394
Chapter 11: The Pacific Ocean
organic material and diel migrants. In Prince William Sound at 60
N, N. plumchrus
together with N. cristatus and E. bungii are the dominant copepods during those months
when they are not overwintering in deep water beyond the shelf (Cooney, 1986). Appearing in spring as nauplii and copepodites over the shelf, many N. cristatus and E. bungi
descend to overwintering depths beyond the shelf edge in October where maturation
and reproduction take place to produce the early larvae of the new generation. However,
many individuals of these oceanic species complete their life cycle within the Sound,
which is sufficiently deep for this to occur, and in the case of Neocalanus at least, may
be distinguished by the isotopically heavy carbon content; their proportion of the species
population in the Sound varies between seasons and years.
Mackas and Coyle (2005) present an important discussion on the mechanisms by
which behavior patterns of individual species are adapted to the rigors of remaining
within a suitable habitat, in three-dimensional space; this, they point out, is no simple
matter for an organism that has (i) very limited motility and (ii) no mechanism for
locating its present position in relation to regions suitable for its continued existence,
although (iii) it does know which way is “up,” as they put it, and (iv) can detect microscale
shear. For these reasons, Mackas and Coyle insist on the importance of vertical motion—
a few tens of meters of relative motion being well within the capabilities of even the
smaller copepod species—because this affords the individual a mechanism for significant
horizontal motion in a layered physical system with differential motion between layers.
They are quick to point out that this is no novel concept: it is usually attributed to
Alister Hardy, who described such organisms as having “seven-league boots, to set them
striding through the sea.” They report differential behavior between Pseudocalanus mimus
and Calanus marshallae, so that the former is unable to avoid offshore transport in
anticyclonic eddies, whereas the latter occurs very rarely in such systems.
Despite these advances, Liu et al. (2005) suggest that we still have inadequate understanding of the nutritional dynamics of even the dominant species, such as Neocalanus
cristatus, adequately to characterize the interaction between copepods and autotrophs.
N. cristatus appears to reject <5-m cells in favor of larger organisms, probably largely
protests in sufficient quantities as to induce a cascade effect in situ: their intake of
autotrophic carbon little more than doubles from ∼30% under nonbloom conditions to
∼70% during blooms. But the measured rates are too small to support observed growth,
and observations of intake of even larger particles are to be anticipated.
The response of biota to the anticyclonic eddies discussed earlier requires separate
consideration and now, fortunately, we have a series of sustained studies of the Heida
eddies to inform us. Matching SeaWiFS-derived chlorophyll and TOPEX/ERS-2 SLA
images over a 5-year period, Crawford et al. (2005) demonstrate that these eddies support
central spring blooms in their “natal year” (as these authors put it) that are stronger
than in surrounding offshore water. This is the anticipated consequence of the difference
in nutrient regime between coastal water contained in the eddy core, and the offshore
water surrounding it; the arcuate streamers of high chlorophyll often observed around
eddies arise by offshore entrainment of eutrophic, chlorophyll-rich coastal water around
the (slowly) whirling eddy. Deployment of expatriated CPRs on lines parallel to the coast
obtained direct evidence of the presence in offshore eddies and streamers of species of
diatoms and copepods characteristic of shelf regions. The zooplankton retained within
a Haida eddy progressively changes its composition during the life of the eddy, so that
abundance within the eddy is higher than outside or in source regions. This appears
to result from aggregation and retention mechanisms favoring those species that preferentially occur relatively deep in the eddy, these being selected by slow divergence and
upwelling within the eddy, by rapid but intermittent flushing of the upper layers by storm
events, and by exchange across geostrophic streamlines at eddy margins by wind-driven
inertial currents (Mackas et al., 2005).
Chapter 11: The Pacific Ocean
organic material and diel migrants. In Prince William Sound at 60
N, N. plumchrus
together with N. cristatus and E. bungii are the dominant copepods during those months
when they are not overwintering in deep water beyond the shelf (Cooney, 1986). Appearing in spring as nauplii and copepodites over the shelf, many N. cristatus and E. bungi
descend to overwintering depths beyond the shelf edge in October where maturation
and reproduction take place to produce the early larvae of the new generation. However,
many individuals of these oceanic species complete their life cycle within the Sound,
which is sufficiently deep for this to occur, and in the case of Neocalanus at least, may
be distinguished by the isotopically heavy carbon content; their proportion of the species
population in the Sound varies between seasons and years.
Mackas and Coyle (2005) present an important discussion on the mechanisms by
which behavior patterns of individual species are adapted to the rigors of remaining
within a suitable habitat, in three-dimensional space; this, they point out, is no simple
matter for an organism that has (i) very limited motility and (ii) no mechanism for
locating its present position in relation to regions suitable for its continued existence,
although (iii) it does know which way is “up,” as they put it, and (iv) can detect microscale
shear. For these reasons, Mackas and Coyle insist on the importance of vertical motion—
a few tens of meters of relative motion being well within the capabilities of even the
smaller copepod species—because this affords the individual a mechanism for significant
horizontal motion in a layered physical system with differential motion between layers.
They are quick to point out that this is no novel concept: it is usually attributed to
Alister Hardy, who described such organisms as having “seven-league boots, to set them
striding through the sea.” They report differential behavior between Pseudocalanus mimus
and Calanus marshallae, so that the former is unable to avoid offshore transport in
anticyclonic eddies, whereas the latter occurs very rarely in such systems.
Despite these advances, Liu et al. (2005) suggest that we still have inadequate understanding of the nutritional dynamics of even the dominant species, such as Neocalanus
cristatus, adequately to characterize the interaction between copepods and autotrophs.
N. cristatus appears to reject <5-m cells in favor of larger organisms, probably largely
protests in sufficient quantities as to induce a cascade effect in situ: their intake of
autotrophic carbon little more than doubles from ∼30% under nonbloom conditions to
∼70% during blooms. But the measured rates are too small to support observed growth,
and observations of intake of even larger particles are to be anticipated.
The response of biota to the anticyclonic eddies discussed earlier requires separate
consideration and now, fortunately, we have a series of sustained studies of the Heida
eddies to inform us. Matching SeaWiFS-derived chlorophyll and TOPEX/ERS-2 SLA
images over a 5-year period, Crawford et al. (2005) demonstrate that these eddies support
central spring blooms in their “natal year” (as these authors put it) that are stronger
than in surrounding offshore water. This is the anticipated consequence of the difference
in nutrient regime between coastal water contained in the eddy core, and the offshore
water surrounding it; the arcuate streamers of high chlorophyll often observed around
eddies arise by offshore entrainment of eutrophic, chlorophyll-rich coastal water around
the (slowly) whirling eddy. Deployment of expatriated CPRs on lines parallel to the coast
obtained direct evidence of the presence in offshore eddies and streamers of species of
diatoms and copepods characteristic of shelf regions. The zooplankton retained within
a Haida eddy progressively changes its composition during the life of the eddy, so that
abundance within the eddy is higher than outside or in source regions. This appears
to result from aggregation and retention mechanisms favoring those species that preferentially occur relatively deep in the eddy, these being selected by slow divergence and
upwelling within the eddy, by rapid but intermittent flushing of the upper layers by storm
events, and by exchange across geostrophic streamlines at eddy margins by wind-driven
inertial currents (Mackas et al., 2005).