96
Chapter 6: Biomes: The Primary Partition
winter mixing is to some extent constrained by a shallow halocline, though convective
cooling extends much deeper.
Increasing irradiance in spring and the onset of stratification may induce an algal
spring bloom. One of the first revelations of the global CZCS images was the degree to
which the North Atlantic spring bloom is a singular feature of the oceans. Nevertheless,
the discipline of biological oceanography is based historically on studies of this bloom,
and what we know about its induction and evolution serves us well as a starting point for
discussing the other, mostly less well researched, regions under the midlatitude westerlies.
The classical vernal sequence, described by the Case 2 model, is too well known to
require more than very brief mention here. Initial pre-bloom conditions for nutrients at
the end of winter are related to the depth of winter mixing and the baroclinicity of the
subsurface nitrate field. There is increasing evidence that near-surface spring blooms may
be initiated in unstratified (winter-mixed) water columns if the mixing rate is below a
critical threshold and if there is sufficient light (e.g., Dandonneau and Gohin, 1984). Once
stratification is established, evolution occurs toward a summer oligotrophic situation
when the summer pycnocline and nitracline lie within the photic zone for 4–6 months
and during this period a deep chlorophyll maximum is formed. The bloom becomes
nutrient-limited when the initial charge of inorganic nitrate (or, in some cases, silicate) in
the mixed layer is exhausted; the summer growth that follows is nutrient-limited, fueled
largely by biological regeneration of nitrogen as NH 3 , with some minor contribution of
NO 3 from entrainment (wind and eddy events) and weak Ekman suction (curl of windstress field). An autumn bloom (in the sense of biomass accumulation), usually weaker
than the spring bloom, may follow when the mixed layer deepens so that vertical nutrient
flux increases, driven by increasing wind stress and reduced surface warming; this may
also result from the reduction of grazing pressure when deep-hibernating herbivorous
copepods descend to overwintering depths.
At lower latitudes where westerly winter winds are relatively light, a Case 3 model
is appropriate. Here, primary production rate is always low, and seasonality in phytoplankton biomass relatively weak, depending on the depth of winter mixing; this is here
rather variable, so that in some years it is sufficiently weak that the nutricline may not
be significantly eroded. From low values in midsummer, the rate of primary production
begins an increase that is sustained through the autumn and culminates in a spring
maximum after a second rate increase in late winter. At these latitudes, the smallest
photosynthetic cells dominate phytoplankton biomass during those long periods when
stratification is strong, nutrient flux is suppressed, and blooms of larger algal cells do
not occur. The picoplankton fraction, dominated by cyanobacteria and prochlorophytes,
may comprise 70–90% of chlorophyll and contribute 80–90% of primary production
at 20–30
latitude. Likewise, the herbivore fraction of the zooplankton is more diverse
than in Case 2 situations and comprises many genera of small copepods, together with
tunicates.
Herbivore ecology is complex in the Westerlies biome, because both the pattern of
seasonal ontogenetic migration typical of the polar seas and the diel vertical migration
pattern typical of trade-wind seas occur here. As at higher latitudes, the timing of the fall
migration to depth appears to be ontogenetic, when a certain instar has been achieved,
rather than a response to lack of food. At all latitudes within this biome, during at least
some part of the oligotrophic summer phase, some large calanoids perform diel vertical
migrations, rising to feed at night from daytime residence depths of around 200–500 m,
in this way presumably escaping predation in the clear water during daylight hours.
Pelagic fish here are not as diverse as they are in the Trades biome. Some migratory
stocks of herring and other shoaling clupeids spend much of the year in the open ocean
and return to the coastal zone to spawn. Others, such as the saury of the North Pacific,
pass the entire year over deep water. Both North Atlantic and North Pacific salmon,
Chapter 6: Biomes: The Primary Partition
winter mixing is to some extent constrained by a shallow halocline, though convective
cooling extends much deeper.
Increasing irradiance in spring and the onset of stratification may induce an algal
spring bloom. One of the first revelations of the global CZCS images was the degree to
which the North Atlantic spring bloom is a singular feature of the oceans. Nevertheless,
the discipline of biological oceanography is based historically on studies of this bloom,
and what we know about its induction and evolution serves us well as a starting point for
discussing the other, mostly less well researched, regions under the midlatitude westerlies.
The classical vernal sequence, described by the Case 2 model, is too well known to
require more than very brief mention here. Initial pre-bloom conditions for nutrients at
the end of winter are related to the depth of winter mixing and the baroclinicity of the
subsurface nitrate field. There is increasing evidence that near-surface spring blooms may
be initiated in unstratified (winter-mixed) water columns if the mixing rate is below a
critical threshold and if there is sufficient light (e.g., Dandonneau and Gohin, 1984). Once
stratification is established, evolution occurs toward a summer oligotrophic situation
when the summer pycnocline and nitracline lie within the photic zone for 4–6 months
and during this period a deep chlorophyll maximum is formed. The bloom becomes
nutrient-limited when the initial charge of inorganic nitrate (or, in some cases, silicate) in
the mixed layer is exhausted; the summer growth that follows is nutrient-limited, fueled
largely by biological regeneration of nitrogen as NH 3 , with some minor contribution of
NO 3 from entrainment (wind and eddy events) and weak Ekman suction (curl of windstress field). An autumn bloom (in the sense of biomass accumulation), usually weaker
than the spring bloom, may follow when the mixed layer deepens so that vertical nutrient
flux increases, driven by increasing wind stress and reduced surface warming; this may
also result from the reduction of grazing pressure when deep-hibernating herbivorous
copepods descend to overwintering depths.
At lower latitudes where westerly winter winds are relatively light, a Case 3 model
is appropriate. Here, primary production rate is always low, and seasonality in phytoplankton biomass relatively weak, depending on the depth of winter mixing; this is here
rather variable, so that in some years it is sufficiently weak that the nutricline may not
be significantly eroded. From low values in midsummer, the rate of primary production
begins an increase that is sustained through the autumn and culminates in a spring
maximum after a second rate increase in late winter. At these latitudes, the smallest
photosynthetic cells dominate phytoplankton biomass during those long periods when
stratification is strong, nutrient flux is suppressed, and blooms of larger algal cells do
not occur. The picoplankton fraction, dominated by cyanobacteria and prochlorophytes,
may comprise 70–90% of chlorophyll and contribute 80–90% of primary production
at 20–30
latitude. Likewise, the herbivore fraction of the zooplankton is more diverse
than in Case 2 situations and comprises many genera of small copepods, together with
tunicates.
Herbivore ecology is complex in the Westerlies biome, because both the pattern of
seasonal ontogenetic migration typical of the polar seas and the diel vertical migration
pattern typical of trade-wind seas occur here. As at higher latitudes, the timing of the fall
migration to depth appears to be ontogenetic, when a certain instar has been achieved,
rather than a response to lack of food. At all latitudes within this biome, during at least
some part of the oligotrophic summer phase, some large calanoids perform diel vertical
migrations, rising to feed at night from daytime residence depths of around 200–500 m,
in this way presumably escaping predation in the clear water during daylight hours.
Pelagic fish here are not as diverse as they are in the Trades biome. Some migratory
stocks of herring and other shoaling clupeids spend much of the year in the open ocean
and return to the coastal zone to spawn. Others, such as the saury of the North Pacific,
pass the entire year over deep water. Both North Atlantic and North Pacific salmon,