THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
55
formation of “marine snow”. A loose sheet of mucus
can act as a nucleus for the formation of a snow
aggregate. Once formed, not only is the snow an
important source of nutriment for larger zooplankton
species, but it is also an important vehicle whereby
particulate organic matter is exported by sedimentation
to the deep ocean (Lampitt et al., 1993; Silver et al.,
1998).
A rich assortment of animals inhabit the epipelagic
zone. However, many of the species are not permanent
residents, but are commuters from deeper water (see
pp. 59–60). These commuting species include the diel
vertical migrants, which each night swim up into the
epipelagic zone to feed, and migrate down again at
dawn; also, the eggs and early larval stages of species
whose adults inhabit deeper depths; and even some
benthic species. Some species invest little if any yolk
in their eggs, so the early larvae have to feed if they
are to grow and develop. For these larvae the invidious
gamble is to risk a perilous existence in the welllit waters where predators abound, but food is more
abundant, rather than to starve deeper down where
the dangers from predation are far less. Despite these
migrations, both by day and by night the numerically
dominant group of mesozooplankton (i.e., plankton
caught in nets with meshes of 200 to 330 mm) are
copepods; >75% of the organisms caught belong to this
group of planktonic crustaceans.
Predators that hunt visually generate a strong selection pressure for the resident zooplankton inhabitants
to minimize their visibility. This is achieved in just two
ways: either by being very small (perforce many of the
grazers have to be very small to feed on picoplankton) or by being highly transparent. Transparency
(or translucence) is almost universal in the many
gelatinous species – salps, siphonophores, medusae,
foraminiferans and chaetognaths, and these groups tend
to be the dominant components of the communities of
larger organisms, by day often contributing >75% of
the micronektonic biovolume (Angel and Pugh, 2000).
Transparency is not an option that is widely adopted in
fishes, but the Leptocephalus larvae of eels are notable
for being almost totally transparent. More usually, fish
are camouflaged by being counter-shaded. Their backs
are dark, their bellies are pale, and their flanks are
patterned with disruptive bars; this relatively simple
colour pattern renders them surprisingly difficult to
see underwater. Very close to the surface, however,
they can be clearly seen by predators approaching
from directly underneath, because they are silhouetted
against a circular patch of bright water. This patch,
called Snell’s circle, results from the refraction of light
as it passes through the surface (Partridge, 1990).
Just after dusk, a vast array of planktonic and
micronektonic commuters arrive. These are the diel
vertical migrants. Their arrival often doubles the
biomass of animals in the epipelagic zone, and also
greatly extends the size range of the inhabitants. The
migrants move up at dusk from daytime depths below
the pycnocline and in many cases from the mesopelagic
zone (see next section), and start on their way back
down at around first light. Although the availability
of food is far higher in the upper layers, the risk of
predation, especially by visually-hunting predators, is
greater. So, by moving up to feed under the cover
of darkness the migrants optimize their chances of
finding enough food, while minimizing the risks of
predation. Since the migrants feed mostly at night, their
movements provide another mechanism for the export
of particulate organic matter out of the euphotic zone
into deep water (see p. 50).
Ecology of the mesopelagic zone
Underlying the epipelagic zone and extending down
to depths of about 1000 m is the mesopelagic zone.
Although the dominant types of pelagic organisms
do not change very much in the mesopelagic zone,
there are marked changes in the species composition;
also, the proportion of the smaller-sized individuals
decreases (Fig. 3.6). Since there is an almost total
absence of viable phytoplankton on which grazers
can feed, the options for feeding become restricted
to detritivory or carnivory. The mesozooplankton is
still numerically dominated by copepods, but in the
upper part of the zone the larger-sized organisms are
predominantly gelatinous, particularly siphonophores.
Many of these gelatinous forms are so fragile that
they are impossible to sample with nets, so it was
not until biologists began to dive in the open ocean
that the abundance and importance of these fragile
animals became recognized. Even now, knowledge of
their role in pelagic processes is sparse; but they must
be very important at depths of 200 to 400 m, where
direct observations from submersibles have shown
them to be extremely abundant. Many of the crustacean
species inhabiting the upper mesopelagic zone are part
transparent and part pigmented, usually tinged with red
and orange carotenoid pigments. In the absence of red
light at these depths, functionally these pigments are
55
formation of “marine snow”. A loose sheet of mucus
can act as a nucleus for the formation of a snow
aggregate. Once formed, not only is the snow an
important source of nutriment for larger zooplankton
species, but it is also an important vehicle whereby
particulate organic matter is exported by sedimentation
to the deep ocean (Lampitt et al., 1993; Silver et al.,
1998).
