46
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
rate of primary production increase. Zooplankton herbivores are often concentrated as
dense layers at the depths of maximum rate of algal growth, though the total vertical
distribution of zooplankton is complicated by the vertical migrations of some species,
usually with diel or seasonal frequency, across the pycnocline. In this way, the migrant
species utilize to their advantage the contrasting ecological conditions of both euphotic
and bathypelagic zones.
Ocean basin-scale baroclinicity defines the topography of pycnocline troughs, ridges,
bowls, and domes that are associated with the geostrophic flow while, everywhere, the
pycnocline itself and the other gradients associated with it remain in the same depth
sequence. Generally, the deeper the pycnocline, the greater the depth interval over which
the features of the oligotrophic profile are spread. In Chapters 9–12, I shall discuss many
regional variations on this general theme, forced by regional topography and climate,
but this introduction is sufficient to demonstrate the ubiquity of this most important
ecological boundary in the ocean, excepting only the sea surface, the sea floor, and the
shoreline.
All this tells us that this boundary layer is also an ecotone, comparable to the examples
discussed previously at vertical frontal zones, and has all the characteristics of one; that
is, not only are ecological conditions different above and below it, but there are very
special ecological conditions within it. If a characteristic flora could be recognized at the
pycnocline, as distinct from occasional shade adaptation of phytoplankton also occurring
in the upper euphotic zone, we could regard it as the shade flora of the ocean and
comparable with that of the forest floor. In fact, for each major group of photosynthetic
cells, there is indeed evidence that an oceanic shade flora must be recognized (Longhurst
and Harrison, 1989). In discussing this evidence, let us begin with the smallest cells.
Cyanobacteria and prochlorophytes are most abundant in the mixed layer across the
entire North Atlantic from the Gulf Stream to Morocco, whereas peak abundance of
small eukaryotes, mostly chlorophytes, prymnesiophytes, and chrysophytes, occurs at the
DCM (Li and Wood, 1988; Li, 1995). In the Northwest Pacific, very small eukaryotes,
especially Micromonas 1–3 m, also dominate the DCM. Thus, there is some evidence
for the existence of a specialized shade flora within the smallest photosynthetic cells.
But for the larger cells, especially dinoflagellates and diatoms, we have much better
evidence. Early in the season in temperate or subtropical regimes, during periods of
relatively high turbulence and diapycnal mixing, the taxa at the DCM resemble those in
the mixed layer above (Venrick et al., 1973; Taniguchi and Kawamura, 1972). However,
at 26
N in the Pacific in summer, after pycnocline and DCM have developed, two
distinct diatom assemblages meet at the top of the nutricline: the shallower assemblage is
nutrient limited while the deeper is light limited (Venrick, 1988). Each assemblage has the
characteristics of a mature, predation-controlled assemblage, and community diversity
increases to a maximum near the DCM.
From the more extensive general literature on phytogeography, conclusions may be
drawn that seem to support the few available floristic profiles. For example, the widespread
existence has been noted of a diverse “shade flora” of four Bacillariophycae, ten Dinophycae, one Prasinophycae, and three Prymnesiophycae. Some of these shade species are
very large organisms, such as the diatom Plantoniella sol (Furuya and Marumo, 1983)
and the widespread prasinophyte Halosphaera viridis. In the Kuroshio region another
11 species, in addition to P. sol, are shade species (including the diatoms Asteromphalus
sarcophagus, Oolithotus fragilis, Thorosphaera flabellata, and Thalassionema spp.). Thus,
the hypothesis that the ordered ecosystem of the euphotic zone is partitioned among two
different assemblages of algal cells, one of which constitutes a shade flora, appears to be
supported.
The protistan consumers of the many size classes of phytoplankton are themselves,
not unexpectedly, likewise distributed in a vertically ordered manner in the upper part of
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
rate of primary production increase. Zooplankton herbivores are often concentrated as
dense layers at the depths of maximum rate of algal growth, though the total vertical
distribution of zooplankton is complicated by the vertical migrations of some species,
usually with diel or seasonal frequency, across the pycnocline. In this way, the migrant
species utilize to their advantage the contrasting ecological conditions of both euphotic
and bathypelagic zones.
Ocean basin-scale baroclinicity defines the topography of pycnocline troughs, ridges,
bowls, and domes that are associated with the geostrophic flow while, everywhere, the
pycnocline itself and the other gradients associated with it remain in the same depth
sequence. Generally, the deeper the pycnocline, the greater the depth interval over which
the features of the oligotrophic profile are spread. In Chapters 9–12, I shall discuss many
regional variations on this general theme, forced by regional topography and climate,
but this introduction is sufficient to demonstrate the ubiquity of this most important
ecological boundary in the ocean, excepting only the sea surface, the sea floor, and the
shoreline.
All this tells us that this boundary layer is also an ecotone, comparable to the examples
discussed previously at vertical frontal zones, and has all the characteristics of one; that
is, not only are ecological conditions different above and below it, but there are very
special ecological conditions within it. If a characteristic flora could be recognized at the
pycnocline, as distinct from occasional shade adaptation of phytoplankton also occurring
in the upper euphotic zone, we could regard it as the shade flora of the ocean and
comparable with that of the forest floor. In fact, for each major group of photosynthetic
cells, there is indeed evidence that an oceanic shade flora must be recognized (Longhurst
and Harrison, 1989). In discussing this evidence, let us begin with the smallest cells.
Cyanobacteria and prochlorophytes are most abundant in the mixed layer across the
entire North Atlantic from the Gulf Stream to Morocco, whereas peak abundance of
small eukaryotes, mostly chlorophytes, prymnesiophytes, and chrysophytes, occurs at the
DCM (Li and Wood, 1988; Li, 1995). In the Northwest Pacific, very small eukaryotes,
especially Micromonas 1–3 m, also dominate the DCM. Thus, there is some evidence
for the existence of a specialized shade flora within the smallest photosynthetic cells.
But for the larger cells, especially dinoflagellates and diatoms, we have much better
evidence. Early in the season in temperate or subtropical regimes, during periods of
relatively high turbulence and diapycnal mixing, the taxa at the DCM resemble those in
the mixed layer above (Venrick et al., 1973; Taniguchi and Kawamura, 1972). However,
at 26
N in the Pacific in summer, after pycnocline and DCM have developed, two
distinct diatom assemblages meet at the top of the nutricline: the shallower assemblage is
nutrient limited while the deeper is light limited (Venrick, 1988). Each assemblage has the
characteristics of a mature, predation-controlled assemblage, and community diversity
increases to a maximum near the DCM.
From the more extensive general literature on phytogeography, conclusions may be
drawn that seem to support the few available floristic profiles. For example, the widespread
existence has been noted of a diverse “shade flora” of four Bacillariophycae, ten Dinophycae, one Prasinophycae, and three Prymnesiophycae. Some of these shade species are
very large organisms, such as the diatom Plantoniella sol (Furuya and Marumo, 1983)
and the widespread prasinophyte Halosphaera viridis. In the Kuroshio region another
11 species, in addition to P. sol, are shade species (including the diatoms Asteromphalus
sarcophagus, Oolithotus fragilis, Thorosphaera flabellata, and Thalassionema spp.). Thus,
the hypothesis that the ordered ecosystem of the euphotic zone is partitioned among two
different assemblages of algal cells, one of which constitutes a shade flora, appears to be
supported.
The protistan consumers of the many size classes of phytoplankton are themselves,
not unexpectedly, likewise distributed in a vertically ordered manner in the upper part of