90
slightly different approach, the change in pore water distribution of ~C02 in intact cores, which have
been incubated with both ends sealed, is determined
relative to the initial profile, and the depth distribution of C mineralization rates is estimated using
a non-steady state transport-reaction model (Aller
and Mackin 1989; Aller et al. 1996). Ammonia accumulation rates in closed incubations also have
been used to estimate mineralization rates based on
experimentally obtained ~C021NH: accumulation
ratios (Mackin and Swider 1989; Canfield et al.
1993b).
As with ~C02 fluxes, the significance of carbonate dissolution and precipitation must be considered. Under anoxic conditions, little CaC0 3 dissolution is expected since little or no acidity is
generated during metal oxide or sulfate (SO~ -) reduction (Canfield and Raiswell 1991). Also, CaC0 3
dissolution has not been observed (Mackin and
Swider 1989; Thamdrup and Canfield 1996). Precipitation of other carbonate phases, for example,
ferrous carbonate, has been indicated during long
incubations of iron-rich sediments (Aller et al.
1996). Generally, though, little is known about the
magnitude of authigenic carbonate formation in
coastal marine sediments.
The main benefit of the closed sediment incubations is that the depth distribution of rates can be
used to quantify pathways and dynamics of C mineralization (see later section on manganese and iron
reduction). In non-steady state systems, the incubations may provide better estimates of mineralization rates than do benthic fluxes (Aller et al.
1996), though this is likely not the case in general,
since less manipulation of the sediment is necessary
for flux measurements. In a direct comparison in
Long Island Sound sediment, very similar rates
were determined with homogenized sediment and
in whole-core incubations, and the depth-integrated
rates were also in close agreement with benthic
~C02 fluxes. This indicated that neither homogenization nor the imposed anoxia affected total mineralization rates (Aller and Mackin 1989; Mackin
and Swider 1989). Also, in sediments underlying
anoxic bottom water, good agreement has been
found with ~C02 fluxes measured in situ (Thamdrup and Canfield 1996).
The heat production of sediments has been used
as a direct measure of total benthic metabolism in
terms of energy flow (Pamatmat et al. 1981; PaBo Thamdrup and Donald E. Canfield
matmat 1982). The technique has mainly been applied to the study of temporal dynamics (Graf
1992). The principle of the method is that heat is
produced during all metabolic processes and can be
measured in a calorimeter, and it is assumed that
heat production from other kinds of reactions in
sediments are negligible. The measured heat can be
converted to carbon or oxidant equivalents if the
overall reaction stoichiometry is known, but the
metabolic pathways producing the heat have not
been separately quantified, and only a conversion
factor for oxic respiration has been applied. The
difficulties in interpreting the results in chemical
equivalents, together with its technical requirements have kept the technique from gaining wider
use.
Electron transport system activity (ETSA) is
used as a proxy for benthic respiration (Christensen
and Packard 1977; Relexans 1996a,b). The ETSA
assay involves the extraction of electron transport
chains from the sediment and an in vitro measurement of their potential activity using synthetic electron donors and acceptors. The potential rate is correlated to bacterial biomass, and is converted to in
situ rates by a calibration factor, which depends on
in situ substrate levels and varies more than an order of magnitude between sites (Relexans 1996b).
The technique is rapid and very sensitive, and
therefore often applied to deep-sea sediments. Due
to the uncertainties in the calibration factor, however, direct determinations of respiration are preferable. The respirationJETSA ratio can possibly
give information on the nutritional state of the microbenthic community (Relexans 1996b).
Respiratory Pathways,
Oxygen Respiration
Oxygen is globally the most important electron acceptor in benthic respiration. In open ocean sediments with deep oxic zones, the process dominates
C oxidation completely, and it has been estimated
to account for 50% of C oxidation in coastal sediments (Jl1lrgensen 1983; Henrichs and Reeburgh
1987; Canfield 1993), though this figure has recently has been questioned (see below).
