Environmental Engineering Reference
In-Depth Information
temperature and salinity, this type of circulation
is also known as thermohaline circulation.
The open oceans in temperate latitudes exhibit
a seasonal effect. During the summer months, the
surface water is warm and the water column is
relatively stable. In the fall, as the surface water
cools, its density increases, the water column
becomes unstable, and the surface water sinks.
The mixing process continues with the aid of
winter storms and the continued cooling of sur-
face waters. The thermocline developed during
the previous summer is ultimately eliminated and
the upper regions become isopycnal to the deeper
water. With spring come warmer temperatures,
and the thermocline begins to re-establish itself,
stabilizing the water column. This process of
stabilization then continues through the summer.
In the open ocean, surface temperature is
more important than salinity in determining the
water density. For instance, the surface water in
tropics has highest salinity in the open ocean, but
the water is so warm that it remains less dense
than the water below it and it does not sink. On
the other hand, water in the North Atlantic has a
lower salinity but is much colder, and as a result,
the surface water is denser and it sinks.
Salinity becomes a more important factor in
determining the density of water close to the
shore. This is especially evident in semi-enclosed
bays that receive a large amount of freshwater
run-off. Wave action also contributes to the
density and mixing of ocean water close to the
shore, where the water is relatively shallow.
moving horizontally into areas where the surface
water is sinking.
Downwelling zones represent areas where
surface water is sinking, whereas upwelling
zones are areas where bottom water rises to the
surface. These processes are extremely important
for the organisms that live at various depths.
Because downwelling carries oxygen-rich sur-
face areas to deeper areas, many organisms can
live in deep-water where downwellings occur.
This process is essentially important for the
organisms that live below the photic zone (dee-
per than sunlight can penetrate). Upwelling on
the other hand brings water rich in nutrients from
sedimentation and decay processes from great
depths to the surface, where it supplies the needs
for photosynthetic organisms and the zooplank-
ton that are vital parts of oceanic food chains.
Upwelling and downwelling can also be pro-
duced by wind-driven surface currents. When
current drives two masses of water together, water
is forced downward. In other regions of the ocean,
surface currents may push two water masses in
opposite directions, drawing deeper water to the
surface. During the slow movement produced by
these currents, adjacent layers of ocean water are
mixed with nutrients and chemicals.
1.3
Chemical Characteristics
of Ocean and Estuarine Water
Current theories hold that our solar system along
with our blue planet Earth was formed about
4.6 billion years ago. It is generally believed that
for the
Upwelling and Downwelling
Since the quantity of water in the oceans is
essentially
rst billion years of its existence, the earth
was mainly composed of silicon compounds,
iron, magnesium oxide, and some other elements
in small amounts. Geologists and earth scientists
believe that originally, this blue planet was
composed of cold matter. Gradually with the
passage of time, several factors, such as energy
from space and the decay of radioactive ele-
ments, contributed to raising its temperature. The
process of heating continued for several hundred
million years until the temperature at the centre
of the earth was high enough to melt iron and
nickel. As these elements melted, they moved to
xed, any movement of water from
one place to another would cause a similar but
opposite movement to replace the water that
leaves an area. For instance, when dense surface
water sinks to a depth at which it no longer
denser than water beneath it, it stops sinking. At
this point, the water begins to move horizontally,
making room for denser water that is sinking
behind it. Eventually, this water rises back to the
surface, replacing the surface water that has been
sinking. At the same time, surface water also
