50
Martin V. ANGEL
adequate supply of nutrients (nitrate, phosphate, silicate
and micronutrients like iron: e.g., de Baar, 1994)
remains available in the surface waters. The biomass of
plant cells continues to increase dramatically, forming
a “spring bloom”. At sea level the sea looks green, and
from space colour sensors mounted in satellites detect
clear changes in ocean colour. However, the bloom is
short-lived. Since the depth to which the wind-driven
mixing is now restricted to above the thermocline, there
is no longer any replenishment of the nutrients from
below the thermocline. Once all the nutrients in the
euphotic zone are used up, the bloom collapses almost
as quickly as it develops. Much of the plant biomass
sinks across the thermocline down into the ocean’s
interior, generating a pulse of organic sedimentation
that takes four to six weeks to reach the sea-bed at
depths of 4000 m, fuelling both the midwater and the
benthic communities. Most of this sedimentary pulse is
either degraded by microbial activity or is intercepted
during its descent by detritivorous pelagic species.
About 10–20% of the carbon fixed by photosynthesis in
the euphotic zone sinks into midwater, and only about
2–3% eventually reaches the benthic communities at
abyssal depths.
In some oceans spring blooms do not occur. The
build-up in plant biomass is either pegged back by the
grazing of herbivorous zooplankton, or, in the highnutrient, low-chlorophyll areas of the North and Central
Pacific and in the Southern Ocean, an inadequate
supply of iron appears to be the factor inhibiting the
growth of the plant cells. It has been estimated that,
if sufficient iron were present in these areas, primary
production in the global ocean would nearly double.
In most of the tropics the upper water column is
permanently stratified. Since the base of the euphotic
zone lies at or above the thermocline and its associated
nutriclines, the re-supply of nutrients by vertical mixing
is limited year-round. The rate of photosynthesis is then
limited to the amount that is supported by nutrients
recycled (regenerated) within the wind-mixed layer, and
relatively little organic material sinks into the deep
water; these conditions are known as oligotrophic. In
a few areas, such as along the equator, the depths of
the isotherms become shallower (“shoal”) so that the
thermocline comes to lie within the euphotic zone.
Since the waters below the thermocline are rich in
nutrients, much higher rates of primary production are
stimulated, and much more organic material sediments
out (is exported) into deep water.
A proportion of the organic material produced is
respired and broken down while it is still in the windmixed layer. Some sinks into deep water, where its
fate is also to be broken down either in midwater or
on the seabed. The degradation releases the nitrogen
and phosphorus content of the organic matter back
into solution – a process known as “regeneration”.
Hence the nutrients required for plant growth tend to
accumulate in deep water. Following strong vertical
mixing or upwelling (see pp. 67–68) nutrients are resupplied into the euphotic zone and stimulate high
rates of primary production. Primary production has
two components – “old” and “new” production. “Old”
production is the component that is supported by
nutrients regenerated within the wind-mixed layer,
whereas “new” production is supported by nutrients resupplied from deep water by vertical mixing. The ratio
between new and old production is the f-ratio (Eppley
and Peterson, 1979). Low f-ratios occur when primary
productivity is predominantly supported by regenerated
nutrients and is dominated by the productivity of
picoplankton, plant cells <2 mm in size. Since these
cells are too small either to be extracted mechanically
from the water by suspension-feeders or to sink under
the influence of gravity, virtually all the organic
material produced flows through the “microbial loop”.
This is the component of the food web in which the
grazers are either mucus-web feeders or exceedingly
tiny themselves (mostly ciliates). Very little of the
organic material flowing through the microbial loop is
exported into deep water; most is respired within the
euphotic zone, so that the nutrients it contains are also
regenerated there.
