52
Chapter 4: Physical Control of Ecological Processes
loss exactly balances growth. He showed that if mixing penetrates deeper than this, then
cells will be light limited because they will experience a time-integrated mean irradiance
lower than I c and their time-integrated growth will take a negative sign. If, on the other
hand, mixed-layer depth is shallower than Z cr , then cells will be light-sufficient, their
time-integrated growth will have a positive sign, and a bloom may ensue. Unstated by
Sverdrup, though well recognized by Riley, is the necessary auxiliary assumption that
deepening of the wind-mixed layer in winter recharges surface layers with inorganic
nutrients.
Sverdrup, of course, was also well aware that his model requires that the loss term
should include not only algal respiration but also excretion, grazing, and sedimentation:
the balance is, as Sathyendranath and Platt (2000) reminded us, between total growth
and total loss of phytoplankton in the superjacent water column. This balance may
be discussed in general terms, but the exact expression offered by Platt, Caverhill, and
Sathyendranath (1991a) leads to a characteristic time scale for the development of a
bloom. Sverdrup’s original formulation invoked very simple assumptions to calculate the
irradiance at the critical depth, and it now understood to be necessary to recognize how
this responds to changes in the optical parameter K. This parameter, of course, itself
responds to the biomass of phytoplankton in the surface mixed layer, so that critical
depth shoals during the growth of a bloom; perhaps the most significant modification of
the original concept is that self-shading by autotrophic cells is now known to account for
a smaller fraction of total shading than had been previously expected. Because individual
taxa of autotrophs have individual characteristic values of I c , this is usually stated as a
community mean.
The validity of the Sverdrup model is not vitiated by observations that blooms can
occur in the absence of significant stratification under special circumstances. This occasionally occurs in the Gulf of Maine (and surely, then, also elsewhere?) when deep
penetration of light in early spring, accompanied by very little wind mixing, permits
exceptionally buoyant phytoplankters to accumulate biomass (Townsend et al., 1992).
This situation has been revisited recently by Ebert et al. (2001) in the context of a “critical
turbulence” rate for buoyant, for slow-sinking, and for fast-sinking phytoplankton cells:
the conclusions reached by these models for the initiation of blooms are intuitive. Further,
since a chlorophyll-rich layer in the water column must induce local warming by absorption of incident radiation at that depth, this may directly induce local stratification (e.g.,
Sathyendranath and Platt, 1994). The consequent changes in the photosynthetic parameters for algal growth will then add new complexity to the overall biological response to
irradiance. The Sverdrup model, therefore, is seen to be but one component of a complex
and coupled biophysical system.
For this reason, more general effects must also be accommodated if the small set of
critical factors for algal ecology required by Sverdrup are to be useful in global analysis, so
as to be relevant to regions where other processes may override the effects of local winds
and sunshine in determining the depth of the surface mixed layer. Two principal effects
must be accommodated as anomalies to Sverdrup, of which we have already discussed
the first: in low latitudes the seasonal changes in mixed-layer depth may be a response to
distant wind forcing rather than to changes in locally induced wind mixing. The second
effect is that of a surface layer of low-salinity water, not always induced by local rainfall,
but which may come to dominate near-surface stratification.
A simple relationship between seasonal values for I c and Z cr results in characteristic
changes in the strength of seasonal blooms at different latitudes (Follows and Dutkiewicz,
2003). The relationship between these two depths may be expressed as the dimensionless
parameter h c /h m that takes values near unity in the subtropics and as low as 0.05 in
polar seas where winter mixing penetrates very deep under dark skies. Increased winter
mixing beyond a threshold depth in high latitudes may result in reduced strength of
Chapter 4: Physical Control of Ecological Processes
loss exactly balances growth. He showed that if mixing penetrates deeper than this, then
cells will be light limited because they will experience a time-integrated mean irradiance
lower than I c and their time-integrated growth will take a negative sign. If, on the other
hand, mixed-layer depth is shallower than Z cr , then cells will be light-sufficient, their
time-integrated growth will have a positive sign, and a bloom may ensue. Unstated by
Sverdrup, though well recognized by Riley, is the necessary auxiliary assumption that
deepening of the wind-mixed layer in winter recharges surface layers with inorganic
nutrients.
Sverdrup, of course, was also well aware that his model requires that the loss term
should include not only algal respiration but also excretion, grazing, and sedimentation:
the balance is, as Sathyendranath and Platt (2000) reminded us, between total growth
and total loss of phytoplankton in the superjacent water column. This balance may
be discussed in general terms, but the exact expression offered by Platt, Caverhill, and
Sathyendranath (1991a) leads to a characteristic time scale for the development of a
bloom. Sverdrup’s original formulation invoked very simple assumptions to calculate the
irradiance at the critical depth, and it now understood to be necessary to recognize how
this responds to changes in the optical parameter K. This parameter, of course, itself
responds to the biomass of phytoplankton in the surface mixed layer, so that critical
depth shoals during the growth of a bloom; perhaps the most significant modification of
the original concept is that self-shading by autotrophic cells is now known to account for
a smaller fraction of total shading than had been previously expected. Because individual
taxa of autotrophs have individual characteristic values of I c , this is usually stated as a
community mean.
The validity of the Sverdrup model is not vitiated by observations that blooms can
occur in the absence of significant stratification under special circumstances. This occasionally occurs in the Gulf of Maine (and surely, then, also elsewhere?) when deep
penetration of light in early spring, accompanied by very little wind mixing, permits
exceptionally buoyant phytoplankters to accumulate biomass (Townsend et al., 1992).
This situation has been revisited recently by Ebert et al. (2001) in the context of a “critical
turbulence” rate for buoyant, for slow-sinking, and for fast-sinking phytoplankton cells:
the conclusions reached by these models for the initiation of blooms are intuitive. Further,
since a chlorophyll-rich layer in the water column must induce local warming by absorption of incident radiation at that depth, this may directly induce local stratification (e.g.,
Sathyendranath and Platt, 1994). The consequent changes in the photosynthetic parameters for algal growth will then add new complexity to the overall biological response to
irradiance. The Sverdrup model, therefore, is seen to be but one component of a complex
and coupled biophysical system.
For this reason, more general effects must also be accommodated if the small set of
critical factors for algal ecology required by Sverdrup are to be useful in global analysis, so
as to be relevant to regions where other processes may override the effects of local winds
and sunshine in determining the depth of the surface mixed layer. Two principal effects
must be accommodated as anomalies to Sverdrup, of which we have already discussed
the first: in low latitudes the seasonal changes in mixed-layer depth may be a response to
distant wind forcing rather than to changes in locally induced wind mixing. The second
effect is that of a surface layer of low-salinity water, not always induced by local rainfall,
but which may come to dominate near-surface stratification.
A simple relationship between seasonal values for I c and Z cr results in characteristic
changes in the strength of seasonal blooms at different latitudes (Follows and Dutkiewicz,
2003). The relationship between these two depths may be expressed as the dimensionless
parameter h c /h m that takes values near unity in the subtropics and as low as 0.05 in
polar seas where winter mixing penetrates very deep under dark skies. Increased winter
mixing beyond a threshold depth in high latitudes may result in reduced strength of