Geoscience Reference
In-Depth Information
cannot use it all. The organic matter that is not used decomposes
locally, thus consuming the O
2
dissolved in the water. Such excessive
nutrient enrichment is called eutrophication, the resulting phytoplankton
bloom often colors the water and is called green or red tide (or another
color depending on the type of phytoplankton), and the accompanying
large decrease in the concentration of dissolved O
2
is qualified,
according to the intensity of the phenomenon, of hypoxia (O
2
deficiency) or anoxia (absence of O
2
). Waters with low or no O
2
are
often called “dead zones”, which is incorrect since anaerobic organisms
can thrive under anoxic conditions.
In between conditions of extreme nutrient deficiency and those of
excessive nutrient richness, described in the previous paragraph, the
availability of nitrogenous nutrients in sea-surface waters of temperate
or polar regions at the end of winter or during spring often cause
phytoplankton blooms, which may be grazed by zooplankton, which
are in turn consumed by fish and other large marine organisms
(mammals, seabirds, etc.). The abundance of nutrients in these regions
at that time of the year is explained by the vertical mixing of the water
column that has taken place in the previous months, bringing to the
surface nutrients that had been regenerated in deep waters. Even when
nutrients are abundant, the phytoplankton bloom cannot occur as long
as photosynthesis is limited by light, e.g. due to low solar radiation in
winter, or strong vertical mixing of the water column that transports
phytoplankton to depth where there is little light. The seasonal
increase in light and stratification provides favorable conditions for
phytoplankton to bloom, which rapidly uses up the surface
nutrients, which explains why blooms are usually of short durations.
During the following summer period, stratification of the water
column often prevents or reduces the transport of nutrients to the
surface, which limits phytoplankton growth despite sufficient solar
radiation. In those coastal zones where upwelling of deep water is
frequent or permanent
,
and commercial fisheries are very prosperous,
e.g in the upwelling zones of Mauritania, Namibia, Chile and
California.
