Geoscience Reference
In-Depth Information
SOD can be determined by
in situ
measurements or by model calibration if direct
measurements from the field are not available.
In situ
measurements are usually
conducted using a chamber. Oxygen uptake in the chamber is measured continuously
over a prespecified period of time, providing the needed data to calculate the oxygen
consumption rate as g O
2
m
−2
d
−1
. In a modeling analysis, sediment oxygen demand
is typically formulated as a zero-order process.
67
SOD is a function of temperature,
and the Arrhenius equation can be used for temperature corrections in the 10-30
°
C
range. Temperature correction coefficients (
θ
) range from 1.040 to 1.130. A typical
value of 1.065 is often used. Below 10
C, SOD probably decreases more rapidly and
approaches zero for water in the temperature range of 0-5
°
C.
64
Actual temperature
dependency of the sediment oxygen demand was observed in Sacca di Goro Lagoon,
Italy. These followed a seasonal trend with pronounced peaks in the warmer months.
Specifically, SOD was 80 mmol O
2
m
2
d
−1
in March, and increased to 365 mmol O
2
m
2
d
−1
in August at a sampling station close to a discharge.
75
°
4.1.4.1.6 Nitrification
Nitrification may be a significant oxygen-demanding process as 4.57 mg O
2
per mg
NH
4
+
is consumed during the oxidation of ammonia to nitrate.
76
Depressed oxygen
levels in water can inhibit nitrification. Therefore, at least 1-2 mg l
−1
of dissolved
oxygen is needed to promote nitrification. Usually in modeling studies, nitrification
rates are multiplied by a factor that shuts down nitrification as the dissolved oxygen
concentration approaches zero.
Changing concentrations of oxygen in the water just above the sediment has
been shown to alter the penetration depth of oxygen within the sediment, and is
believed to be a major controlling factor for nitrification and denitrification processes
in sediment. Detailed information on nitrification is given in Section 4.1.1.2.
4.1.4.2
Redox Potential
Redox potential,
E
h
, refers to the relative degree of oxidation and reduction in an
environment; high values indicate more oxidized conditions.
27
E
h
controls the change
in the oxidation state of many metal ions and some nutrients.
23
In coastal marine
sediment, redox potential is largely controlled by the sulfide concentration.
20
Redox potential is one of the most important parameters characterizing surface
sediment because it provides information about organic matter oxidation. The
sequence of organic matter oxidation reactions is controlled by the energy gained
from each particular reaction. Aerobic respiration is the most energetic redox reaction
and proceeds first. When the oxygen is exhausted, denitrification, Mn-oxide reduc-
tion, Fe-oxide reduction, sulfate reduction, and methane production occur sequen-
tially.
29
The progression to less energetic reactions also reflects a general decrease
in sediment redox potential, with each reaction occurring within a certain redox
potential range. Redox potential differs according to the sediment type and its
location. In the Gulf of Gdansk, Poland, the sediment redox potentials were in the
range of -365 -
246 mV within the system. As a result of better oxygenation,
higher
E
h
values were obtained in the sandy sediment as compared to the silty-clay
sediment. These patterns in redox potentials were similar to those observed in other
coastal areas like the Mediterranean and North Seas.
77
+
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