Environmental Engineering Reference
In-Depth Information
limiting levels, particularly for conditions that produced high concentrations of
algae, because photosynthesis naturally increases the pH level. Decreased pH and
increased absolute CO
2
levels arising from current conditions might reduce this
limitation. Because there is substantial variability among species of phytoplankton
in their response to increased CO
2
, planktonic biodiversity is at risk [
8
]. However,
certain algal functional groups, such as nitrogen-fixing cyanobacteria, positively
respond to increased CO
2
concentrations by increasing their growth and photosyn-
thesis, whereas others can not. Similarly, at least one species of toxin-producing
dinoflagellate demonstrated increased growth and modified elemental ratios under
increased CO
2
conditions [
9
], suggesting the possibility of an enhancement of
occurrences of harmful algal blooms in the future. Because the marine carbon
cycle is intimately linked with the biogeochemical cycles of nitrogen, phosphorus,
silicon, and iron, these interactive effects make it extremely difficult to predict what
future decreases in oceanic pH will generate. Oceanographers have recognized that
increased inorganic carbon levels can have subtle effects on the biota, and much
work is being done to document and quantify these effects.
The Biogeochemical Cycles of Nitrogen and Phosphorus
Although the early work of Redfield [
1
] clearly differentiated between the sources
of nitrogen and phosphorus and the regulation of their turnover, they are linked in
nature by the processes operating in the biological pump (
Fig. 12.2
). Despite this
coupling, as well as their linkage to carbon, there are a number of features that
distinguish them.
Nitrogen occurs in three reactive, inorganic forms in the ocean: nitrate (
NO
3
),
nitrite (
NO
2
), and ammonium (
NH
4
) and the processes that transform and modify
these forms make up the nitrogen cycle (
Fig. 12.3
). The nitrogen cycle has five
major pathways that result in changes in the availability of nitrogen that can be used
by plants. Nitrogen fixation removes gaseous nitrogen from the atmosphere, which
is then converted by a series of reactions to forms that can be used for plant growth.
In the ocean this process occurs primarily in tropical and semitropical
environments, and the major algal species responsible for this transformation is
Trichodesmium
. Denitrification results in the reduction of
NO
3
to gaseous nitrogen,
usually mediated by bacteria, and results in the loss of nitrogen available for
phytoplankton in oceanic systems. These two processes are the primary means by
which the ocean biota controls nitrogen biogeochemistry.
Nitrogen assimilation is the process by which nitrate (
NO
3
) and ammonium
NH
4
) are removed from the water by phytoplankton. Ammonium is energetically
favored for uptake because it does not have to be reduced intracellularly, but nitrate
often occurs in greater concentrations, particularly in areas of upwelling or deep
vertical mixing. Ammonium inhibits nitrate uptake, but the degree of inhibition
varies with the relative concentration of the two nutrients. Ammonification
generates
NH
4
by the cleaving of amine groups from organic nitrogen. Because
many marine organisms excrete ammonium, the vertical distribution of
NH
4
can
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