Environmental Engineering Reference
In-Depth Information
coccolithophorids, the genus coccolithophorid, which produce and shed calcium
carbonate shells, making them a major contributor to the inorganic carbon cycle
[
2
]. The calcium carbonate not immediately dissolved back into the water column
is removed by sinking, with coccolithophorids comprising a major component of
the carbon found in marine sediments. A by-product of calcification is CO
2
,which
either remains in the water column or reenters the biological pump through
photosynthesis [
2
].
The importance of iron (Fe) and silicic acid (Si(OH)
4
) in regulating carbon
production in oceanic systems has also been established. Iron is required by all
living organisms for a variety of metabolic processes, and silicon (Si) is needed by
an important phytoplankton functional group, the diatoms, which are characterized
by a hard silica shell. Diatoms remove silicic acid in approximately a 1:1 ratio to N,
and the P:Fe ratio is approximately 1,000:1. Both ratios show considerable plastic-
ity and their uptake ratios are related to other environmental variables as well [
3
,
4
].
The organic material produced in the upper water column via photosynthesis is
used by heterotrophic organisms (e.g., bacteria, zooplankton) and transformed by
their metabolism and growth processes. The unassimilated ingestion of these
organisms (fecal pellet production) sinks and is oxidized below the euphotic zone
by a host of heterotrophic organisms (from bacteria to ciliates to scavenging,
mobile animals), thereby converting the organic matter to CO
2
. Also, particle
aggregates formed from phytoplankton cells, detritus, and dead organisms sink
from the euphotic zone and are oxidized. The unidirectional movement of large
particles to depth and their remineralization defines the biological pump (
Fig. 12.2
),
which also contributes to the generation of “nutrient-like” profiles in the ocean. The
processes that contribute to the fluxes within the biological pump are critical to
understanding the marine carbon cycle.
Atmospheric fluxes of CO
2
into and out of the ocean vary spatially. In general,
equatorial waters tend to be large sources of CO
2
(net fluxes are from the ocean to
the atmosphere). The equatorial Pacific is a large source because it is the site of
large-scale upwelling, a process which brings cold water from depth to the surface.
These waters are in turn heated by solar radiation, and because the solubility of CO
2
is strongly temperature dependent (CO
2
is less soluble in warm water), it is lost to
the atmosphere. Conversely, polar waters are in general sinks for CO
2
. Waters there
lose heat to the atmosphere, and thus are able to absorb more CO
2
. A topic of
intense debate is the possible decrease in carbon flux to the waters of the Southern
Ocean resulting from recent increases in wind strength, which may have altered the
ocean's ability to remove CO
2
[
5
]. Such changes potentially would have profound
impacts on the global carbon budget. At the present time the ocean is a net sink for
atmospheric carbon dioxide, and has sequestered at least 25% of all anthropogenic
emissions to date.
Ocean Acidification
Recently, great concern has been expressed about the increasing concentrations of
CO
2
in the ocean, since its absorption decreases the pH,
leading to ocean
