6. Benthic Respiration in Aquatic Sediments
reactions (see Fig. 6.1). There are no assays for the
separate determination of any of these pathways. In
early studies of benthic O2 consumption, it was attempted to distinguish oxic organotrophic respiration and reoxidation of inorganic compounds by
poisoning sediment cores with, for example, formaldehyde since all reoxidation was assumed to be
chemical and therefore not affected by poisoning
(Pamatmat 1977). In the following years the role of
lithotrophic microorganisms in reoxidation has become apparent (e.g., Jj2jrgensen 1989) and such approaches are not recommended.
In active sediments, where all three O2 sinks may
be important, oxic organotrophic respiration rates
are estimated as the difference between the total
benthic O 2 uptake (see previous discussion) and the
O 2 demand from inorganic reoxidation. The largest
uncertainties are associated with the reoxidation
term. Until recently, SO~- reduction has been considered the only significant anaerobic respiration in
sediments, and the reoxidation term has therefore
been calculated from the balance of SO~ - reduction rates and sulfide burial (Jj2jrgensen 1982; see
also later section on sulfate reduction). Recent studies have shown that the reoxidation of Mn(I!) and
Fe (I!) from organotrophic Mn and Fe reduction
contributes significantly to O2 consumption in
many continental margin sediments, so that less O2
is left for aerobic respiration than previously
thought (Aller 1990; Canfield et al. 1993a; Thamdrup and Canfield 1996; see also later section on
manganese and iron reduction). The best estimate
of the O 2 consumption coupled to inorganic reoxidation is therefore obtained from the sum of Mn,
Fe, and SO~ - reduction rates plus O 2 consumption
by nitrification less the burial flux of reduced Mn,
Fe, and reduced S (Canfield et al. 1993a). As discussed for the use of total O2 uptake as a measure
of total C mineralization, a major uncertainty lies
in the poorly validated assumption of a tight temporal coupling between anaerobic mineralization
and reoxidation.
The depth distribution of O2 consumption can be
determined from the O 2 profiles obtained with microsensors (Revsbech 1989). This may aid in distinguishing pathways of consumption, since the oxidation of dissolved inorganic species is often
focused near the oxic-anoxic interface (e.g., J0rgensen and Revsbech 1983, Rasmussen and J0rgensen 1992). The analysis can be strengthened by
91
microsensor measurements of dissolved Mn2+,
Fe2+, and H 2 S distributions (Revsbech et al. 1983;
Brendel and Luther 1995). The reoxidation of solid
reduced inorganic phases is not included in such
measurements, however, and since these phases
dominate in bioturbated sediments (see later section
on manganese and iron reduction), this approach
may not allow a separate determination of organotrophic O 2 consumption there.
Nitrate Reduction
Bacterial NO; reduction follows two main pathways: denitrification with N2 as the final product,
and reduction to ammonium (NHt). In recent studies, the latter pathway has been found to be of little
importance in sediments (Binnerup et al. 1992;
Rysgaard et al. 1993). In addition to organic C, reduced inorganic compounds (Fe2+ , H 2 S) may serve
as electron donors for both pathways (Kuenen et al.
1985; Dannenberg et al. 1992; Straub et al. 1996).
A separate quantification of organotrophic and
lithotrophic NO; reduction is mostly not attempted
due to the relatively minor role of NO; as electron
acceptor in sediments (but see Fossing et al. 1995;
Thamdrup and Canfield 1996).
The main interest in NO; reduction measurements arises from the role of benthic denitrification
as an important sink in the marine nitrogen cycle
which influences primary productivity on both local
and global scales (Christensen et al. 1987; Middelburg et al. 1996b). Global estimates of the contribution of benthic denitrification to C oxidation fall
in the range 3 to 11 % (J0rgensen 1983; Middelburg
et al. 1996b). The process may be of greater importance in sediments underlying 02-depleted water with associated elevated NO; levels or in areas
with high NO; levels due to eutrophication (1j2jrgens en and Sj2jrensen 1985; Canfield 1993).
