The Four Primary Biomes of the Upper Ocean
95
whales, you may be surprised by Table 6.1. Contrary to tradition, the polar oceans are not
dominated by euphausiids (“krill”), although obviously these occur in great abundance in
some restricted regions. In fact, in both northern and southern hemispheres, euphausiids
form <1% numerically and <8% by biomass of all zooplankton in regional surveys of
polar oceans. In tropical latitudes, euphausiids comprise 30% of zooplankton biomass
overall.
The herbivore response to the availability of phytoplankton may not be as direct as in
lower latitudes because of relatively long generation times; some of the larger species of
copepods require more than a single season to complete their growth and reproductive
cycles, passing the winter in water as deep as 1000 m. In their first season, having reached
the fifth-stage copepodite stage (the pre-adult instar) they cease to feed and migrate down.
Apart from the consequences for the standing stock of phytoplankton cells, the annual
cycle of the large copepods has another, often neglected, consequence. Where water depth
is insufficient for the seasonal ontogenetic migration to occur, large copepods do not
complete their life cycles successfully and exist principally as expatriate populations. A
difference between northern and southern hemisphere polar zones is that in the latter
there are relatively more important populations of herbivorous euphausiids that, unlike
the large calanoid copepods of both north and south, do not perform seasonal migrations:
they remain relatively near the surface during winter, usually associated with sea ice.
At the upper trophic levels, the marine mammal fauna is more diverse than in
lower latitudes and includes several whale species that feed directly on plankton and
micronekton. Less than 1% of the approximately 20,000 species of marine fish occur
south of the antarctic convergence, and there are no antarctic species of densely schooling
pelagic fish. Small shoaling fish occur only in the southerly parts of the boreal polar biome:
for instance, capelin (Mallotus villosus) is a significant component of the ecosystem of
the Labrador and Barents Seas. In the high arctic, the mesopelagic polar cod Boreogadus
saida is a key species in the transfer of material from planktonic production to birds and
mammals and occupies a niche similar to that of schooling pelagic euphausiids Euphausia
superba and E. crystalophorius in the antarctic, together with another small gadoid, the
silverfish Pleurogramma antarcticum.
Westerlies Biome
Here, either the Case 2 or Case 3 model describes the seasonal production cycle, depending on latitude and on the relative depth of winter mixing by the westerly winds.
These, of course, extend equatorward to the Subtropical Convergence zones between
the opposing hemispheric Trades and Westerly wind systems; because the westerlies are
globe-encircling, stronger, and more consistent in location in the southern hemisphere,
the subtropical convergence (STC) there is more readily located. It also occurs at much
higher latitudes than in the northern hemisphere, being constrained by the geography
of South America and Australia-New Zealand to encircle the globe at around 60
S. In
the northern hemisphere, the STC crosses the oceans SW-NE at between 30 and 40
of
latitude.
Wind stress (see Color plate 3) is most strongly seasonal across the North Atlantic
(winter <30 and summer <5 × 10
−1 dyn cm
−1 ) and least seasonal in the Southern Ocean
(<20–40 × 10
−1 dyn cm
−1 at all seasons). Westerly storm tracks in the North Atlantic
track across the ocean from Florida to the Europe and, in the North Pacific, more zonally
from southern Japan to the west coast of Canada. In the Southern Ocean in winter, the
track is circumpolar, crossing Patagonia at 45–50
S.
Mixed-layer depth is strongly seasonal, deepening in winter with increased wind stress
and convective cooling forced by the seasonal changes in sun angle, cloudiness, and
strength of the zonal westerly winds. In the North Pacific, and especially in the Alaska gyre,
95
whales, you may be surprised by Table 6.1. Contrary to tradition, the polar oceans are not
dominated by euphausiids (“krill”), although obviously these occur in great abundance in
some restricted regions. In fact, in both northern and southern hemispheres, euphausiids
form <1% numerically and <8% by biomass of all zooplankton in regional surveys of
polar oceans. In tropical latitudes, euphausiids comprise 30% of zooplankton biomass
overall.
The herbivore response to the availability of phytoplankton may not be as direct as in
lower latitudes because of relatively long generation times; some of the larger species of
copepods require more than a single season to complete their growth and reproductive
cycles, passing the winter in water as deep as 1000 m. In their first season, having reached
the fifth-stage copepodite stage (the pre-adult instar) they cease to feed and migrate down.
Apart from the consequences for the standing stock of phytoplankton cells, the annual
cycle of the large copepods has another, often neglected, consequence. Where water depth
is insufficient for the seasonal ontogenetic migration to occur, large copepods do not
complete their life cycles successfully and exist principally as expatriate populations. A
difference between northern and southern hemisphere polar zones is that in the latter
there are relatively more important populations of herbivorous euphausiids that, unlike
the large calanoid copepods of both north and south, do not perform seasonal migrations:
they remain relatively near the surface during winter, usually associated with sea ice.
At the upper trophic levels, the marine mammal fauna is more diverse than in
lower latitudes and includes several whale species that feed directly on plankton and
micronekton. Less than 1% of the approximately 20,000 species of marine fish occur
south of the antarctic convergence, and there are no antarctic species of densely schooling
pelagic fish. Small shoaling fish occur only in the southerly parts of the boreal polar biome:
for instance, capelin (Mallotus villosus) is a significant component of the ecosystem of
the Labrador and Barents Seas. In the high arctic, the mesopelagic polar cod Boreogadus
saida is a key species in the transfer of material from planktonic production to birds and
mammals and occupies a niche similar to that of schooling pelagic euphausiids Euphausia
superba and E. crystalophorius in the antarctic, together with another small gadoid, the
silverfish Pleurogramma antarcticum.
Westerlies Biome
Here, either the Case 2 or Case 3 model describes the seasonal production cycle, depending on latitude and on the relative depth of winter mixing by the westerly winds.
These, of course, extend equatorward to the Subtropical Convergence zones between
the opposing hemispheric Trades and Westerly wind systems; because the westerlies are
globe-encircling, stronger, and more consistent in location in the southern hemisphere,
the subtropical convergence (STC) there is more readily located. It also occurs at much
higher latitudes than in the northern hemisphere, being constrained by the geography
of South America and Australia-New Zealand to encircle the globe at around 60
S. In
the northern hemisphere, the STC crosses the oceans SW-NE at between 30 and 40
of
latitude.
Wind stress (see Color plate 3) is most strongly seasonal across the North Atlantic
(winter <30 and summer <5 × 10
−1 dyn cm
−1 ) and least seasonal in the Southern Ocean
(<20–40 × 10
−1 dyn cm
−1 at all seasons). Westerly storm tracks in the North Atlantic
track across the ocean from Florida to the Europe and, in the North Pacific, more zonally
from southern Japan to the west coast of Canada. In the Southern Ocean in winter, the
track is circumpolar, crossing Patagonia at 45–50
S.
Mixed-layer depth is strongly seasonal, deepening in winter with increased wind stress
and convective cooling forced by the seasonal changes in sun angle, cloudiness, and
strength of the zonal westerly winds. In the North Pacific, and especially in the Alaska gyre,