6. Benthic Respiration in Aquatic Sediments
CaC0 3 dissolution may account for most of the
~C02 flux (Berelson et al. 1987; Reimers et al.
1992). Significant carbonate p.recipitation has been
described for iron-rich Amazon shelf sediments,
where a large part of the ~C02 production precipitates as ferrous carbonate (FeC03) and ~C02 is
released during vigorous reworking events (Aller et
al. 1996).
For the comparison of measurements of C mineralization based on oxidant consumption or ~C02
production, the oxidation state of the metabolized
organic C must be known. Estimates based on a
variety of approaches including both aerobic and
anaerobic mineralization range from - 0.7 to 0 corresponding to 02/~C02 ratios of 1.0 to 1.2 (Hammond et al. 1996; Roden and Wetzel 1996; Thamdrup and Canfield 1996; and references therein).
The Ozl~C02 flux ratio will be somewhat higher
than this due to the additional oxygen consumption
by nitrification (see above).
Flux measurements are made either with benthic
flux chambers in situ (e.g., Pamatmat 1977; Smith
1978; Tengberg et al. 1995) or with retrieved cores
in closed or continuous flow incubations (Pamatmat
1977; Rasmussen and Jorgensen 1992; Miller-Way
and Twilley 1996). Sediment retrieved from depths
greater than approximately 1 Ian and incubated on
shipboard has strongly enhanced O2 uptake compared with in situ rates, which has been ascribed to
physical and biological decompression and temperature effects (Smith 1978; Reimers et al. 1986;
Glud et al. 1994). Conversely, higher in situ than
laboratory O 2 uptake rates have been reported from
shallower depths (Devol and Christensen 1993;
Glud et al. 1994, 1998; Jorgensen 1996). In this
case, the difference between in situ and laboratory
results correlated with faunal biomass, and was interpreted to result from decreased bioirrigation due
to both disturbance of the fauna by coring and to
selection against larger fauna through the relatively
smaller area of individual cores (20 to 80 cm 2 ) relative to benthic chambers (400 to 900 cm 2 ). Thus,
in lab incubations, the effects of core area and possible selectivity during coring should be carefully
considered.
It is important that in situ hydrodynamic conditions are simulated during flux determinations. The
thickness of the diffusive boundary layer (DBL) influences diffusive exchange rates, particularly of O2
(Boudreau and Guinasso 1982), and the water
89
phase should therefore be stirred to create a DBL
of a thickness comparable to the natural conditions
(Sweerts et al. 1989; Gundersen and Jorgensen
1990; Rasmussen and Jorgensen 1992). Optimally,
flow rates or DBL thicknesses in the experimental
set-up can be mapped, so that the effect of deviations between the natural and experimental conditions can be evaluated (Buchholtz-ten Brink et al.
1989; Glud et al. 1995). Most set-ups involve rotational stirring, which induces radial hydrostatic
pressure gradients. In sandy sediments with high
permeability, this induces substantial advective
pore water flow, and it can cause flushing of
U-shaped burrows in less permeable sediments
(Huettel and Gust 1992; Glud et al. 1996). To avoid
this problem, benthic fluxes from permeable sediments may alternatively be determined in flumetype set-ups under a unidirectional flow (Forster et
al. 1996; Huettel et al. 1996).
In closed incubations, the incubation time should
be chosen to yield the minimum necessary changes
in the overlying water. The diffusional O2 flux in
active sediments decreases with decreasing bottom
water concentration (Hall et al. 1989; Rasmussen
and Jorgensen 1992), and changes in O 2 concentration may also affect the irrigating activity of infauna (Forster et al. 1995). Furthermore, pumping
activity by suspension-feeding organisms may stop
when the overlying water has been cleared for phytoplankton, and clearing of the shallow water column typically enclosed in flux determinations can
be extremely rapid (e.g., Vedel et al. 1994).
Other Total Mineralization Assays
As an alternative to benthic fluxes, total mineralization rates can be determined from the accumulation rate of ~C02 in pore water in closed, anoxic
"jar" incubations. Sediment sectioned in discrete
depth intervals and homogenized under anoxic conditions can be incubated in centrifuge tubes that are
sacrificed for sampling serially (Mackin and Swider
1989; Aller et al. 1996) or as a single batch in gastight plastic bags each of which is sampled several
times (Canfield et al. 1993b; Kruse 1993; Thamdrup and Canfield 1996). By reducing the variability between subsamples, the bag incubations give a
higher accuracy and thereby require shorter incubation times (1 to 5 days vs. 2 to 30+ days). In a
CaC0 3 dissolution may account for most of the
~C02 flux (Berelson et al. 1987; Reimers et al.
