60
Martin V. ANGEL
an increasing amount of evidence that these migrations
contribute to the downward fluxes of organic matter
from the surface (Longhurst and Harrison, 1988;
Dam et al., 1995; Zhang and Dam, 1998; Steinberg
et al., 2000; Angel and Pugh, 2000). The majority of
planktonic and micronektonic animals which perform
diel vertical migrations only feed during the shallow
phase of their migrations, where and when food is
more available. They migrate down with full stomachs,
and when they return the following dusk their guts are
empty. Thus, any organic material they do not retain
after ingestion but defaecate contributes to a downward
flux. Since most of what they consume comes from
shallow depths, anything they excrete at depth, be it
carbon dioxide, ammonia, or dissolved organic matter,
will have been derived from near the surface. If they
moult, die or are eaten, there is a downward flux of
material. Recent data from the equatorial Pacific show
that diel migrant mesozooplankton were exporting
0.6 and 1.1 mmol C m
−2 d
−1 in the two seasons of
observations, respectively, contributing 31% and 44%
to the sedimentary flux (Zhang and Dam, 1998). To
this estimate must be added the export by migrating
micronekton, whose biomass is roughly about half that
of the mesozooplankton (Angel and Pugh, 2000), but
whose migrations extend over much greater vertical
distances. These values imply that the active flux by
migrants may amount to about half that resulting from
particle sedimentation.
Diel migrations are generally restricted to the upper
kilometre of the water column (i.e., they involve
exchanges between the epipelagic and the deep and
shallow mesopelagic zones), but in the central oligotrophic gyres pelagic decapods (Domanski, 1986)
and some myctophid fish migrate even deeper. The
myctophid fish Ceratoscopelus warmingeri was found
to be migrating to depths of 1600–1700 m in the region
of the Azores Front (Angel, 1989b).
Ontogenetic migrations are carried out by species
that change their depth ranges during their life cycles.
Many deep-living species produce buoyant eggs, which
float up to the surface so that the larvae hatch and
feed in the euphotic zone. As they reach the stage
when they are ready to metamorphose into the adult
form they sink back down into deep water. Euphausiids
of the genus Euphausia show the reverse pattern of
development. They lay yolk-rich eggs that are heavier
than water. These eggs sink to depths of 1000–
2000 m, where they hatch into non-feeding larvae.
These larvae begin the long swim back up towards the
surface, their early development being fuelled by the
yolk. As they near the surface their development has
progressed to their first feeding stage as they exhaust
their yolk supplies. Presumably this strategy ensures
that the return of the high investment of resources in
providing each egg with so much yolk is optimized
by the larvae spending so much time at depths where
predation is relatively low. A special example is seen
in the copepod Calanoides carinatus, which abounds in
newly upwelled water in coastal upwelling regions such
as the Somali Current (Smith, 1984) and the Benguela
Region (Verheye, 1991) (see pp. 67–68). Its ontogenetic
migrations enable the species to stay within the cell
of circulation associated with the upwelling, so that it
seeds the newly upwelled water with larvae which can
then exploit the phytoplankton bloom which follows an
upwelling event.
Seasonal migrations are particularly common at high
latitudes, where the season of high productivity is too
brief for many of the species to be able to complete
their life cycles within a year. In late summer/autumn
they migrate down into deep water and overwinter,
very often in a state of diapause during which the gut
regresses and they do not feed. In spring they migrate
back to the surface to mature, breed, and start the
cycle again. In the North Atlantic, the dominant highlatitude large copepod species Calanus finmarchicus
overwinters as a diapausing Stage v copepodite, at
depths as deep as 2000 m, and it needs to complete
a further instar before it matures and can breed. On
the other hand, in the North Pacific, the dominant
copepods belong to the genus Neocalanus, some of
which diapause as Stage vi copepodites and mature as
soon as they come out of diapause and return to the
upper layers. There are also some smaller species which
undergo an alternation of generations. Their growth
rates are fast enough for them to have a short-lived
summer generation, which is then followed by a longerlived winter generation which spends several months
in deep water. However, in terms of gross fluxes these
seasonal migrations result in trivial transfers of organic
material.
