64
Chapter 4: Physical Control of Ecological Processes
real ocean. From it, one may infer only four states of phytoplankton ecology: red tides,
spring blooms, oligotrophic central gyres, and (just possibly) austral high latitudes. This,
it seems to me, is an inadequate characterization of the ecological states that we can
observe at sea.
Furthermore, I believe that a partition based solely on interaction between varying
nutrients and turbulence conceals more than it reveals. Perhaps one could construct a
“Margalef diagram” that interpreted the interaction between changing seasonal illumination at the sea surface and seasonal changes of pycnocline depth. Another informative
diagram might be constructed to represent the consequences of the fact that nutrients
known to be limiting to phytoplankton have three sources: atmospheric, riverine, and
from subpycnocline water. I am not sure that such suggestions are useful, but one simple
Margalef-type relationship cannot perform predictively for the variety of ecological conditions that we now recognize in the oceans, in the same way that such diagrams can
apparently perform in the terrestrial realm.
So, if this what we want (and I think it is), then we shall have to proceed in a slightly
less reductionist manner. To this end, I have proposed (Longhurst, 1985b) that we might
classify the ecology of oceanic phytoplankton into six primary cases, or models, defined
below, to be partitioned among the biogeochemical provinces that will be discussed later:
Case 1—Polar Irradiance-Mediated Production Peak
This represents the ideal production cycle beyond the Polar Front in all oceans. A halocline
defines the surface mixed layer within a relatively isothermal water mass, especially at
very high boreal latitudes. The pycnocline mixes deepest in winter in the open ocean
regions and in the absence of ice cover. The shallow polar halocline may induce stability
earlier in the spring than at lower latitude.
The extreme range of irradiance in polar regions forces a unique seasonal cycle of
primary production rate, having a single, light-limited maximum at the summer solstice
that may be independent of the concentration of nitrate within the photic zone. Once
sustained algal growth is initiated, a shallow subsurface chlorophyll maximum develops at
the halocline and summer oligotrophic conditions with irradiance of the pycnocline are
very briefly established. Chlorophyll accumulates during the period when productivity is
increasing and tracks its initial decline. A secondary accumulation of chlorophyll during
the late summer period of declining primary production rate is consistent with reduced
consumption as herbivores descend out of the photic zone to their overwintering depths.
This sequence is not necessarily observed at all individual locations; blooms occur
locally when winter ice cover breaks up, when irradiance within the photic zone is at a
local annual maximum, and the date of breakup is determined not only by latitude but
also by topography and circulation.
Case 2—Nutrient-Limited Spring Production Peak
This model represents the cycle of productivity within the regions of maximum westerly
wind stress, where winter deepening of the mixed layer may reach depths >300 m. It may
be convenient to specify the mixed layer either by a temperature discontinuity (as in the
Atlantic) or by a salinity discontinuity (as in the Pacific Subarctic gyre).
The rate of primary production is minimal in winter, and about 1–2 months after
the winter solstice the rate begins to increase toward a unimodal maximum about 2–3
months later. This spring bloom is usually thought to conform closely to the Sverdrup
critical depth model, but there is increasing evidence that near-surface spring blooms
may be initiated in unstratified (winter-mixed) water columns if the mixing rate is
Chapter 4: Physical Control of Ecological Processes
real ocean. From it, one may infer only four states of phytoplankton ecology: red tides,
spring blooms, oligotrophic central gyres, and (just possibly) austral high latitudes. This,
it seems to me, is an inadequate characterization of the ecological states that we can
observe at sea.
Furthermore, I believe that a partition based solely on interaction between varying
nutrients and turbulence conceals more than it reveals. Perhaps one could construct a
“Margalef diagram” that interpreted the interaction between changing seasonal illumination at the sea surface and seasonal changes of pycnocline depth. Another informative
diagram might be constructed to represent the consequences of the fact that nutrients
known to be limiting to phytoplankton have three sources: atmospheric, riverine, and
from subpycnocline water. I am not sure that such suggestions are useful, but one simple
Margalef-type relationship cannot perform predictively for the variety of ecological conditions that we now recognize in the oceans, in the same way that such diagrams can
apparently perform in the terrestrial realm.
So, if this what we want (and I think it is), then we shall have to proceed in a slightly
less reductionist manner. To this end, I have proposed (Longhurst, 1985b) that we might
classify the ecology of oceanic phytoplankton into six primary cases, or models, defined
below, to be partitioned among the biogeochemical provinces that will be discussed later:
Case 1—Polar Irradiance-Mediated Production Peak
This represents the ideal production cycle beyond the Polar Front in all oceans. A halocline
defines the surface mixed layer within a relatively isothermal water mass, especially at
very high boreal latitudes. The pycnocline mixes deepest in winter in the open ocean
regions and in the absence of ice cover. The shallow polar halocline may induce stability
earlier in the spring than at lower latitude.
The extreme range of irradiance in polar regions forces a unique seasonal cycle of
primary production rate, having a single, light-limited maximum at the summer solstice
that may be independent of the concentration of nitrate within the photic zone. Once
sustained algal growth is initiated, a shallow subsurface chlorophyll maximum develops at
the halocline and summer oligotrophic conditions with irradiance of the pycnocline are
very briefly established. Chlorophyll accumulates during the period when productivity is
increasing and tracks its initial decline. A secondary accumulation of chlorophyll during
the late summer period of declining primary production rate is consistent with reduced
consumption as herbivores descend out of the photic zone to their overwintering depths.
This sequence is not necessarily observed at all individual locations; blooms occur
locally when winter ice cover breaks up, when irradiance within the photic zone is at a
local annual maximum, and the date of breakup is determined not only by latitude but
also by topography and circulation.
Case 2—Nutrient-Limited Spring Production Peak
This model represents the cycle of productivity within the regions of maximum westerly
wind stress, where winter deepening of the mixed layer may reach depths >300 m. It may
be convenient to specify the mixed layer either by a temperature discontinuity (as in the
Atlantic) or by a salinity discontinuity (as in the Pacific Subarctic gyre).
The rate of primary production is minimal in winter, and about 1–2 months after
the winter solstice the rate begins to increase toward a unimodal maximum about 2–3
months later. This spring bloom is usually thought to conform closely to the Sverdrup
critical depth model, but there is increasing evidence that near-surface spring blooms
may be initiated in unstratified (winter-mixed) water columns if the mixing rate is