1992). Significant carbonate p.recipitation has been
described for iron-rich Amazon shelf sediments,
where a large part of the ~C02 production precipitates as ferrous carbonate (FeC03) and ~C02 is
released during vigorous reworking events (Aller et
al. 1996).
For the comparison of measurements of C mineralization based on oxidant consumption or ~C02
production, the oxidation state of the metabolized
organic C must be known. Estimates based on a
variety of approaches including both aerobic and
anaerobic mineralization range from - 0.7 to 0 corresponding to 02/~C02 ratios of 1.0 to 1.2 (Hammond et al. 1996; Roden and Wetzel 1996; Thamdrup and Canfield 1996; and references therein).
The Ozl~C02 flux ratio will be somewhat higher
than this due to the additional oxygen consumption
by nitrification (see above).
Flux measurements are made either with benthic
flux chambers in situ (e.g., Pamatmat 1977; Smith
1978; Tengberg et al. 1995) or with retrieved cores
in closed or continuous flow incubations (Pamatmat
1977; Rasmussen and Jorgensen 1992; Miller-Way
and Twilley 1996). Sediment retrieved from depths
greater than approximately 1 Ian and incubated on
shipboard has strongly enhanced O2 uptake compared with in situ rates, which has been ascribed to
physical and biological decompression and temperature effects (Smith 1978; Reimers et al. 1986;
Glud et al. 1994). Conversely, higher in situ than
laboratory O 2 uptake rates have been reported from
shallower depths (Devol and Christensen 1993;
Glud et al. 1994, 1998; Jorgensen 1996). In this
case, the difference between in situ and laboratory
results correlated with faunal biomass, and was interpreted to result from decreased bioirrigation due
to both disturbance of the fauna by coring and to
selection against larger fauna through the relatively
smaller area of individual cores (20 to 80 cm 2 ) relative to benthic chambers (400 to 900 cm 2 ). Thus,
in lab incubations, the effects of core area and possible selectivity during coring should be carefully
considered.
It is important that in situ hydrodynamic conditions are simulated during flux determinations. The
thickness of the diffusive boundary layer (DBL) influences diffusive exchange rates, particularly of O2
(Boudreau and Guinasso 1982), and the water
89
phase should therefore be stirred to create a DBL
of a thickness comparable to the natural conditions
(Sweerts et al. 1989; Gundersen and Jorgensen
1990; Rasmussen and Jorgensen 1992). Optimally,
flow rates or DBL thicknesses in the experimental
set-up can be mapped, so that the effect of deviations between the natural and experimental conditions can be evaluated (Buchholtz-ten Brink et al.
1989; Glud et al. 1995). Most set-ups involve rotational stirring, which induces radial hydrostatic
pressure gradients. In sandy sediments with high
permeability, this induces substantial advective
pore water flow, and it can cause flushing of
U-shaped burrows in less permeable sediments
(Huettel and Gust 1992; Glud et al. 1996). To avoid
this problem, benthic fluxes from permeable sediments may alternatively be determined in flumetype set-ups under a unidirectional flow (Forster et
al. 1996; Huettel et al. 1996).
In closed incubations, the incubation time should
be chosen to yield the minimum necessary changes
in the overlying water. The diffusional O2 flux in
active sediments decreases with decreasing bottom
water concentration (Hall et al. 1989; Rasmussen
and Jorgensen 1992), and changes in O 2 concentration may also affect the irrigating activity of infauna (Forster et al. 1995). Furthermore, pumping
activity by suspension-feeding organisms may stop
when the overlying water has been cleared for phytoplankton, and clearing of the shallow water column typically enclosed in flux determinations can
be extremely rapid (e.g., Vedel et al. 1994).
Other Total Mineralization Assays
As an alternative to benthic fluxes, total mineralization rates can be determined from the accumulation rate of ~C02 in pore water in closed, anoxic
"jar" incubations. Sediment sectioned in discrete
depth intervals and homogenized under anoxic conditions can be incubated in centrifuge tubes that are
sacrificed for sampling serially (Mackin and Swider
1989; Aller et al. 1996) or as a single batch in gastight plastic bags each of which is sampled several
times (Canfield et al. 1993b; Kruse 1993; Thamdrup and Canfield 1996). By reducing the variability between subsamples, the bag incubations give a
higher accuracy and thereby require shorter incubation times (1 to 5 days vs. 2 to 30+ days). In a