Chapter 6
Biomes: The Primary
Partition
W
herever the wind blows and the sun shines over the ocean, turbulent diffusion
and thermal stratification may be imposed locally and may interact locally,
as in all one-dimensional models of the spring bloom. Wherever distantly
forced adjustment of mixed-layer depth occurs, the elements of such algal growth models
will be shoaled or deepened.
That these two statements apply uniquely to fundamentally different and characteristic
latitudinal zones in the oceans, as was discussed in Chapter 4, has only recently become
clear. That both local and distant forcing must be accommodated in analyses of local
algal dynamics is perhaps still not generally understood, though the relative importance
of local and distant forcing is what principally distinguishes algal dynamics at low and
high latitudes. This is the principal distinction between the two most extensive biomes
of the pelagos: those regions generally lying below the high-latitude westerly winds and
those below the easterly trades characteristic of low latitudes.
Wherever the winter sun lies so low on the horizon that the ocean freezes, brine is
extracted from the freezing seawater. When sea ice melts with the climbing sun in spring,
fresh water is released to form a brackish surface layer above the deeper, more saline
layers so that the two water bodies are separated by a sharp density gradient. Because this
pycnocline constrains wind mixing, the polar brackish-water layer retains its integrity
while spreading in the surface circulation. The oceanic polar fronts of each hemisphere
define the extent of its dispersal and in this way serve to define a third primary biome of
the pelagos.
Circulation and stratification of surface water masses is modified where the continental
topography and coastal wind patterns are encountered. These changes, for Mittelstaedt
(1991), define a coastal habitat within a shelf-break front that lies above the upper part
of the continental slope. This coastal zone is here considered to be the fourth biome,
characteristically more diverse than the other three. As already noted, we exclude from
this biome the narrow alongshore zone that is very difficult to generalize except in a few
exceptional regions, such as those coasts dominated by the effluents of great rivers where
the turbid littoral zone is relatively wide in relation to the entire shelf.
Because it is a principal argument of this book that the processes that force stratification
of the surface layers thereby also determine characteristically different phytoplankton
regimes, recognized as the primary partitions of pelagic ecology, it will be useful to
consider in greater detail the mechanisms and processes that specify the four primary
biomes before proceeding to consider the secondary division of these biomes into the
provinces of a biogeographic system.
89
Biomes: The Primary
Partition
W
herever the wind blows and the sun shines over the ocean, turbulent diffusion
and thermal stratification may be imposed locally and may interact locally,
as in all one-dimensional models of the spring bloom. Wherever distantly
forced adjustment of mixed-layer depth occurs, the elements of such algal growth models
will be shoaled or deepened.
That these two statements apply uniquely to fundamentally different and characteristic
latitudinal zones in the oceans, as was discussed in Chapter 4, has only recently become
clear. That both local and distant forcing must be accommodated in analyses of local
algal dynamics is perhaps still not generally understood, though the relative importance
of local and distant forcing is what principally distinguishes algal dynamics at low and
high latitudes. This is the principal distinction between the two most extensive biomes
of the pelagos: those regions generally lying below the high-latitude westerly winds and
those below the easterly trades characteristic of low latitudes.
Wherever the winter sun lies so low on the horizon that the ocean freezes, brine is
extracted from the freezing seawater. When sea ice melts with the climbing sun in spring,
fresh water is released to form a brackish surface layer above the deeper, more saline
layers so that the two water bodies are separated by a sharp density gradient. Because this
pycnocline constrains wind mixing, the polar brackish-water layer retains its integrity
while spreading in the surface circulation. The oceanic polar fronts of each hemisphere
define the extent of its dispersal and in this way serve to define a third primary biome of
the pelagos.
Circulation and stratification of surface water masses is modified where the continental
topography and coastal wind patterns are encountered. These changes, for Mittelstaedt
(1991), define a coastal habitat within a shelf-break front that lies above the upper part
of the continental slope. This coastal zone is here considered to be the fourth biome,
characteristically more diverse than the other three. As already noted, we exclude from
this biome the narrow alongshore zone that is very difficult to generalize except in a few
exceptional regions, such as those coasts dominated by the effluents of great rivers where
the turbid littoral zone is relatively wide in relation to the entire shelf.
Because it is a principal argument of this book that the processes that force stratification
of the surface layers thereby also determine characteristically different phytoplankton
regimes, recognized as the primary partitions of pelagic ecology, it will be useful to
consider in greater detail the mechanisms and processes that specify the four primary
biomes before proceeding to consider the secondary division of these biomes into the
provinces of a biogeographic system.
89