
Figure 6.5, the fit of the measured nitrate profile
was achieved by determining an indirect nitrification rate (coupled oxidation and nitrification as
described by Equation 6.1 and the C/N ratio) and
denitrification occurring with a distinct rate at a
depth of about 3 cm. A reduction of the denitrification rate would result in a greater nitrate
penetration depth, i.e. not all the nitrate can be
consumed in this depth zone. On the other hand,
an increase of the denitrification rate would lead
to a depression of the nitrate maximum and
therefore reduce upward and downward fluxes,
and finally induce a total nitrate flux from the
bottom water into the sediment. To clarify these
interactions we plotted three nitrate profiles from
different regions of the South Atlantic in
Figure 6.17. Two profiles indicate high respiration
rates with a nitrate penetration depth of approximately 3 cm, but a distinct peak is visible only at
one station. The station with the nearly linear
gradient into the sediment is indicative for strong
denitrification. The third profile shows a low
gradient into the bottom water which is due to
nitrification and remains nearly constant with
depth indicating that denitrification does not
occur close to the sediment surface.
Depletion of nitrate and the formation of
dinitrogen are, however, not exclusively coupled
to denitrification, since the microbially mediated
reduction utilizing reduced species like Mn
2+
or
Fe
2+
might occur. Based on field observations, a
number of studies invoke the reduction of nitrate
by Mn
2+
to form N 2 instead of organic matter
respiration (Aller 1990; Schulz et al. 1994; Luther
et al. 1997,1998).
Benthic Cycling of Oxygen, Nitrogen and Phosphorus
226
most areas of the global ocean. With regard to
oxygen a possible intensification of organic matter
decay, triggered by the temperature increase or
the lysis of cells, might result in a reduction of the
oxygen penetration depth due to higher oxygen
consumption. This effect has intensely been
studied by Glud et al. (1994) and Wenzhöfer and
Glud (2002) who could show that there might be
significant discrepancies between in-situ and exsitu measured fluxes and oxygen penetration
depths. This effect is illustrated in Figure 6.16 and
shows that the difference obviously increases
with increasing water depth. At or above 1,000 m
the sampling effect seems to be more or less
negligible.
In analogy to oxygen, such artifacts are
believed to exist also for nitrate profiles determined ex-situ. The occurrence of increased
(compared to Redfield stoichiometry in Eq. 6.1)
subsurface nitrate concentrations has been
described in a number of studies (e.g. Hammond et
al. 1996; Martin and Sayles 1996), but is mostly
attributed to the centrifugation method in pore
water extraction (see Section 3.3.2). Artificially
increased subsurface nitrate concentration would
consequently lead to an overestimation of benthic
fluxes of nitrate. Berelson et al. (1990) and
Hammond et al. (1996) found evidence for increased nitrate fluxes after pore water centrifugation
compared to lander measurements, which were up
to a factor of 3, but also agreement between both
methods for quite a number of stations was found.
However, the possibility of low C/N organic matter
or oxidation of reduced nitrogen species
(diffusing upwards from deeper layers) can be
important natural factors increasing nitrate
concentrations deviating from the general
expected stoichiometry. Following results of
Luther et al. (1997) the amount of subsurface
nitrate production due to nitrification can also be
regulated by the manganese oxide content of the
sediment. As discussed in Section 6.3.2.3 high
MnO 2 concentrations favor a catalytic reduction
of nitrate to N 2 already in the oxic zone of the
sediment so that nitrate peaks only occur when
the solid-phase manganese content is low.
A further aspect of decompression that should
be kept in mind is the degassing of CO 2 and the
resulting precipitation of CaCO 3 , which might
affect pore water concentrations of phosphate by
adsorption or co-precipitation (Jahnke et al. 1982).
This effect can even imply negative fluxes as discussed below (see Section 6.5.1).
In general, fluxes, which are calculated on the
basis of ex-situ data should be interpreted with
caution; the overall result, however, in most cases
reveals a reasonable approximation to real conditions (cf. Section 12.2).
6.4.3
Determination of Denitrification
Rates
The downward flux of nitrate (e.g Fig. 6.5)
indicates the depth of active denitrification, but is
no measure for the total rate of denitrification. As
mentioned above, denitrifying bacteria are facultative anaerobic and denitrification is generally
located directly below the oxic zone (Christensen
et al. 1989). The shape of a nitrate profile depends
on the total rate of nitrification and the denitrification rate. Considering the example shown in
Figure 6.5, the fit of the measured nitrate profile
was achieved by determining an indirect nitrification rate (coupled oxidation and nitrification as
described by Equation 6.1 and the C/N ratio) and
denitrification occurring with a distinct rate at a
depth of about 3 cm. A reduction of the denitrification rate would result in a greater nitrate
penetration depth, i.e. not all the nitrate can be
consumed in this depth zone. On the other hand,
an increase of the denitrification rate would lead
to a depression of the nitrate maximum and
therefore reduce upward and downward fluxes,
and finally induce a total nitrate flux from the
bottom water into the sediment. To clarify these
interactions we plotted three nitrate profiles from
different regions of the South Atlantic in
Figure 6.17. Two profiles indicate high respiration
rates with a nitrate penetration depth of approximately 3 cm, but a distinct peak is visible only at
one station. The station with the nearly linear
gradient into the sediment is indicative for strong
denitrification. The third profile shows a low
gradient into the bottom water which is due to
nitrification and remains nearly constant with
depth indicating that denitrification does not
occur close to the sediment surface.
Depletion of nitrate and the formation of
dinitrogen are, however, not exclusively coupled
to denitrification, since the microbially mediated
reduction utilizing reduced species like Mn
2+
or
Fe
2+
might occur. Based on field observations, a
number of studies invoke the reduction of nitrate
by Mn
2+
to form N 2 instead of organic matter
respiration (Aller 1990; Schulz et al. 1994; Luther
et al. 1997,1998).