A rich assortment of animals inhabit the epipelagic
zone. However, many of the species are not permanent
residents, but are commuters from deeper water (see
pp. 59–60). These commuting species include the diel
vertical migrants, which each night swim up into the
epipelagic zone to feed, and migrate down again at
dawn; also, the eggs and early larval stages of species
whose adults inhabit deeper depths; and even some
benthic species. Some species invest little if any yolk
in their eggs, so the early larvae have to feed if they
are to grow and develop. For these larvae the invidious
gamble is to risk a perilous existence in the welllit waters where predators abound, but food is more
abundant, rather than to starve deeper down where
the dangers from predation are far less. Despite these
migrations, both by day and by night the numerically
dominant group of mesozooplankton (i.e., plankton
caught in nets with meshes of 200 to 330 mm) are
copepods; >75% of the organisms caught belong to this
group of planktonic crustaceans.
Predators that hunt visually generate a strong selection pressure for the resident zooplankton inhabitants
to minimize their visibility. This is achieved in just two
ways: either by being very small (perforce many of the
grazers have to be very small to feed on picoplankton) or by being highly transparent. Transparency
(or translucence) is almost universal in the many
gelatinous species – salps, siphonophores, medusae,
foraminiferans and chaetognaths, and these groups tend
to be the dominant components of the communities of
larger organisms, by day often contributing >75% of
the micronektonic biovolume (Angel and Pugh, 2000).
Transparency is not an option that is widely adopted in
fishes, but the Leptocephalus larvae of eels are notable
for being almost totally transparent. More usually, fish
are camouflaged by being counter-shaded. Their backs
are dark, their bellies are pale, and their flanks are
patterned with disruptive bars; this relatively simple
colour pattern renders them surprisingly difficult to
see underwater. Very close to the surface, however,
they can be clearly seen by predators approaching
from directly underneath, because they are silhouetted
against a circular patch of bright water. This patch,
called Snell’s circle, results from the refraction of light
as it passes through the surface (Partridge, 1990).
Just after dusk, a vast array of planktonic and
micronektonic commuters arrive. These are the diel
vertical migrants. Their arrival often doubles the
biomass of animals in the epipelagic zone, and also
greatly extends the size range of the inhabitants. The
migrants move up at dusk from daytime depths below
the pycnocline and in many cases from the mesopelagic
zone (see next section), and start on their way back
down at around first light. Although the availability
of food is far higher in the upper layers, the risk of
predation, especially by visually-hunting predators, is
greater. So, by moving up to feed under the cover
of darkness the migrants optimize their chances of
finding enough food, while minimizing the risks of
predation. Since the migrants feed mostly at night, their
movements provide another mechanism for the export
of particulate organic matter out of the euphotic zone
into deep water (see p. 50).
Ecology of the mesopelagic zone
Underlying the epipelagic zone and extending down
to depths of about 1000 m is the mesopelagic zone.
Although the dominant types of pelagic organisms
do not change very much in the mesopelagic zone,
there are marked changes in the species composition;
also, the proportion of the smaller-sized individuals
decreases (Fig. 3.6). Since there is an almost total
absence of viable phytoplankton on which grazers
can feed, the options for feeding become restricted
to detritivory or carnivory. The mesozooplankton is
still numerically dominated by copepods, but in the
upper part of the zone the larger-sized organisms are
predominantly gelatinous, particularly siphonophores.
Many of these gelatinous forms are so fragile that
they are impossible to sample with nets, so it was
not until biologists began to dive in the open ocean
that the abundance and importance of these fragile
animals became recognized. Even now, knowledge of
their role in pelagic processes is sparse; but they must
be very important at depths of 200 to 400 m, where
direct observations from submersibles have shown
them to be extremely abundant. Many of the crustacean
species inhabiting the upper mesopelagic zone are part
transparent and part pigmented, usually tinged with red
and orange carotenoid pigments. In the absence of red
light at these depths, functionally these pigments are