98
Chapter 6: Biomes: The Primary Partition
Productivity is rarely light-limited and responds rapidly to nutrient entrainment into
the photic zone by coastal upwelling, offshore Ekman suction, and geostrophic adjustment of pycnocline associated with reversal of the monsoon winds. The pycnocline is
illuminated during the oligotrophic season, but the mixed layer shoals above the photic
zone during upwelling episodes. Changes in phytoplankton biomass match the seasonal
changes in production rate, consistent with consumption and production rates being
closely matched. Seasonal algal blooms are diatom-dominated, with picocyanobacteria
and other prochlorophytes contributing only 30–40% of phytoplankton carbon, compared to >90% in the oligotrophic regions/seasons.
The trade-winds regime covers about 45% of the total area of the ocean, and for that
reason alone it will be useful here to review very briefly its principal physical characteristics
that determine its singular ecology:
The radiation balance is such that there is a mean annual positive downward heat flux
across the surface. The area so defined has its widest latitudinal extent in the Atlantic.
The seasonal radiation flux is such that surface mixed layer is maintained continuously;
at no season is there sufficient loss of heat to produce convective deepening of the
permanent tropical thermocline.
The level of solar radiation is such that autotrophs are less commonly light-limited
than they are at higher latitudes.
The Rossby radius of internal deformation increases into the tropical zone as a consequence of diminishing Coriolis force; therefore, eddies are increasingly larger but
fewer toward the equator.
Baroclinic time scales are weeks in the tropics rather than years at higher latitudes.
The equatorward-diminishing Coriolis parameter (f ) means that the slope of the sea
surface required to force horizontal motion diminishes toward the equator.
The pycnocline is coincident with permanent tropical thermocline and has very high
stability (N ; the Brunt-Väisälä frequency takes high values) equatorward of latitude
at which winter convective mixing becomes trivial.
The tropical Ekman layer [Z ekman = friction of imposed stress/(Nf )] lies above a very
sharp density discontinuity where turbulence is weak. Most energy from wind friction
becomes kinetic and little is used to further deepen Ekman layer.
For all these reasons, the locally forced Z m model of Sverdrup is insufficient to account
for observed algal blooms in the tropical ocean, although it is still often invoked for this
purpose (e.g., Wroblewski et al., 1988; Yentsch, 1990; Obata et al., 1996). The seasonal
changes in the depth of the mixed layer that occur in responses to distant geostrophic
forcing are relatively smaller than those caused by deep winter convection in the westerlies
biome. Nevertheless, some of these distantly forced changes do result directly in a bloom,
as in the case of the spin-up of geostrophic domes and where a basin-scale thermocline
tilt shoals the nutricline into the photic zone. Vertical Ekman velocities in the tropical
ocean are similar to rates at high latitudes (Isemer and Hasse, 1987) but may act over
larger areas and may be effective in sustaining relatively high chlorophyll at the surface
for extended periods, as occurs in the Arabian Sea (Brock et al., 1991).
An increase in the rate of primary production is normally accompanied by a simultaneous accumulation of chlorophyll because growth and loss terms are closely coupled,
so that standing stock or biomass values vary over only a relatively small dynamic range.
It here that the very small cell fraction of the phytoplankton is most important, and
even the eukaryotic cells themselves are of relatively small size, while the submicron
picoplankton fraction dominates: <90% of all plant biomass and <80% of its production
may be attributable to this fraction. The perennially relatively low nutrient concentrations
have induced the evolution of special strategies, such as the symbiotic relationships of
heliozoans and their autotrophic plastids, and some large cells have evolved special forms
Chapter 6: Biomes: The Primary Partition
Productivity is rarely light-limited and responds rapidly to nutrient entrainment into
the photic zone by coastal upwelling, offshore Ekman suction, and geostrophic adjustment of pycnocline associated with reversal of the monsoon winds. The pycnocline is
illuminated during the oligotrophic season, but the mixed layer shoals above the photic
zone during upwelling episodes. Changes in phytoplankton biomass match the seasonal
changes in production rate, consistent with consumption and production rates being
closely matched. Seasonal algal blooms are diatom-dominated, with picocyanobacteria
and other prochlorophytes contributing only 30–40% of phytoplankton carbon, compared to >90% in the oligotrophic regions/seasons.
The trade-winds regime covers about 45% of the total area of the ocean, and for that
reason alone it will be useful here to review very briefly its principal physical characteristics
that determine its singular ecology:
The radiation balance is such that there is a mean annual positive downward heat flux
across the surface. The area so defined has its widest latitudinal extent in the Atlantic.
The seasonal radiation flux is such that surface mixed layer is maintained continuously;
at no season is there sufficient loss of heat to produce convective deepening of the
permanent tropical thermocline.
The level of solar radiation is such that autotrophs are less commonly light-limited
than they are at higher latitudes.
The Rossby radius of internal deformation increases into the tropical zone as a consequence of diminishing Coriolis force; therefore, eddies are increasingly larger but
fewer toward the equator.
Baroclinic time scales are weeks in the tropics rather than years at higher latitudes.
The equatorward-diminishing Coriolis parameter (f ) means that the slope of the sea
surface required to force horizontal motion diminishes toward the equator.
The pycnocline is coincident with permanent tropical thermocline and has very high
stability (N ; the Brunt-Väisälä frequency takes high values) equatorward of latitude
at which winter convective mixing becomes trivial.
The tropical Ekman layer [Z ekman = friction of imposed stress/(Nf )] lies above a very
sharp density discontinuity where turbulence is weak. Most energy from wind friction
becomes kinetic and little is used to further deepen Ekman layer.
For all these reasons, the locally forced Z m model of Sverdrup is insufficient to account
for observed algal blooms in the tropical ocean, although it is still often invoked for this
purpose (e.g., Wroblewski et al., 1988; Yentsch, 1990; Obata et al., 1996). The seasonal
changes in the depth of the mixed layer that occur in responses to distant geostrophic
forcing are relatively smaller than those caused by deep winter convection in the westerlies
biome. Nevertheless, some of these distantly forced changes do result directly in a bloom,
as in the case of the spin-up of geostrophic domes and where a basin-scale thermocline
tilt shoals the nutricline into the photic zone. Vertical Ekman velocities in the tropical
ocean are similar to rates at high latitudes (Isemer and Hasse, 1987) but may act over
larger areas and may be effective in sustaining relatively high chlorophyll at the surface
for extended periods, as occurs in the Arabian Sea (Brock et al., 1991).
An increase in the rate of primary production is normally accompanied by a simultaneous accumulation of chlorophyll because growth and loss terms are closely coupled,
so that standing stock or biomass values vary over only a relatively small dynamic range.
It here that the very small cell fraction of the phytoplankton is most important, and
even the eukaryotic cells themselves are of relatively small size, while the submicron
picoplankton fraction dominates: <90% of all plant biomass and <80% of its production
may be attributable to this fraction. The perennially relatively low nutrient concentrations
have induced the evolution of special strategies, such as the symbiotic relationships of
heliozoans and their autotrophic plastids, and some large cells have evolved special forms