Environmental Engineering Reference
In-Depth Information
Table 1.16
Dissolved gases in sea water
Gas molecule
% in
atmosphere
% in surface
sea water
ml/l of
sea water
mg/kg (ppm)
in sea water
Molecular
weight
M mol/kg
Nitrogen N
2
78
47.5
10
12.5
28.014
0.446
Oxygen O
2
21
36.0
5
7
31.998
0.219
Carbon dioxide CO
2
0.03
15.1
40
90
42.009
2.142
Argon
1
1.4
-
0.4
39.948
0.01
Dissolved Gases in Sea Water
The gases dissolved in sea water are in constant
equilibrium with the atmosphere, but their rela-
tive concentrations depend on each gas solubil-
ity, which depends also on salinity and
temperature. As salinity increases, the amount of
gas dissolved decreases because more water
molecules are immobilized by the salt ion. As
water temperature increases, the increased
mobility of gas molecules makes them escape
from the water, thereby reducing the amount of
gas dissolved.
Inert gases such as nitrogen and argon do not
take part in the process of life and are thus not
affected by plant and animal life. However, non-
conservative gases such as oxygen and carbon
dioxide are in
remain as in air, according to Henry
s Law,
except where life changes this. Plants increase
oxygen content while decreasing carbon dioxide
and animals do the reverse. Bacteria are even
capable of using up all oxygen.
All gases are less soluble as temperature
increases, particularly nitrogen, oxygen and car-
bon dioxide which become about 40
'
50 % less
-
soluble with an increase of 25
C. When water is
warmed, it becomes more saturated, eventually
resulting in bubbles leaving the liquid. Fish like
sunbathing or resting near the warm surface or in
warm water outfalls because oxygen levels there
are higher. The elevated temperature also
enhances their metabolism, resulting in faster
growth and perhaps a sense of well-being.
Likewise, if the whole ocean was to warm up,
the equilibrium with the atmosphere would
change towards more carbon dioxide (and oxy-
gen) being released to the atmosphere, thereby
exacerbating global warming.
°
uenced by sea life. Plants reduce
the concentration of carbon dioxide in the pres-
ence of sunlight, whereas animals do the oppo-
site in either light or darkness.
Among the gases stated in Table
1.16
, the
conservative gases nitrogen and argon do not
contribute to life processes, even though nitrogen
gas can be converted by some bacteria into fer-
tilizing nitrogen compounds (NO
3
,NH
4
). Sur-
prisingly, the world under water is very much
different from the above in the availability of the
most important gases for life: oxygen and carbon
dioxide. Whereas in air about one in
fl
Sediments of Oceans and Estuaries
A large portion of the sea bottom is covered with
sediments, which contains rock fragments, ani-
mal debris, sand, silt, clay, etc. Sediments may be
categorized as sand, if the diameter of the indi-
vidual particle exceeds 62
m, or mud, if the
diameter of the individual particle is less than
62
μ
ve mole-
cules is oxygen, in sea water, this is only about 4
in every thousand million water molecules.
Whereas air contains about one carbon dioxide
molecule in 3,000 air molecules, in sea water,
this ratio becomes 4 in every 100 million water
molecules, which make carbon dioxide much
more common (available) in sea water than
oxygen. Note that even though their concentra-
tions in solution differ due to differences in sol-
ubility (ability to dissolve), their partial pressures
scale, based on
the negative of the power of 2 to differentiate
between the sizes of
m. Geologists use a phi or
Ø
μ
the sediment particles
(Table
1.17
).
Apart from describing the marine sediments
as mud or sands, the particles may be categorized
by origin such as biogenous, hydrogenous and
lithogenous. The biogenous sediment is derived
from the marine biota, particularly the debris of
the shell. If the sediment contains more than
