6. Benthic Respiration in Aquatic Sediments
Conclusions
Although aquatic sediments may appear rather dull
to the eye, they are highly structured communities
harboring a metabolic diversity that is matched by
few other environments. Steep chemical gradients
at the surface and around animal burrows and plant
roots provide a range of different microenvironments, and much remains to be learned about the
interactions between micro- and macroorganisms.
It must therefore be a prime concern in measurements of sediment processes to minimize the disturbance of the sediment community. The techniques described in this chapter achieve this to a
quite variable extent. Clearly, in situ flux measurements involve the least manipulation and should be
applied as widely as possible, and in situ techniques
should be extended to other processes as well. Their
cost in both time and funds is large, however, and
they are not a priori free of experimental artifacts.
In the deep sea, in situ flux measurements are indispensable due to decompression artifacts. The indications that laboratory incubations with shallowwater sediments underestimate mineralization rates
compared with benthic flux chambers deserve more
attention. Paradoxically, efforts to obtain "undisturbed" sediment cores may cause an undersampIing of sediment with larger infauna and therefore,
in essence, a serious disturbance of the sediment
community. The behavioral response of the infauna
to coring and incubation is another main concern.
Information on the lifestyle of dominating fauna
may help to constrain such effects as well as patterns of bioturbation in the sediment in general
(e.g., Blair et al. 1996; Wheatcroft and Martin
1996), and thereby provide insights to the regulation of carbon oxidation pathways.
The partitioning of carbon oxidation according
to individual respiratory pathways has been improved through recent developments in assays for
denitrification and Mn and Fe reduction. For denitrification, the Nz-flux and isotope pairing techniques complement each other and should be used
as mutual controls. The bag incubation technique
for Mn and Fe reduction has closed a methodological gap and shown that these processes must be
considered in benthic C oxidation. The technique
is rather crude compared with the other assays, and
it is important that results are verified by compar97
ison with other rate determinations and through
modeling in combination with quantifications of
sediment reworking. The measurement of Mn and
Fe reduction is particularly dependent on an accurate determination of SO~ - reduction rates. The radiotracer technique for measuring such rates is well
tested and widely used. Still, a possible underestimation of sulfate reduction rates in oxidized sediment due to reoxidation needs further investigation.
The contribution of oxic respiration to C oxidation can only be estimated from the difference between total and anaerobic mineralization rates, and
therefore this pathway probably has the largest associated uncertainty. The suggestion, raised by the
inclusion of Mn and Fe reduction in anaerobic mineralization rates, that oxic mineralization plays a
lesser role than previously thought needs further
investigation. The role of oxic mineralization is an
important question in benthic ecology, and furthermore, the balance between oxic and anoxic mineralization has been suggested to playa key role in
the regulation of carbon preservation in sediments
with implications for the global C cycle (cf. Canfield 1994). Hence, a better quantification of oxic
mineralization rates is an important task for future
research.
Acknowledgments During the writing of this chapter, we were supported by the Max Planck Society
and the Danish National Science and Research
Foundations.
References
Aller, R.C. Bioturbation and manganese cycling in hemipelagic sediments. Philos. Trans. R. Soc. Lond.
A331:51-58; 1990.
Aller, R.c. The sedimentary Mn cycle in Long Island
Sound: Its role as intermediate oxidant and the influence of bioturbation, O2, and Corg flux on diagenetic
reaction balances. l Mar. Res. 52:259-295; 1994.
Aller, R.C.; Blair, N.B.; Xia, Q.; Rude, P.D. Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments. Continental Shelf Res. 16:753786; 1996.
Aller, R.C.; Mackin, lE. Open-incubation, diffusion
methods for measuring solute reaction rates in sediments. J. Mar. Res. 47:411-440; 1989.
Aller, R.c.; Rude, P.D. Complete oxidation of solid phase
sulfides by manganese and bacteria in anoxic marine
Conclusions
Although aquatic sediments may appear rather dull
to the eye, they are highly structured communities
harboring a metabolic diversity that is matched by
few other environments. Steep chemical gradients
at the surface and around animal burrows and plant
roots provide a range of different microenvironments, and much remains to be learned about the
interactions between micro- and macroorganisms.
It must therefore be a prime concern in measurements of sediment processes to minimize the disturbance of the sediment community. The techniques described in this chapter achieve this to a
quite variable extent. Clearly, in situ flux measurements involve the least manipulation and should be
applied as widely as possible, and in situ techniques
should be extended to other processes as well. Their
cost in both time and funds is large, however, and
they are not a priori free of experimental artifacts.
In the deep sea, in situ flux measurements are indispensable due to decompression artifacts. The indications that laboratory incubations with shallowwater sediments underestimate mineralization rates
compared with benthic flux chambers deserve more
attention. Paradoxically, efforts to obtain "undisturbed" sediment cores may cause an undersampIing of sediment with larger infauna and therefore,
in essence, a serious disturbance of the sediment
community. The behavioral response of the infauna
to coring and incubation is another main concern.
Information on the lifestyle of dominating fauna
may help to constrain such effects as well as patterns of bioturbation in the sediment in general
(e.g., Blair et al. 1996; Wheatcroft and Martin
1996), and thereby provide insights to the regulation of carbon oxidation pathways.
The partitioning of carbon oxidation according
to individual respiratory pathways has been improved through recent developments in assays for
denitrification and Mn and Fe reduction. For denitrification, the Nz-flux and isotope pairing techniques complement each other and should be used
as mutual controls. The bag incubation technique
for Mn and Fe reduction has closed a methodological gap and shown that these processes must be
considered in benthic C oxidation. The technique
is rather crude compared with the other assays, and
it is important that results are verified by compar97
ison with other rate determinations and through
modeling in combination with quantifications of
sediment reworking. The measurement of Mn and
Fe reduction is particularly dependent on an accurate determination of SO~ - reduction rates. The radiotracer technique for measuring such rates is well
tested and widely used. Still, a possible underestimation of sulfate reduction rates in oxidized sediment due to reoxidation needs further investigation.
The contribution of oxic respiration to C oxidation can only be estimated from the difference between total and anaerobic mineralization rates, and
therefore this pathway probably has the largest associated uncertainty. The suggestion, raised by the
inclusion of Mn and Fe reduction in anaerobic mineralization rates, that oxic mineralization plays a
lesser role than previously thought needs further
investigation. The role of oxic mineralization is an
important question in benthic ecology, and furthermore, the balance between oxic and anoxic mineralization has been suggested to playa key role in
the regulation of carbon preservation in sediments
with implications for the global C cycle (cf. Canfield 1994). Hence, a better quantification of oxic
mineralization rates is an important task for future
research.
Acknowledgments During the writing of this chapter, we were supported by the Max Planck Society
and the Danish National Science and Research
Foundations.
References
Aller, R.C. Bioturbation and manganese cycling in hemipelagic sediments. Philos. Trans. R. Soc. Lond.
A331:51-58; 1990.
Aller, R.c. The sedimentary Mn cycle in Long Island
Sound: Its role as intermediate oxidant and the influence of bioturbation, O2, and Corg flux on diagenetic
reaction balances. l Mar. Res. 52:259-295; 1994.
Aller, R.C.; Blair, N.B.; Xia, Q.; Rude, P.D. Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments. Continental Shelf Res. 16:753786; 1996.
Aller, R.C.; Mackin, lE. Open-incubation, diffusion
methods for measuring solute reaction rates in sediments. J. Mar. Res. 47:411-440; 1989.
Aller, R.c.; Rude, P.D. Complete oxidation of solid phase
sulfides by manganese and bacteria in anoxic marine