In sediments, O 2 is consumed by organotrophic
respiration, lithotrophic respiration, and by abiotic
slightly different approach, the change in pore water distribution of ~C02 in intact cores, which have
been incubated with both ends sealed, is determined
relative to the initial profile, and the depth distribution of C mineralization rates is estimated using
a non-steady state transport-reaction model (Aller
and Mackin 1989; Aller et al. 1996). Ammonia accumulation rates in closed incubations also have
been used to estimate mineralization rates based on
experimentally obtained ~C021NH: accumulation
ratios (Mackin and Swider 1989; Canfield et al.
1993b).
As with ~C02 fluxes, the significance of carbonate dissolution and precipitation must be considered. Under anoxic conditions, little CaC0 3 dissolution is expected since little or no acidity is
generated during metal oxide or sulfate (SO~ -) reduction (Canfield and Raiswell 1991). Also, CaC0 3
dissolution has not been observed (Mackin and
Swider 1989; Thamdrup and Canfield 1996). Precipitation of other carbonate phases, for example,
ferrous carbonate, has been indicated during long
incubations of iron-rich sediments (Aller et al.
1996). Generally, though, little is known about the
magnitude of authigenic carbonate formation in
coastal marine sediments.
The main benefit of the closed sediment incubations is that the depth distribution of rates can be
used to quantify pathways and dynamics of C mineralization (see later section on manganese and iron
reduction). In non-steady state systems, the incubations may provide better estimates of mineralization rates than do benthic fluxes (Aller et al.
1996), though this is likely not the case in general,
since less manipulation of the sediment is necessary
for flux measurements. In a direct comparison in
Long Island Sound sediment, very similar rates
were determined with homogenized sediment and
in whole-core incubations, and the depth-integrated
rates were also in close agreement with benthic
~C02 fluxes. This indicated that neither homogenization nor the imposed anoxia affected total mineralization rates (Aller and Mackin 1989; Mackin
and Swider 1989). Also, in sediments underlying
anoxic bottom water, good agreement has been
found with ~C02 fluxes measured in situ (Thamdrup and Canfield 1996).
The heat production of sediments has been used
as a direct measure of total benthic metabolism in
terms of energy flow (Pamatmat et al. 1981; PaBo Thamdrup and Donald E. Canfield
matmat 1982). The technique has mainly been applied to the study of temporal dynamics (Graf
1992). The principle of the method is that heat is
produced during all metabolic processes and can be
measured in a calorimeter, and it is assumed that
heat production from other kinds of reactions in
sediments are negligible. The measured heat can be
converted to carbon or oxidant equivalents if the
overall reaction stoichiometry is known, but the
metabolic pathways producing the heat have not
been separately quantified, and only a conversion
factor for oxic respiration has been applied. The
difficulties in interpreting the results in chemical
equivalents, together with its technical requirements have kept the technique from gaining wider
use.
Electron transport system activity (ETSA) is
used as a proxy for benthic respiration (Christensen
and Packard 1977; Relexans 1996a,b). The ETSA
assay involves the extraction of electron transport
chains from the sediment and an in vitro measurement of their potential activity using synthetic electron donors and acceptors. The potential rate is correlated to bacterial biomass, and is converted to in
situ rates by a calibration factor, which depends on
in situ substrate levels and varies more than an order of magnitude between sites (Relexans 1996b).
The technique is rapid and very sensitive, and
therefore often applied to deep-sea sediments. Due
to the uncertainties in the calibration factor, however, direct determinations of respiration are preferable. The respirationJETSA ratio can possibly
give information on the nutritional state of the microbenthic community (Relexans 1996b).
Respiratory Pathways,
Oxygen Respiration
Oxygen is globally the most important electron acceptor in benthic respiration. In open ocean sediments with deep oxic zones, the process dominates
C oxidation completely, and it has been estimated
to account for 50% of C oxidation in coastal sediments (Jl1lrgensen 1983; Henrichs and Reeburgh
1987; Canfield 1993), though this figure has recently has been questioned (see below).
In sediments, O 2 is consumed by organotrophic
respiration, lithotrophic respiration, and by abiotic