Conversely, a very much higher proportion of the
organic material resulting from new production is
synthesized by larger phytoplankton cells, which are
exploited by larger suspension-feeding zooplankton,
and flows along pathways in the food-web that
ultimately lead to carnivorous fishes, cephalopods
and marine mammals. A much larger proportion of
new production is exported into deep water via the
“biological pump”; this is a significant mechanism
whereby carbon dioxide is taken up into solution at
the surface of the ocean and becomes transferred into
deep water. The sedimentation of particulate organic
matter (POM) under the influence of gravity is probably
the most important component of this pump, and the
quantity of organic matter thus removed from the upper
ocean across the pycnocline into deep water is often
described as “export production”. Export production is
often correlated with the quantities of dissolved silicate
Martin V. ANGEL
adequate supply of nutrients (nitrate, phosphate, silicate
and micronutrients like iron: e.g., de Baar, 1994)
remains available in the surface waters. The biomass of
plant cells continues to increase dramatically, forming
a “spring bloom”. At sea level the sea looks green, and
from space colour sensors mounted in satellites detect
clear changes in ocean colour. However, the bloom is
short-lived. Since the depth to which the wind-driven
mixing is now restricted to above the thermocline, there
is no longer any replenishment of the nutrients from
below the thermocline. Once all the nutrients in the
euphotic zone are used up, the bloom collapses almost
as quickly as it develops. Much of the plant biomass
sinks across the thermocline down into the ocean’s
interior, generating a pulse of organic sedimentation
that takes four to six weeks to reach the sea-bed at
depths of 4000 m, fuelling both the midwater and the
benthic communities. Most of this sedimentary pulse is
either degraded by microbial activity or is intercepted
during its descent by detritivorous pelagic species.
About 10–20% of the carbon fixed by photosynthesis in
the euphotic zone sinks into midwater, and only about
2–3% eventually reaches the benthic communities at
abyssal depths.
In some oceans spring blooms do not occur. The
build-up in plant biomass is either pegged back by the
grazing of herbivorous zooplankton, or, in the highnutrient, low-chlorophyll areas of the North and Central
Pacific and in the Southern Ocean, an inadequate
supply of iron appears to be the factor inhibiting the
growth of the plant cells. It has been estimated that,
if sufficient iron were present in these areas, primary
production in the global ocean would nearly double.
In most of the tropics the upper water column is
permanently stratified. Since the base of the euphotic
zone lies at or above the thermocline and its associated
nutriclines, the re-supply of nutrients by vertical mixing
is limited year-round. The rate of photosynthesis is then
limited to the amount that is supported by nutrients
recycled (regenerated) within the wind-mixed layer, and
relatively little organic material sinks into the deep
water; these conditions are known as oligotrophic. In
a few areas, such as along the equator, the depths of
the isotherms become shallower (“shoal”) so that the
thermocline comes to lie within the euphotic zone.
Since the waters below the thermocline are rich in
nutrients, much higher rates of primary production are
stimulated, and much more organic material sediments
out (is exported) into deep water.
A proportion of the organic material produced is
respired and broken down while it is still in the windmixed layer. Some sinks into deep water, where its
fate is also to be broken down either in midwater or
on the seabed. The degradation releases the nitrogen
and phosphorus content of the organic matter back
into solution – a process known as “regeneration”.
Hence the nutrients required for plant growth tend to
accumulate in deep water. Following strong vertical
mixing or upwelling (see pp. 67–68) nutrients are resupplied into the euphotic zone and stimulate high
rates of primary production. Primary production has
two components – “old” and “new” production. “Old”
production is the component that is supported by
nutrients regenerated within the wind-mixed layer,
whereas “new” production is supported by nutrients resupplied from deep water by vertical mixing. The ratio
between new and old production is the f-ratio (Eppley
and Peterson, 1979). Low f-ratios occur when primary
productivity is predominantly supported by regenerated
nutrients and is dominated by the productivity of
picoplankton, plant cells <2 mm in size. Since these
cells are too small either to be extracted mechanically
from the water by suspension-feeders or to sink under
the influence of gravity, virtually all the organic
material produced flows through the “microbial loop”.
This is the component of the food web in which the
grazers are either mucus-web feeders or exceedingly
tiny themselves (mostly ciliates). Very little of the
organic material flowing through the microbial loop is
exported into deep water; most is respired within the
euphotic zone, so that the nutrients it contains are also
regenerated there.
Conversely, a very much higher proportion of the
organic material resulting from new production is
synthesized by larger phytoplankton cells, which are
exploited by larger suspension-feeding zooplankton,
and flows along pathways in the food-web that
ultimately lead to carnivorous fishes, cephalopods
and marine mammals. A much larger proportion of
new production is exported into deep water via the
“biological pump”; this is a significant mechanism
whereby carbon dioxide is taken up into solution at
the surface of the ocean and becomes transferred into
deep water. The sedimentation of particulate organic
matter (POM) under the influence of gravity is probably
the most important component of this pump, and the
quantity of organic matter thus removed from the upper
ocean across the pycnocline into deep water is often
described as “export production”. Export production is
often correlated with the quantities of dissolved silicate