R
REDOX CONDITIONS
Evgeniy Yakushev
Section of Oceanography and Remote Sensing,
Norwegian Institute for Water Research (NIVA),
Oslo, Norway
Synonyms
Oxic-anoxic conditions; Oxidation-reduction conditions
Definition
Redox conditions refer to media dominated by oxidants
(oxidizers, oxidizing agents, electron acceptors), the substances that have ability to oxidize other substances
(cause them to lose electrons), or reductants (reducers,
reducing agents, electron donors), the substances that have
ability to reduce other substances (cause them to gain
electrons).
Description
The content of dissolved oxygen (DO), the quintessential
oxidizer in the natural waters, is used to define the following redox conditions:
Normoxic or oxic conditions correspond to high concentrations of oxygen (greater than 80 % saturation).
Oxygen deficient is a term used to characterize waters
with oxygen concentrations below 80 % saturation or
approximately 3 mg O 2 L
À1 (about 95 mM O 2 ) (Renaud,
1986).
Hypoxic conditions correspond to the threshold of
tolerance, stress responses, and morbidity of pelagic and
benthic animals to low dissolved oxygen levels, usually
in the range of 1–4 mg O 2 L
À1 (e.g., Diaz, 2001;
Vaquer-Sunyer and Duarte, 2008; Savchuk, 2010).
A formal boundary is set at 2 mg O 2 L
À1 ($63 mM O 2 ,
i.e., CENR, 2000), or 2 ml O 2 L
À1 ($89 mM O 2 , i.e.,
Diaz and Rosenberg, 2008; Savchuk, 2010), or $75 mM
O 2 (Yakushev and Newton, 2013).
Suboxic conditions correspond to DO levels $15 mM
O 2 , after which DO becomes an auxiliary oxidant, and
the dominant electron acceptors are the oxidized ions of
N (nitrate, nitrite) or oxidized species of metals: Mn(IV),
Mn(III), and Fe(III). In the Black Sea, the reported suboxic
values range from 4.5 mM O 2 (Lam et al., 2007) to 10 mM
(Murray, 1991) and to 15 mM O 2 (Zubkov et al., 1992).
Suboxic conditions may be further divided into
suboxidized conditions, where DO is present in nanomolar
concentrations, and subreduced conditions, where DO is
absent (Stunzhas, 2005). The subreduced conditions,
where there is no longer any DO and H 2 S is not yet present, should correspond to the onset of processes such as
anammox (Kuypers et al., 2003) or formation of Mn(III)
(Trouwborst et al., 2006), which are inhibited by both
DO and H 2 S.
Once the oxidized species of nitrogen, manganese, or
iron are completely depleted, OM microbial decomposition uses sulfate as the next available electron acceptor.
This is the appropriate threshold for the term anoxic,
where H 2 S appears and the Eh potential becomes
negative.
Bibliography
CENR, 2000. Integrated Assessment of Hypoxia in the Northern
Gulf of Mexico. Washington, DC: National Science and
Technology Council Committee on Environment and Natural
Resources.
Diaz, R. J., 2001. Overview of hypoxia around the world. Journal of
Environmental Quality, 30, 275–281.
Diaz, R., and Rosenberg, R., 2008. Spreading dead zones and consequences for marine ecosystems. Science, 321, 926–929.
Kuypers, M. M. M., Sliekers, A. O., Lavik, G., Schmid, M.,
Jorgensen, B. B., Kuenen, J. G., Damste, J. S., Strous, M., and
Jetten, M. S. M., 2003. Anaerobic ammonium oxidation by
anammox bacteria in the Black Sea. Nature, 422, 608–611.
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016
REDOX CONDITIONS
Evgeniy Yakushev
Section of Oceanography and Remote Sensing,
Norwegian Institute for Water Research (NIVA),
Oslo, Norway
Synonyms
Oxic-anoxic conditions; Oxidation-reduction conditions
Definition
Redox conditions refer to media dominated by oxidants
(oxidizers, oxidizing agents, electron acceptors), the substances that have ability to oxidize other substances
(cause them to lose electrons), or reductants (reducers,
reducing agents, electron donors), the substances that have
ability to reduce other substances (cause them to gain
electrons).
Description
The content of dissolved oxygen (DO), the quintessential
oxidizer in the natural waters, is used to define the following redox conditions:
Normoxic or oxic conditions correspond to high concentrations of oxygen (greater than 80 % saturation).
Oxygen deficient is a term used to characterize waters
with oxygen concentrations below 80 % saturation or
approximately 3 mg O 2 L
À1 (about 95 mM O 2 ) (Renaud,
1986).
Hypoxic conditions correspond to the threshold of
tolerance, stress responses, and morbidity of pelagic and
benthic animals to low dissolved oxygen levels, usually
in the range of 1–4 mg O 2 L
À1 (e.g., Diaz, 2001;
Vaquer-Sunyer and Duarte, 2008; Savchuk, 2010).
A formal boundary is set at 2 mg O 2 L
À1 ($63 mM O 2 ,
i.e., CENR, 2000), or 2 ml O 2 L
À1 ($89 mM O 2 , i.e.,
Diaz and Rosenberg, 2008; Savchuk, 2010), or $75 mM
O 2 (Yakushev and Newton, 2013).
Suboxic conditions correspond to DO levels $15 mM
O 2 , after which DO becomes an auxiliary oxidant, and
the dominant electron acceptors are the oxidized ions of
N (nitrate, nitrite) or oxidized species of metals: Mn(IV),
Mn(III), and Fe(III). In the Black Sea, the reported suboxic
values range from 4.5 mM O 2 (Lam et al., 2007) to 10 mM
(Murray, 1991) and to 15 mM O 2 (Zubkov et al., 1992).
Suboxic conditions may be further divided into
suboxidized conditions, where DO is present in nanomolar
concentrations, and subreduced conditions, where DO is
absent (Stunzhas, 2005). The subreduced conditions,
where there is no longer any DO and H 2 S is not yet present, should correspond to the onset of processes such as
anammox (Kuypers et al., 2003) or formation of Mn(III)
(Trouwborst et al., 2006), which are inhibited by both
DO and H 2 S.
Once the oxidized species of nitrogen, manganese, or
iron are completely depleted, OM microbial decomposition uses sulfate as the next available electron acceptor.
This is the appropriate threshold for the term anoxic,
where H 2 S appears and the Eh potential becomes
negative.
Bibliography
CENR, 2000. Integrated Assessment of Hypoxia in the Northern
Gulf of Mexico. Washington, DC: National Science and
Technology Council Committee on Environment and Natural
Resources.
Diaz, R. J., 2001. Overview of hypoxia around the world. Journal of
Environmental Quality, 30, 275–281.
Diaz, R., and Rosenberg, R., 2008. Spreading dead zones and consequences for marine ecosystems. Science, 321, 926–929.
Kuypers, M. M. M., Sliekers, A. O., Lavik, G., Schmid, M.,
Jorgensen, B. B., Kuenen, J. G., Damste, J. S., Strous, M., and
Jetten, M. S. M., 2003. Anaerobic ammonium oxidation by
anammox bacteria in the Black Sea. Nature, 422, 608–611.
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016