Rule-Based Models of Ecological Response to External Forcing
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below a critical threshold and there is sufficient light. Once stratification is established,
evolution occurs toward a summer oligotrophic situation. The summer pycnocline and
nitracline lie within the photic zone for 4–6 months and during this period a deep
chlorophyll maximum is formed. Production there varies from about 5% to 25% of total
production.
Phytoplankton biomass tracks increasing production rate in early spring, but the
rate of biomass increase is suppressed when hibernating migrant herbivores (copepods,
euphausiids) return to the surface. When the main biomass of these organisms descends
again toward the end of summer, a secondary chlorophyll accumulation is initiated that
reaches a weak peak a few weeks later. The subsequent progressive recharging of the
mixed layer with nitrate at the end of summer may be accompanied by a brief surge of
primary production before being dissipated by autumnal mixing and declining light.
Case 3—Winter–Spring Production with Nutrient
Limitation
This model is applicable to the open ocean at midlatitudes where winter winds are
relatively light, primary production rates are always low, and seasonality in phytoplankton
biomass is weak. Such regions cover almost half the area of the ocean, so there are
significant regional differences that cannot be ignored. However, most of these regions
share the common attribute that 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.
Characteristically, the winter increase in production rate is accompanied by an increase
in chlorophyll values that exhibit a broad peak in midwinter, but fail to track the spring
surge in primary production rate. The nutricline is illuminated for a longer period each
summer than in Case 2 situations. A Case 3 seasonal cycle seems not to be associated
with accumulation of chlorophyll at any season when the rate of primary production is
declining, as occurs in Case 1 and 2 situations. This suggests a greater degree of biological
coupling between growth and loss rates than occurs in higher latitudes.
Because of latitudinal and between-ocean effects it will useful to consider the Case 3
regions in three groups: (i) the North Atlantic and Pacific Oceans and the Tasman Sea and,
diverging somewhat from this type, (ii) the southern gyres of the three ocean basins, and
(iii) the Southern Ocean. These groups differ in their characteristic depths of winter mixing,
in relative productivity, and in the timing of the winter increase in production rate.
Case 4—Small-Amplitude Response to Trade Wind
Seasonality
This model is appropriate to that part of the ocean at low latitudes (forming 22% of the
whole ocean) that lies under the influence of the trade winds, where seasonality is imposed
principally by geostrophic adjustment of the zonal equatorial current systems. There is weak
seasonality in mixed-layer depth, while primary production rate and phytoplankton biomass
take characteristically low values; a deep chlorophyll maximum is almost always present.
The nutricline is perennially shoaler than the photic depth, except during exceptional
events and, consequently, the vertically integrated production rate is not light limited,
although nutrients or trace elements may be limiting. Minor rate increases are forced by
geostrophic response of the pycnocline to seasonality in trade wind stress, and to openocean Ekman suction and divergence, particularly at the equator. Rates of phytoplankton
chlorophyll accumulation and consumption are closely balanced at all seasons.
65
below a critical threshold and there is sufficient light. Once stratification is established,
evolution occurs toward a summer oligotrophic situation. The summer pycnocline and
nitracline lie within the photic zone for 4–6 months and during this period a deep
chlorophyll maximum is formed. Production there varies from about 5% to 25% of total
production.
Phytoplankton biomass tracks increasing production rate in early spring, but the
rate of biomass increase is suppressed when hibernating migrant herbivores (copepods,
euphausiids) return to the surface. When the main biomass of these organisms descends
again toward the end of summer, a secondary chlorophyll accumulation is initiated that
reaches a weak peak a few weeks later. The subsequent progressive recharging of the
mixed layer with nitrate at the end of summer may be accompanied by a brief surge of
primary production before being dissipated by autumnal mixing and declining light.
Case 3—Winter–Spring Production with Nutrient
Limitation
This model is applicable to the open ocean at midlatitudes where winter winds are
relatively light, primary production rates are always low, and seasonality in phytoplankton
biomass is weak. Such regions cover almost half the area of the ocean, so there are
significant regional differences that cannot be ignored. However, most of these regions
share the common attribute that 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.
Characteristically, the winter increase in production rate is accompanied by an increase
in chlorophyll values that exhibit a broad peak in midwinter, but fail to track the spring
surge in primary production rate. The nutricline is illuminated for a longer period each
summer than in Case 2 situations. A Case 3 seasonal cycle seems not to be associated
with accumulation of chlorophyll at any season when the rate of primary production is
declining, as occurs in Case 1 and 2 situations. This suggests a greater degree of biological
coupling between growth and loss rates than occurs in higher latitudes.
Because of latitudinal and between-ocean effects it will useful to consider the Case 3
regions in three groups: (i) the North Atlantic and Pacific Oceans and the Tasman Sea and,
diverging somewhat from this type, (ii) the southern gyres of the three ocean basins, and
(iii) the Southern Ocean. These groups differ in their characteristic depths of winter mixing,
in relative productivity, and in the timing of the winter increase in production rate.
Case 4—Small-Amplitude Response to Trade Wind
Seasonality
This model is appropriate to that part of the ocean at low latitudes (forming 22% of the
whole ocean) that lies under the influence of the trade winds, where seasonality is imposed
principally by geostrophic adjustment of the zonal equatorial current systems. There is weak
seasonality in mixed-layer depth, while primary production rate and phytoplankton biomass
take characteristically low values; a deep chlorophyll maximum is almost always present.
The nutricline is perennially shoaler than the photic depth, except during exceptional
events and, consequently, the vertically integrated production rate is not light limited,
although nutrients or trace elements may be limiting. Minor rate increases are forced by
geostrophic response of the pycnocline to seasonality in trade wind stress, and to openocean Ekman suction and divergence, particularly at the equator. Rates of phytoplankton
chlorophyll accumulation and consumption are closely balanced at all seasons.