90
Chapter 6: Biomes: The Primary Partition
The Four Primary Biomes of the Upper Ocean
The partition discussed here is of the upper ocean and rests principally on observed or
inferred regional discontinuities in physical processes, particularly those that affect the
stability of the upper kilometer of the ocean; regional differences in other ecologically
significant variables, such as irradiance, are also considered. In short, the partition is based
largely on the factors invoked by the Sverdrup model and proceeds from the suggestion
made in Chapter 4 that six simple models, or cases, are sufficient to accommodate the
observed range of pelagic production mechanisms. You will remember that I suggested
that this might be more useful in partitioning oceanic phytoplankton ecology than the
simpler analysis of Margalef, who defined only four quadrants in the nutrient/turbulence
relationship.
Little rearrangement and only modest consolidation is required to apply these six cases
to my earlier suggestions to recognize four biomes, or basic vegetation types, within the
pelagial realm of the oceans; these are very similar in concept to those of Beklemishev
(1969) and to the eco-regions of Bailey (1983). With each biome is associated one or
more of the six models already discussed, as in the following arrangement:
Polar biome: where the mixed-layer depth is constrained by a surface brackish layer
that forms each spring in the marginal ice zone:
Case 1—Polar irradiance-mediated production peak.
Westerlies biome: where the mixed-layer depth is forced largely by local winds and
by local irradiance:
Case 2—Nutrient-limited spring production peak.
Case 3—Winter-spring production with nutrient limitation.
Trades biome: where the mixed-layer depth is forced by geostrophic adjustment on
an ocean-basin scale to local or distant wind forcing:
Case 4—Small-amplitude response to trade wind seasonality.
Case 5—Large-amplitude response to monsoon reversal of trade winds.
Coastal biome: where many diverse coastal processes modify the mixed-layer depth
and nutrient inputs:
Case 6—Intermittent production at coastal divergences and upwellings.
While reviewing the general properties and boundaries of these biomes it will be
useful to bear in mind that only the trades biome represents a continuous body of water
in each ocean. The polar and westerlies biomes each exist as two separated boreal and
austral units and, furthermore, because land masses are not uniformly distributed in each
hemisphere, their boreal and austral expressions have individual characteristics: thus,
the boreal polar biome comprises a mediterranean sea containing a major archipelago,
whereas the austral polar biome comprises an annular open ocean surrounding a central
continent. Despite such asymmetries, the degree of ecological commonality is sufficient
to support the biome concept presented again here.
In putting the case for this partition, I want to emphasize that the definition offered
refers to an ideal ocean on a landless globe. In applying it to the real ocean, pragmatism
must be used in setting limits to the individual partitions, this being especially true for
the Coastal Biome, where the distribution of shallow continental shelves and deep basins
may be too fractal for the dogmatic application of the definitions. Similarly, partially
isolated basins such as the Mediterranean and Caribbean, or archipelagic regions such as
the SW Pacific, also require to be treated pragmatically.
There is, obviously, a wider set of ecological factors that, if they were available and
if we could identify discontinuities in their global fields, could be used to define more
precisely the ecological characteristics and boundaries of each biome. For instance, it
would be useful to quantify how plankton diversity (and hence the complexity of trophic
Chapter 6: Biomes: The Primary Partition
The Four Primary Biomes of the Upper Ocean
The partition discussed here is of the upper ocean and rests principally on observed or
inferred regional discontinuities in physical processes, particularly those that affect the
stability of the upper kilometer of the ocean; regional differences in other ecologically
significant variables, such as irradiance, are also considered. In short, the partition is based
largely on the factors invoked by the Sverdrup model and proceeds from the suggestion
made in Chapter 4 that six simple models, or cases, are sufficient to accommodate the
observed range of pelagic production mechanisms. You will remember that I suggested
that this might be more useful in partitioning oceanic phytoplankton ecology than the
simpler analysis of Margalef, who defined only four quadrants in the nutrient/turbulence
relationship.
Little rearrangement and only modest consolidation is required to apply these six cases
to my earlier suggestions to recognize four biomes, or basic vegetation types, within the
pelagial realm of the oceans; these are very similar in concept to those of Beklemishev
(1969) and to the eco-regions of Bailey (1983). With each biome is associated one or
more of the six models already discussed, as in the following arrangement:
Polar biome: where the mixed-layer depth is constrained by a surface brackish layer
that forms each spring in the marginal ice zone:
Case 1—Polar irradiance-mediated production peak.
Westerlies biome: where the mixed-layer depth is forced largely by local winds and
by local irradiance:
Case 2—Nutrient-limited spring production peak.
Case 3—Winter-spring production with nutrient limitation.
Trades biome: where the mixed-layer depth is forced by geostrophic adjustment on
an ocean-basin scale to local or distant wind forcing:
Case 4—Small-amplitude response to trade wind seasonality.
Case 5—Large-amplitude response to monsoon reversal of trade winds.
Coastal biome: where many diverse coastal processes modify the mixed-layer depth
and nutrient inputs:
Case 6—Intermittent production at coastal divergences and upwellings.
While reviewing the general properties and boundaries of these biomes it will be
useful to bear in mind that only the trades biome represents a continuous body of water
in each ocean. The polar and westerlies biomes each exist as two separated boreal and
austral units and, furthermore, because land masses are not uniformly distributed in each
hemisphere, their boreal and austral expressions have individual characteristics: thus,
the boreal polar biome comprises a mediterranean sea containing a major archipelago,
whereas the austral polar biome comprises an annular open ocean surrounding a central
continent. Despite such asymmetries, the degree of ecological commonality is sufficient
to support the biome concept presented again here.
In putting the case for this partition, I want to emphasize that the definition offered
refers to an ideal ocean on a landless globe. In applying it to the real ocean, pragmatism
must be used in setting limits to the individual partitions, this being especially true for
the Coastal Biome, where the distribution of shallow continental shelves and deep basins
may be too fractal for the dogmatic application of the definitions. Similarly, partially
isolated basins such as the Mediterranean and Caribbean, or archipelagic regions such as
the SW Pacific, also require to be treated pragmatically.
There is, obviously, a wider set of ecological factors that, if they were available and
if we could identify discontinuities in their global fields, could be used to define more
precisely the ecological characteristics and boundaries of each biome. For instance, it
would be useful to quantify how plankton diversity (and hence the complexity of trophic