The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline
47
the water column. Unpublished pumped profiles that I obtained at oligotrophic stations
in the North Atlantic (31
and 34
N) in 1987 showed that protist taxa (both genera and
species) occurred preferentially across restricted depths within the mixed layer and the
thermocline, in the same way as the larger zooplankton. Unlike these, however, protist
cells have negligible powers of locomotion. The depth-differential distribution of protists
is best illustrated by the tintinnids, a group whose taxa are relatively simple to identify:
at Station PURPLE, south of the Convergence, 62 species of 31 genera were tallied at
10-m intervals from 0 to 110 m. The vertical distribution of these showed that the simple hypothesis—that within the mixed layer, all species should be distributed randomly
by wind-induced turbulence—must be rejected. Some genera (Salpingia, Tintinnoposis)
occurred preferentially within the subsurface chlorophyll maximum, others (Xystonella,
Eutintinnus) within the mixed layer. Of three species of Dictyocystis, one occurred preferentially above, and two within, the subsurface chlorophyll maximum. The mechanism
by which these taxon-specific layers of single-celled consumers are maintained is yet to
be explained, but one presumes that depth-differential growth rates must be invoked in
some way.
For the larger zooplankton, the pycnocline represents a special depth zone that not only
has unique characteristics but also lies close to the separation between the two principal
life zones of the ocean: lighted and dark (Longhurst and Harrison, 1989). The increase
of zooplankton biomass (five or six orders of magnitude) over the vertical distance (5 or
6 km) from the ocean floor to the sea surface would represent an unprecedented degree
of variability if translated into horizontal change within the mixed layer; it would also
have an unprecedented predictability (Longhurst, 1985a). In the interior of the ocean,
the vertical rate of change of biomass is very small, and the gradient is greatest over a
few tens of meters of the pycnocline, where the sparse bathypelagic plankton is separated
across a planktocline from the much more abundant epiplankton above. The epiplankton
and the deeper acoustic scattering layers of diel migrants are the most prominent features
in full-depth profiles of pelagic biota and, like the DCM and the pycnocline, they may
be traced across ocean basins.
At some depth within the epiplankton, and most often also within the pycnocline, a
maximum of zooplankton abundance (Z max ) usually occurs. Where the water column has
stabilized, Z max lies somewhat shallower than the DCM, especially at night, and closer to
the depth of the productivity maximum (Pt). Where there is a very shallow mixed layer,
as in upwelling regions or at the start of a spring bloom, Z max occurs very close to both
PM and DCM, which are coincident in these circumstances. The depth difference between
Z max and DCM is positively correlated with the absolute depth of DCM. Such observations
lead one to enquire whether all taxa aggregate at the Z max , or is it only certain species
that are specialized for life in the ecotone we are considering? It is convenient to discuss
this question by reference to special investigations of vertical distribution of zooplankton
species made at the BIOSTAT station in the eastern tropical Pacific (Longhurst, 1985b),
where there was a shoal pycnocline together with all the features of a TTP. Groups
of species could be identified that feed similarly and that occur within common depth
horizons, though some rearrangement of the vertical pattern occurs at dawn and dusk,
because several of these groups are diel migrants (Fig. 3.5).
The following characteristic groups of copepods were identified by species-specific
depths of maximum abundance (“preferred depths”) and depth ranges (“layer depths”)
of the central 50% of the populations of the 72 most abundant species:
Small herbivores (<20 mm): All species (genera: Calanus, Clausocalanus, Calocalanus,
Undinula, Nanocalanus, Acrocalanus, Paracalanus, Ischnocalanus, Acartia, Eucalanus,
Oncaea, and Corycaeus) had preferred day depths on the upper shoulder of the
DCM and thus close to the Pt. Some of these shifted a few meters upward into the
lower mixed layer at night. Oithona lay deeper than the other genera—closer to
47
the water column. Unpublished pumped profiles that I obtained at oligotrophic stations
in the North Atlantic (31
and 34
N) in 1987 showed that protist taxa (both genera and
species) occurred preferentially across restricted depths within the mixed layer and the
thermocline, in the same way as the larger zooplankton. Unlike these, however, protist
cells have negligible powers of locomotion. The depth-differential distribution of protists
is best illustrated by the tintinnids, a group whose taxa are relatively simple to identify:
at Station PURPLE, south of the Convergence, 62 species of 31 genera were tallied at
10-m intervals from 0 to 110 m. The vertical distribution of these showed that the simple hypothesis—that within the mixed layer, all species should be distributed randomly
by wind-induced turbulence—must be rejected. Some genera (Salpingia, Tintinnoposis)
occurred preferentially within the subsurface chlorophyll maximum, others (Xystonella,
Eutintinnus) within the mixed layer. Of three species of Dictyocystis, one occurred preferentially above, and two within, the subsurface chlorophyll maximum. The mechanism
by which these taxon-specific layers of single-celled consumers are maintained is yet to
be explained, but one presumes that depth-differential growth rates must be invoked in
some way.
For the larger zooplankton, the pycnocline represents a special depth zone that not only
has unique characteristics but also lies close to the separation between the two principal
life zones of the ocean: lighted and dark (Longhurst and Harrison, 1989). The increase
of zooplankton biomass (five or six orders of magnitude) over the vertical distance (5 or
6 km) from the ocean floor to the sea surface would represent an unprecedented degree
of variability if translated into horizontal change within the mixed layer; it would also
have an unprecedented predictability (Longhurst, 1985a). In the interior of the ocean,
the vertical rate of change of biomass is very small, and the gradient is greatest over a
few tens of meters of the pycnocline, where the sparse bathypelagic plankton is separated
across a planktocline from the much more abundant epiplankton above. The epiplankton
and the deeper acoustic scattering layers of diel migrants are the most prominent features
in full-depth profiles of pelagic biota and, like the DCM and the pycnocline, they may
be traced across ocean basins.
At some depth within the epiplankton, and most often also within the pycnocline, a
maximum of zooplankton abundance (Z max ) usually occurs. Where the water column has
stabilized, Z max lies somewhat shallower than the DCM, especially at night, and closer to
the depth of the productivity maximum (Pt). Where there is a very shallow mixed layer,
as in upwelling regions or at the start of a spring bloom, Z max occurs very close to both
PM and DCM, which are coincident in these circumstances. The depth difference between
Z max and DCM is positively correlated with the absolute depth of DCM. Such observations
lead one to enquire whether all taxa aggregate at the Z max , or is it only certain species
that are specialized for life in the ecotone we are considering? It is convenient to discuss
this question by reference to special investigations of vertical distribution of zooplankton
species made at the BIOSTAT station in the eastern tropical Pacific (Longhurst, 1985b),
where there was a shoal pycnocline together with all the features of a TTP. Groups
of species could be identified that feed similarly and that occur within common depth
horizons, though some rearrangement of the vertical pattern occurs at dawn and dusk,
because several of these groups are diel migrants (Fig. 3.5).
The following characteristic groups of copepods were identified by species-specific
depths of maximum abundance (“preferred depths”) and depth ranges (“layer depths”)
of the central 50% of the populations of the 72 most abundant species:
Small herbivores (<20 mm): All species (genera: Calanus, Clausocalanus, Calocalanus,
Undinula, Nanocalanus, Acrocalanus, Paracalanus, Ischnocalanus, Acartia, Eucalanus,
Oncaea, and Corycaeus) had preferred day depths on the upper shoulder of the
DCM and thus close to the Pt. Some of these shifted a few meters upward into the
lower mixed layer at night. Oithona lay deeper than the other genera—closer to