Diversity in pelagic communities
The diversity that one sees today is the result of
a dynamic interaction between speciation, whereby
the global inventory of species has evolved, and
the subsequent dispersion of the species, and the
currently prevailing environmental conditions which
Martin V. ANGEL
an increasing amount of evidence that these migrations
contribute to the downward fluxes of organic matter
from the surface (Longhurst and Harrison, 1988;
Dam et al., 1995; Zhang and Dam, 1998; Steinberg
et al., 2000; Angel and Pugh, 2000). The majority of
planktonic and micronektonic animals which perform
diel vertical migrations only feed during the shallow
phase of their migrations, where and when food is
more available. They migrate down with full stomachs,
and when they return the following dusk their guts are
empty. Thus, any organic material they do not retain
after ingestion but defaecate contributes to a downward
flux. Since most of what they consume comes from
shallow depths, anything they excrete at depth, be it
carbon dioxide, ammonia, or dissolved organic matter,
will have been derived from near the surface. If they
moult, die or are eaten, there is a downward flux of
material. Recent data from the equatorial Pacific show
that diel migrant mesozooplankton were exporting
0.6 and 1.1 mmol C m
−2 d
−1 in the two seasons of
observations, respectively, contributing 31% and 44%
to the sedimentary flux (Zhang and Dam, 1998). To
this estimate must be added the export by migrating
micronekton, whose biomass is roughly about half that
of the mesozooplankton (Angel and Pugh, 2000), but
whose migrations extend over much greater vertical
distances. These values imply that the active flux by
migrants may amount to about half that resulting from
particle sedimentation.
Diel migrations are generally restricted to the upper
kilometre of the water column (i.e., they involve
exchanges between the epipelagic and the deep and
shallow mesopelagic zones), but in the central oligotrophic gyres pelagic decapods (Domanski, 1986)
and some myctophid fish migrate even deeper. The
myctophid fish Ceratoscopelus warmingeri was found
to be migrating to depths of 1600–1700 m in the region
of the Azores Front (Angel, 1989b).
Ontogenetic migrations are carried out by species
that change their depth ranges during their life cycles.
Many deep-living species produce buoyant eggs, which
float up to the surface so that the larvae hatch and
feed in the euphotic zone. As they reach the stage
when they are ready to metamorphose into the adult
form they sink back down into deep water. Euphausiids
of the genus Euphausia show the reverse pattern of
development. They lay yolk-rich eggs that are heavier
than water. These eggs sink to depths of 1000–
2000 m, where they hatch into non-feeding larvae.
These larvae begin the long swim back up towards the
surface, their early development being fuelled by the
yolk. As they near the surface their development has
progressed to their first feeding stage as they exhaust
their yolk supplies. Presumably this strategy ensures
that the return of the high investment of resources in
providing each egg with so much yolk is optimized
by the larvae spending so much time at depths where
predation is relatively low. A special example is seen
in the copepod Calanoides carinatus, which abounds in
newly upwelled water in coastal upwelling regions such
as the Somali Current (Smith, 1984) and the Benguela
Region (Verheye, 1991) (see pp. 67–68). Its ontogenetic
migrations enable the species to stay within the cell
of circulation associated with the upwelling, so that it
seeds the newly upwelled water with larvae which can
then exploit the phytoplankton bloom which follows an
upwelling event.
Seasonal migrations are particularly common at high
latitudes, where the season of high productivity is too
brief for many of the species to be able to complete
their life cycles within a year. In late summer/autumn
they migrate down into deep water and overwinter,
very often in a state of diapause during which the gut
regresses and they do not feed. In spring they migrate
back to the surface to mature, breed, and start the
cycle again. In the North Atlantic, the dominant highlatitude large copepod species Calanus finmarchicus
overwinters as a diapausing Stage v copepodite, at
depths as deep as 2000 m, and it needs to complete
a further instar before it matures and can breed. On
the other hand, in the North Pacific, the dominant
copepods belong to the genus Neocalanus, some of
which diapause as Stage vi copepodites and mature as
soon as they come out of diapause and return to the
upper layers. There are also some smaller species which
undergo an alternation of generations. Their growth
rates are fast enough for them to have a short-lived
summer generation, which is then followed by a longerlived winter generation which spends several months
in deep water. However, in terms of gross fluxes these
seasonal migrations result in trivial transfers of organic
material.
Diversity in pelagic communities
The diversity that one sees today is the result of
a dynamic interaction between speciation, whereby
the global inventory of species has evolved, and
the subsequent dispersion of the species, and the
currently prevailing environmental conditions which