The Four Primary Biomes of the Upper Ocean
99
that enhance the rate of uptake of scarce nutrient molecules across their cell walls, and
minimize their sinking rate.
Although nitrogen fixation occurs in the pelagos of the Westerlies biome, especially by
epiphytic cells on Sargassum of the North Atlantic and as endosymbionts within diatoms,
the blooms of nitrogen-fixing cyanophytes of this biome have become the paradigm
for N 2 fixation among phytoplankton cells. The apparent coincidence of these blooms
with trajectories of aeolian dust may be related to the high demand for cellular iron
during N 2 fixation (de Baar et al., 1997). Four pelagic species of the cyanobacterium
Trichodesmium (= Oscillatoria) contribute macroscopic cellular mats to these blooms,
which occur seasonally during the most windless period of the year and may color the
sea surface red as far as the eye can see; the global distribution of actively growing Trichodesmium populations is limited by the seasonal 20
C sea-surface isotherm (Carpenter,
1989). It has a remarkable ability to fix gaseous nitrogen and release oxygen under aerobic
conditions as well as to offer a substrate for a wide range of consorting organisms within
its loose colonial mats (Capone et al., 1997).
Though diel vertical migration of planktonic organisms is a global phenomenon,
occurring in all kinds of waters from small freshwater ponds to the deep ocean, it is in
the Trades biome that it extends over greater depth intervals and is most ubiquitous. At
all seasons and in all areas, we may expect that a substantial fraction of the zooplankton
and nekton, especially copepods (Pleuromamma Metridia, and Euchaeta), euphausiids,
myctophid fish, and squids, will arrive at or close under the surface soon after dusk and
to descend again to 200–500 m at dawn. Watching the swarming of deep red bathypelagic
squid and black or silvery myctophid fish under the lights of a research ship on station
at night, apparently feeding on migrant euphausiids, is one of the pleasures of tropical
oceanography.
In this biome, the pelagic ecosystem is at its most taxonomically diverse and represents
the climax community of the pelagos. All other pelagic ecosystems can be viewed as
derivatives from it, constrained or stressed in different ways that suppress some of the
complexities of the tropical pelagos. As shown in Table 6.1, it is in the tropical oceans that
the overall dominance of copepods, both numerically and as relative plankton biomass,
is weakest; copepods are reduced to only 33% of total biomass. The difference between
this and the 68% of polar oceans is accounted for by relatively greater biomass of several
groups in tropical seas.
It is also in this biome that pelagic fish reach their greatest development, and another
pleasure of tropical oceanography is to stand on the foredeck under way and watch the
scatter of flying fish from under the bows. A great variety of shoaling clupeids, schooling
tuna and other scombroids, and solitary sharks also inhabit the tropical pelagos so that
there are multiple food chains, each more complex and longer than those in high latitudes.
This is reflected in the temporal stability of the tropical biome and the climax nature
of the ecosystem. Populations are more sustainable over the long term in tropical seas,
recruitment in fish species between years usually having a variance of less than 3 whereas,
for high-latitude fish, recruitment may vary by a factor of 30—or even more.
Coastal Biome
The physical forcing of water motion and stratification over continental shelves is very
similar at all latitudes equatorward of the Polar Fronts, beyond which the presence of sea
ice induces unique conditions. I suggest that this commonality of physical processes on
shelves is more significant than the consequences of latitude; for this reason, I suggest
that all continental shelf regions, save only within the Polar biome, should comprise a
single unit, the Coastal biome.
99
that enhance the rate of uptake of scarce nutrient molecules across their cell walls, and
minimize their sinking rate.
Although nitrogen fixation occurs in the pelagos of the Westerlies biome, especially by
epiphytic cells on Sargassum of the North Atlantic and as endosymbionts within diatoms,
the blooms of nitrogen-fixing cyanophytes of this biome have become the paradigm
for N 2 fixation among phytoplankton cells. The apparent coincidence of these blooms
with trajectories of aeolian dust may be related to the high demand for cellular iron
during N 2 fixation (de Baar et al., 1997). Four pelagic species of the cyanobacterium
Trichodesmium (= Oscillatoria) contribute macroscopic cellular mats to these blooms,
which occur seasonally during the most windless period of the year and may color the
sea surface red as far as the eye can see; the global distribution of actively growing Trichodesmium populations is limited by the seasonal 20
C sea-surface isotherm (Carpenter,
1989). It has a remarkable ability to fix gaseous nitrogen and release oxygen under aerobic
conditions as well as to offer a substrate for a wide range of consorting organisms within
its loose colonial mats (Capone et al., 1997).
Though diel vertical migration of planktonic organisms is a global phenomenon,
occurring in all kinds of waters from small freshwater ponds to the deep ocean, it is in
the Trades biome that it extends over greater depth intervals and is most ubiquitous. At
all seasons and in all areas, we may expect that a substantial fraction of the zooplankton
and nekton, especially copepods (Pleuromamma Metridia, and Euchaeta), euphausiids,
myctophid fish, and squids, will arrive at or close under the surface soon after dusk and
to descend again to 200–500 m at dawn. Watching the swarming of deep red bathypelagic
squid and black or silvery myctophid fish under the lights of a research ship on station
at night, apparently feeding on migrant euphausiids, is one of the pleasures of tropical
oceanography.
In this biome, the pelagic ecosystem is at its most taxonomically diverse and represents
the climax community of the pelagos. All other pelagic ecosystems can be viewed as
derivatives from it, constrained or stressed in different ways that suppress some of the
complexities of the tropical pelagos. As shown in Table 6.1, it is in the tropical oceans that
the overall dominance of copepods, both numerically and as relative plankton biomass,
is weakest; copepods are reduced to only 33% of total biomass. The difference between
this and the 68% of polar oceans is accounted for by relatively greater biomass of several
groups in tropical seas.
It is also in this biome that pelagic fish reach their greatest development, and another
pleasure of tropical oceanography is to stand on the foredeck under way and watch the
scatter of flying fish from under the bows. A great variety of shoaling clupeids, schooling
tuna and other scombroids, and solitary sharks also inhabit the tropical pelagos so that
there are multiple food chains, each more complex and longer than those in high latitudes.
This is reflected in the temporal stability of the tropical biome and the climax nature
of the ecosystem. Populations are more sustainable over the long term in tropical seas,
recruitment in fish species between years usually having a variance of less than 3 whereas,
for high-latitude fish, recruitment may vary by a factor of 30—or even more.
Coastal Biome
The physical forcing of water motion and stratification over continental shelves is very
similar at all latitudes equatorward of the Polar Fronts, beyond which the presence of sea
ice induces unique conditions. I suggest that this commonality of physical processes on
shelves is more significant than the consequences of latitude; for this reason, I suggest
that all continental shelf regions, save only within the Polar biome, should comprise a
single unit, the Coastal biome.