As a tight coupling between nitrification and denitrification is often observed (Nishio et al. 1983),
with a rapid turnover of the pore water NO; pool
and NO; depletion very close to the oxic-anoxic
interface (Christensen et al. 1989; Brandes and Devol 1995), accurate determinations of denitrification rates require that the natural O2 and NO; gradients and associated microenvironments are not
disturbed. A multitude of techniques have been
used to determine denitrification and NO; reduc-
reactions (see Fig. 6.1). There are no assays for the
separate determination of any of these pathways. In
early studies of benthic O2 consumption, it was attempted to distinguish oxic organotrophic respiration and reoxidation of inorganic compounds by
poisoning sediment cores with, for example, formaldehyde since all reoxidation was assumed to be
chemical and therefore not affected by poisoning
(Pamatmat 1977). In the following years the role of
lithotrophic microorganisms in reoxidation has become apparent (e.g., Jj2jrgensen 1989) and such approaches are not recommended.
In active sediments, where all three O2 sinks may
be important, oxic organotrophic respiration rates
are estimated as the difference between the total
benthic O 2 uptake (see previous discussion) and the
O 2 demand from inorganic reoxidation. The largest
uncertainties are associated with the reoxidation
term. Until recently, SO~- reduction has been considered the only significant anaerobic respiration in
sediments, and the reoxidation term has therefore
been calculated from the balance of SO~ - reduction rates and sulfide burial (Jj2jrgensen 1982; see
also later section on sulfate reduction). Recent studies have shown that the reoxidation of Mn(I!) and
Fe (I!) from organotrophic Mn and Fe reduction
contributes significantly to O2 consumption in
many continental margin sediments, so that less O2
is left for aerobic respiration than previously
thought (Aller 1990; Canfield et al. 1993a; Thamdrup and Canfield 1996; see also later section on
manganese and iron reduction). The best estimate
of the O 2 consumption coupled to inorganic reoxidation is therefore obtained from the sum of Mn,
Fe, and SO~ - reduction rates plus O 2 consumption
by nitrification less the burial flux of reduced Mn,
Fe, and reduced S (Canfield et al. 1993a). As discussed for the use of total O2 uptake as a measure
of total C mineralization, a major uncertainty lies
in the poorly validated assumption of a tight temporal coupling between anaerobic mineralization
and reoxidation.
The depth distribution of O2 consumption can be
determined from the O 2 profiles obtained with microsensors (Revsbech 1989). This may aid in distinguishing pathways of consumption, since the oxidation of dissolved inorganic species is often
focused near the oxic-anoxic interface (e.g., J0rgensen and Revsbech 1983, Rasmussen and J0rgensen 1992). The analysis can be strengthened by
91
microsensor measurements of dissolved Mn2+,
Fe2+, and H 2 S distributions (Revsbech et al. 1983;
Brendel and Luther 1995). The reoxidation of solid
reduced inorganic phases is not included in such
measurements, however, and since these phases
dominate in bioturbated sediments (see later section
on manganese and iron reduction), this approach
may not allow a separate determination of organotrophic O 2 consumption there.
Nitrate Reduction
Bacterial NO; reduction follows two main pathways: denitrification with N2 as the final product,
and reduction to ammonium (NHt). In recent studies, the latter pathway has been found to be of little
importance in sediments (Binnerup et al. 1992;
Rysgaard et al. 1993). In addition to organic C, reduced inorganic compounds (Fe2+ , H 2 S) may serve
as electron donors for both pathways (Kuenen et al.
1985; Dannenberg et al. 1992; Straub et al. 1996).
A separate quantification of organotrophic and
lithotrophic NO; reduction is mostly not attempted
due to the relatively minor role of NO; as electron
acceptor in sediments (but see Fossing et al. 1995;
Thamdrup and Canfield 1996).
The main interest in NO; reduction measurements arises from the role of benthic denitrification
as an important sink in the marine nitrogen cycle
which influences primary productivity on both local
and global scales (Christensen et al. 1987; Middelburg et al. 1996b). Global estimates of the contribution of benthic denitrification to C oxidation fall
in the range 3 to 11 % (J0rgensen 1983; Middelburg
et al. 1996b). The process may be of greater importance in sediments underlying 02-depleted water with associated elevated NO; levels or in areas
with high NO; levels due to eutrophication (1j2jrgens en and Sj2jrensen 1985; Canfield 1993).
As a tight coupling between nitrification and denitrification is often observed (Nishio et al. 1983),
with a rapid turnover of the pore water NO; pool
and NO; depletion very close to the oxic-anoxic
interface (Christensen et al. 1989; Brandes and Devol 1995), accurate determinations of denitrification rates require that the natural O2 and NO; gradients and associated microenvironments are not
disturbed. A multitude of techniques have been
used to determine denitrification and NO; reduc-