Pacific Westerly Winds Biome
341
North Atlantic. Neocalanus plumchrus, N. cristatus, and E. bungii are the most important
of these and comprise 80–95% of total mesozooplankton biomass. E. bungii has a 2-year
life cycle, but the others undergo maturation from C5s to adults below the surface layer
without a long overwintering period. Consequently, as the recent JGOFS investigations
confirmed, the annual biomass maximum of Neocalanus copepods in the surface layers
occurs in spring and copepodites are present in surface waters at all stations along Line
P during the winter (Goldblatt et al., 1999).
Neocalanus plumchrus adults are present throughout the year at 400 m and some
reproduction occurs in all months, peaking in winter. Female N. cristatus lie deeper (800–
900 m) and reproduce in all months, though peak egg production occurs in November.
Eucalanus bungii has a 2- to 3-year life cycle, the complex details of which place the whole
population in the upper 100 m in summer and at 250–500 m in winter. Development
from eggs to C4s occurs in the first year, with the C3s and C4s overwintering at depth
and developing as far as C5s during their second summer. They pass their second winter
as C5s and, maturing at depth, the new adults produce a new generation of eggs near the
surface. Some of the females may enter diapause for a third winter and produce another
batch of eggs near-surface the following spring.
The seasonal changes in mesozooplankton biomass in surface layers are largely forced
by the annual life cycles of Neocalanus flemingeri and N. plumchrus that reproduce at
depth. Their nauplii and early copepodites are present at the surface in winter, and the
copepod biomass maximum occurs when 50% of the individuals of these species exist as
the C4 stage and declines when the C5s migrate back to deep water in summer, there to
mature and spawn. This migration reduces the near-surface mesozooplankton biomass
by about 65%.
The important result of all these complexities is that some individuals of each species
are present in the surface layers at all seasons. It is this fact that prompted the classical
explanation of the OSW P observations that sufficient grazing pressure was already
available to suppress the accumulation of phytoplankton biomass during the spring
increase in production rate. As Goldblatt et al. (1999) show, mesozooplankton biomass
in spring (∼20 mg C m
−3 ) is high relative to that of autotrophic cells (10–20 mg C m
−3 ).
Further, the biomass of microzooplankton in spring is about equivalent to that of the
larger animals.
However, when the JGOFS investigators examined more carefully than had been
done before the relative vertical distribution of copepods in relation to biomass of
autotrophs and microherbivores that is concentrated in the upper 50 m, they found
strong vertical niche separation among copepods. Thus, N. cristatus and E. bungii—both
important contributors to mesozooplankton biomass—occur only deeper than 50 m and
feed on sinking aggregates, whereas N. flemingeri and N. plumchrus occur shallower than
this depth. Similar vertical separation at 50 m was observed among species of Oithona,
whereas other copepods (e.g., Microcalanus, Scolecithricella) occurred mostly below 75 m.
Consequently, at least in summer, much of the mesozooplankton can have little impact
on phytoplankton and microbial biomass. Thus the JGOFS investigations of 1996–97
confirmed earlier studies that suggested that the large copepods probably have only a
minor grazing impact on phytoplankton. However, small species of copepod (< 1 mm)
are not well sampled in standard nets and assumptions concerning their abundance may
be too low by more than an order of magnitude. Their specific activity is high, so they
may be a neglected source of mortality for small cells as they are in other comparable
situations: Goldblatt et al. remind us that in the subantarctic, Oithona spp. are responsible
for 50% of the mortality of small cells, though forming a much smaller biomass than the
large copepods. Oithona, of course, is the most abundant mesozooplankter along Line P.
Uncertainty such as this is our common lot when we come to examine a problem like
that of the phytoplankton at OWS P closely, and with truly critical intent.
341
North Atlantic. Neocalanus plumchrus, N. cristatus, and E. bungii are the most important
of these and comprise 80–95% of total mesozooplankton biomass. E. bungii has a 2-year
life cycle, but the others undergo maturation from C5s to adults below the surface layer
without a long overwintering period. Consequently, as the recent JGOFS investigations
confirmed, the annual biomass maximum of Neocalanus copepods in the surface layers
occurs in spring and copepodites are present in surface waters at all stations along Line
P during the winter (Goldblatt et al., 1999).
Neocalanus plumchrus adults are present throughout the year at 400 m and some
reproduction occurs in all months, peaking in winter. Female N. cristatus lie deeper (800–
900 m) and reproduce in all months, though peak egg production occurs in November.
Eucalanus bungii has a 2- to 3-year life cycle, the complex details of which place the whole
population in the upper 100 m in summer and at 250–500 m in winter. Development
from eggs to C4s occurs in the first year, with the C3s and C4s overwintering at depth
and developing as far as C5s during their second summer. They pass their second winter
as C5s and, maturing at depth, the new adults produce a new generation of eggs near the
surface. Some of the females may enter diapause for a third winter and produce another
batch of eggs near-surface the following spring.
The seasonal changes in mesozooplankton biomass in surface layers are largely forced
by the annual life cycles of Neocalanus flemingeri and N. plumchrus that reproduce at
depth. Their nauplii and early copepodites are present at the surface in winter, and the
copepod biomass maximum occurs when 50% of the individuals of these species exist as
the C4 stage and declines when the C5s migrate back to deep water in summer, there to
mature and spawn. This migration reduces the near-surface mesozooplankton biomass
by about 65%.
The important result of all these complexities is that some individuals of each species
are present in the surface layers at all seasons. It is this fact that prompted the classical
explanation of the OSW P observations that sufficient grazing pressure was already
available to suppress the accumulation of phytoplankton biomass during the spring
increase in production rate. As Goldblatt et al. (1999) show, mesozooplankton biomass
in spring (∼20 mg C m
−3 ) is high relative to that of autotrophic cells (10–20 mg C m
−3 ).
Further, the biomass of microzooplankton in spring is about equivalent to that of the
larger animals.
However, when the JGOFS investigators examined more carefully than had been
done before the relative vertical distribution of copepods in relation to biomass of
autotrophs and microherbivores that is concentrated in the upper 50 m, they found
strong vertical niche separation among copepods. Thus, N. cristatus and E. bungii—both
important contributors to mesozooplankton biomass—occur only deeper than 50 m and
feed on sinking aggregates, whereas N. flemingeri and N. plumchrus occur shallower than
this depth. Similar vertical separation at 50 m was observed among species of Oithona,
whereas other copepods (e.g., Microcalanus, Scolecithricella) occurred mostly below 75 m.
Consequently, at least in summer, much of the mesozooplankton can have little impact
on phytoplankton and microbial biomass. Thus the JGOFS investigations of 1996–97
confirmed earlier studies that suggested that the large copepods probably have only a
minor grazing impact on phytoplankton. However, small species of copepod (< 1 mm)
are not well sampled in standard nets and assumptions concerning their abundance may
be too low by more than an order of magnitude. Their specific activity is high, so they
may be a neglected source of mortality for small cells as they are in other comparable
situations: Goldblatt et al. remind us that in the subantarctic, Oithona spp. are responsible
for 50% of the mortality of small cells, though forming a much smaller biomass than the
large copepods. Oithona, of course, is the most abundant mesozooplankter along Line P.
Uncertainty such as this is our common lot when we come to examine a problem like
that of the phytoplankton at OWS P closely, and with truly critical intent.