94
Chapter 6: Biomes: The Primary Partition
discontinuity across the ocean at between 40 and 50
N. Taxonomic richness, supported
by five separate diversity indices, followed very closely, as the authors commented, the
boundaries between the partition between biomes discussed here. The authors note that
the poleward decrease of taxonomic diversity anticipated in the pelagic ecosystem is
discontinuous, and that the gradient is interrupted by local variability where different
surface water bodies are interleaved.
Such information, although very restricted in scope, does gives some confidence that
the primary partition discussed here is based on reality.
Polar Biome
Beyond the polar fronts, strong near-surface stratification is induced by the effects of the
freeze-thaw cycle of sea ice: thus, brine rejection occurs when surface water freezes so
that the dense water sinks, and fresh water is released when sea ice melts in the spring.
Deep winter mixing occurs only in the absence of ice cover and, after the spring
thaw, the superficial low-salinity layer stabilizes the upper water column so that seasonal
change in mixed-layer depth is relatively small during the open-water season. Despite
these constraints, sufficient mixing occurs to recharge the surface layers seasonally with
inorganic nutrients, and this is critical because the stability imposed by the near-surface
halocline may be sufficient to allow algal growth to occur as soon as there is sufficient
sunlight, even though values of N max (commonly 2–8 cycles hr
−1 ) within the shallow
polar pycnocline are not as high in the tropical pycnocline. In this connection, note that
because of its low salinity, equivalent changes in the density of polar water are forced by
salinity changes of 0.1/ml or temperature changes of 5
C.
The extreme range of irradiance in polar regions forces a unique seasonal cycle of
primary production rate, represented by the simple Case 1 model and having a single,
light-limited maximum at the summer solstice. Poleward of 66
latitude, after the vernal
equinox, day length increases exponentially with latitude so that there is insignificant
effect of latitude on the timing of a spring bloom in open water. A shallow chlorophyll
maximum develops at the halocline in open water, and summer oligotrophic conditions
with irradiance of the pycnocline may be very briefly established. Paradoxically, one might
think, early spring thaws produce less strong ice-edge blooms than late thaws when the
sun is high in the sky. This is because the accumulation of an ice-edge bloom requires
sufficient irradiance that algal growth will exceed its local sinking rate, given that it may
occur prior to the establishment of stratification in the water column.
The polar biome is characterized by low taxonomic diversity at all trophic levels; during
the short pulse of primary production that occurs during the brief period of open water
and high sun angle, phytoplankton biomass is dominated by large cells (>90% diatoms
and coccolithophores) although the smaller fractions of pico- and nano-phytoplankton
are nevertheless responsible for 10–25% of production. Diatom frustules form a diatom
ooze on the deep ocean floor, especially in the Southern Ocean where the northern limit
of ooze corresponds very closely with the location of the Antarctic Divergence, itself
defining the equatorward boundary of the austral polar biome. When a brief oligotrophic
phase follows the spring bloom, as it may do in open water in Baffin Bay, smaller cells
dominate as everywhere when nitrate is no longer available and biologically regenerated
ammonium is utilized.
A second accumulation of chlorophyll that occurs in some regions, in some years,
during the late summer period of declining primary production rate is consistent with
the effect of reduced herbivore consumption as these organisms descend to overwintering
depths. Herbivores are dominated by large copepods in both hemispheres; if you contribute to the popular belief that the polar seas are unique because euphausiids dominate
the plankton and provide the abundant and easily strained food required to sustain baleen
Chapter 6: Biomes: The Primary Partition
discontinuity across the ocean at between 40 and 50
N. Taxonomic richness, supported
by five separate diversity indices, followed very closely, as the authors commented, the
boundaries between the partition between biomes discussed here. The authors note that
the poleward decrease of taxonomic diversity anticipated in the pelagic ecosystem is
discontinuous, and that the gradient is interrupted by local variability where different
surface water bodies are interleaved.
Such information, although very restricted in scope, does gives some confidence that
the primary partition discussed here is based on reality.
Polar Biome
Beyond the polar fronts, strong near-surface stratification is induced by the effects of the
freeze-thaw cycle of sea ice: thus, brine rejection occurs when surface water freezes so
that the dense water sinks, and fresh water is released when sea ice melts in the spring.
Deep winter mixing occurs only in the absence of ice cover and, after the spring
thaw, the superficial low-salinity layer stabilizes the upper water column so that seasonal
change in mixed-layer depth is relatively small during the open-water season. Despite
these constraints, sufficient mixing occurs to recharge the surface layers seasonally with
inorganic nutrients, and this is critical because the stability imposed by the near-surface
halocline may be sufficient to allow algal growth to occur as soon as there is sufficient
sunlight, even though values of N max (commonly 2–8 cycles hr
−1 ) within the shallow
polar pycnocline are not as high in the tropical pycnocline. In this connection, note that
because of its low salinity, equivalent changes in the density of polar water are forced by
salinity changes of 0.1/ml or temperature changes of 5
C.
The extreme range of irradiance in polar regions forces a unique seasonal cycle of
primary production rate, represented by the simple Case 1 model and having a single,
light-limited maximum at the summer solstice. Poleward of 66
latitude, after the vernal
equinox, day length increases exponentially with latitude so that there is insignificant
effect of latitude on the timing of a spring bloom in open water. A shallow chlorophyll
maximum develops at the halocline in open water, and summer oligotrophic conditions
with irradiance of the pycnocline may be very briefly established. Paradoxically, one might
think, early spring thaws produce less strong ice-edge blooms than late thaws when the
sun is high in the sky. This is because the accumulation of an ice-edge bloom requires
sufficient irradiance that algal growth will exceed its local sinking rate, given that it may
occur prior to the establishment of stratification in the water column.
The polar biome is characterized by low taxonomic diversity at all trophic levels; during
the short pulse of primary production that occurs during the brief period of open water
and high sun angle, phytoplankton biomass is dominated by large cells (>90% diatoms
and coccolithophores) although the smaller fractions of pico- and nano-phytoplankton
are nevertheless responsible for 10–25% of production. Diatom frustules form a diatom
ooze on the deep ocean floor, especially in the Southern Ocean where the northern limit
of ooze corresponds very closely with the location of the Antarctic Divergence, itself
defining the equatorward boundary of the austral polar biome. When a brief oligotrophic
phase follows the spring bloom, as it may do in open water in Baffin Bay, smaller cells
dominate as everywhere when nitrate is no longer available and biologically regenerated
ammonium is utilized.
A second accumulation of chlorophyll that occurs in some regions, in some years,
during the late summer period of declining primary production rate is consistent with
the effect of reduced herbivore consumption as these organisms descend to overwintering
depths. Herbivores are dominated by large copepods in both hemispheres; if you contribute to the popular belief that the polar seas are unique because euphausiids dominate
the plankton and provide the abundant and easily strained food required to sustain baleen