Environmental Engineering Reference
In-Depth Information
are critical in the regulation of carbon cycling in the ocean, and hence their study
forms the basis for most biogeochemical research.
The biogeochemical cycles described in subsequent sections use carbon as a
“common denominator.” Carbon is the basic component of organic matter, and with
the advent of industrialization is being added to the atmosphere at an unprecedented,
rapid rate, which is changing atmospheric temperatures and impacting the thermal
equilibrium of the ocean. Also, carbon is absorbed from the atmosphere at the ocean
surface where it reacts with ocean water to produce carbonic acid, thereby making
ocean waters more acidic (reducing the pH), which has profound impacts on oceanic
chemistry and biological activity. Thus, the production and oxidation of organic
matter in the ocean has numerous critical interactions with all other elemental cycles,
and is a major regulator of all marine biogeochemical cycles.
The Biogeochemical Cycle of Carbon
Carbon is the primary building block for all life because of its chemical ability to
form a myriad of covalent bonds with itself and numerous other elements. As
a result, the numerous complex organic compounds that form the basis of life
systems are based on carbon. In the present-day ocean, synthesis of organic
molecules (photosynthesis) is done largely by phytoplankton, which converts the
radiant energy from the sun into chemical energy in the form of adenosine triphos-
phate (ATP). The ATP, along with reductant, is used to reduce CO
2
into simple
sugars, which are in turn modified into all of the compounds required for cellular
metabolism, growth, and division. Photosynthesis is dependent on energy from the
sun, thereby confining this process to the euphotic zone, which is the part of the
upper water column that receives at least 1% of the irradiance that reaches the sea
surface. Phytoplankton require energy for the uptake and assimilation of nearly all
elements. This dependence on light generally results in vertical distributions of
nutrients that are characterized by reduced concentrations in the euphotic zone,
where photosynthesis and growth are most active, and increased concentrations at
depth, where photosynthesis and growth are reduced or absent (
Fig. 12.1
). This
vertical profile is a typical of nutrient distributions throughout the oceans. The
organic matter generated by photosynthesis and growth has roughly an inverse
relationship to that of the inorganic building blocks (
Fig. 12.1
).
Redfield [
1
] suggested that organic matter (carbon,
C
) production in the sea
occurs in relatively constant elemental ratios given by the relationship:
16
H
þ
þ
16
NO
3
þ
106
CO
2
þ
H
3
PO
4
þ
122
H
2
O
$ð
CH
2
O
Þ
106
ð
NH
3
Þ
16
ð
H
3
PO
4
Þþ
138
O
2
This relationship describes the reaction of
CO
2
with hydrogen (
H
), nitrate (
NO
3
),
phosphate (
H
3
PO
4
), and water (
H
2
O
) within the photosynthetic process to produce
