The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline
49
elsewhere in the Pacific Ocean, where the mixed layer is both extremely shallow (Costa
Rica Dome) or very deep (at the CLIMAX station in the North Pacific central gyre).
We should not expect to find such a consistent pattern of vertical distribution in
latitudes with deep winter mixing, where plankton profiles are driven alternately by
diapausing and feeding strategies. However, available summer profiles from all oceans
suggest that, as the summer progresses, plankton profiles approach the vertical pattern
of the tropical climax community described previously.
The North Atlantic from 44
to 62
N in August exemplifies this situation; at this
time, many genera have depth distributions remarkably similar to their occurrence in
tropical profiles (Longhurst and Williams, 1979). Small herbivorous copepods (Acartia,
Paracalanus, Pseudocalanus, Oithona, and Clausocalanus) and cladocerans occur most
abundantly in the upper thermocline, near the DCM, with only scattered individuals
deeper. Already in this season, ontogenetic migrations had carried diapausing individuals
of the large herbivores to 1000 m. At night, the interzonal copepods and diel migrant
euphausiids occurred at the same depths as the epiplanktonic filter-feeding copepods.
Therefore, there seems to be abundant evidence from observations of plankton organisms from the smallest autotrophic cells to herbivorous mesozooplankton to support the
original statement. We should, indeed, consider the horizontal front represented by the
pycnocline not only as a way station where sinking particles may aggregate, but also as an
ecotone that is the preferred living space of a characteristic group of plankton organisms.
49
elsewhere in the Pacific Ocean, where the mixed layer is both extremely shallow (Costa
Rica Dome) or very deep (at the CLIMAX station in the North Pacific central gyre).
We should not expect to find such a consistent pattern of vertical distribution in
latitudes with deep winter mixing, where plankton profiles are driven alternately by
diapausing and feeding strategies. However, available summer profiles from all oceans
suggest that, as the summer progresses, plankton profiles approach the vertical pattern
of the tropical climax community described previously.
The North Atlantic from 44
to 62
N in August exemplifies this situation; at this
time, many genera have depth distributions remarkably similar to their occurrence in
tropical profiles (Longhurst and Williams, 1979). Small herbivorous copepods (Acartia,
Paracalanus, Pseudocalanus, Oithona, and Clausocalanus) and cladocerans occur most
abundantly in the upper thermocline, near the DCM, with only scattered individuals
deeper. Already in this season, ontogenetic migrations had carried diapausing individuals
of the large herbivores to 1000 m. At night, the interzonal copepods and diel migrant
euphausiids occurred at the same depths as the epiplanktonic filter-feeding copepods.
Therefore, there seems to be abundant evidence from observations of plankton organisms from the smallest autotrophic cells to herbivorous mesozooplankton to support the
original statement. We should, indeed, consider the horizontal front represented by the
pycnocline not only as a way station where sinking particles may aggregate, but also as an
ecotone that is the preferred living space of a characteristic group of plankton organisms.