The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline
45
averaging of shoal and deep mixed layer values during spring, thus indicating a false
mean value somewhere between the two.
At all latitudes, vertical velocity (Ekman’s WE) is imparted to the water column by
the curl of the local wind stress at the sea surface, and this motion shoals or deepens the
pycnocline. Cyclonic stress imparts positive values; anticyclonic stress imparts negative
values. In this book I shall follow the notation of Isemer and Hasse (1987). Therefore,
positive values of WE (upward motion) shall be labeled “Ekman suction” and negative
values (downward motion) shall be labeled “Ekman pumping.” You should be aware
that in many papers the latter term is loosely used without definition and often in the
opposite sense to that used by Isemer and Hasse, or by Tomczak and Godfrey (1994).
The pycnocline, whether seasonal or permanent, lies at a depth generally predictable
for each region and has a gradient that is predictable from its depth: generally, the
shoaler the pycnocline, the sharper the gradient. From its depth (and some knowledge of
regional oceanography) the vertical gradients in other properties of ecological significance
that are associated with it may also be predicted, especially nutrients, light, and the
biomass of both phytoplankton and zooplankton. The interactions between nutrients,
light, turbulence, and the biota in the mixed layer and pycnocline have been a central
theme of biological oceanography in recent decades: for a good discussion of all this, see
Mann and Lazier (2006) or Banse and English (1994).
The simplest expression of the feature is the typical tropical profile (TTP) of Herbland
and Voituriez (1977); this is a description of an oligotrophic profile that is either the end
member of plankton succession in midlatitudes from spring to summer, or the permanent
condition in tropical seas. Under these conditions, the depths of nutricline, pycnocline,
deep chlorophyll maximum (DCM), and productivity maximum do not differ by more
than a few meters, and the vertical distribution of zooplankton and micronekton also
conforms in a predictable manner (Longhurst and Harrison, 1989). Below the pycnocline,
in regions where the TTP is a permanent regional feature of the water column, bacterial
oxidation of sinking organic material commonly leads to the development of an oxygen
minimum zone and a characteristic anomaly in the usual vertical distribution of plankton
profiles (Longhurst, 1967; Saltzman and Wishner, 1997a,b).
It is now generally accepted that in many situations the appropriate model for an
oligotrophic profile has a two-layered euphotic zone. Where the mixed layer water is
sufficiently clear (few algal cells and little suspended particulates) that light attenuation
is dominated by seawater absorption, the 1% isolume often lies within the pycnocline.
In this case, the upper (mixed layer) zone is well lit and nitrate poor, while the deeper
(pycnocline) zone is poorly lit and nutrient rich. Here we have a sufficient model for
the steady-state DCM in which algae are larger, shade-adapted, and receive a constant
vertical flux of fresh nitrate so that new production is high relative to production based on
regenerated ammonium. On the contrary, the well-lit and nitrate-poor upper mixed layer
has low rates of new production relative to ammonium-based, regenerated production.
Production in the DCM relative to the production in the mixed layer is complex to
compute but usually lies within the range 5–50%. Though the shade-adapted cells of the
DCM may have access to adequate nitrate, their P max (photosynthetic rate per unit of
chlorophyll at light saturation) may be 10 times lower than for near-surface cells. The
plots of seasonal cycles for individual provinces (see Chapters 9–12) show that whereas an
illuminated pycnocline is the normal condition in low latitudes, it is ephemeral in polar
seas. These graphs thus illustrate the potential for increase in the absolute rate of primary
production in the deep chlorophyll maximum during seasons when the pycnocline lies
shoaler than the photic depth.
In short, all this describes an ecosystem vertically ordered about a discontinuity in
the density, nutrient, and biotic gradients, as shall be discussed later. Nitrate diminishes
to very low values upward across the pycnocline as both chlorophyll biomass and the
45
averaging of shoal and deep mixed layer values during spring, thus indicating a false
mean value somewhere between the two.
At all latitudes, vertical velocity (Ekman’s WE) is imparted to the water column by
the curl of the local wind stress at the sea surface, and this motion shoals or deepens the
pycnocline. Cyclonic stress imparts positive values; anticyclonic stress imparts negative
values. In this book I shall follow the notation of Isemer and Hasse (1987). Therefore,
positive values of WE (upward motion) shall be labeled “Ekman suction” and negative
values (downward motion) shall be labeled “Ekman pumping.” You should be aware
that in many papers the latter term is loosely used without definition and often in the
opposite sense to that used by Isemer and Hasse, or by Tomczak and Godfrey (1994).
The pycnocline, whether seasonal or permanent, lies at a depth generally predictable
for each region and has a gradient that is predictable from its depth: generally, the
shoaler the pycnocline, the sharper the gradient. From its depth (and some knowledge of
regional oceanography) the vertical gradients in other properties of ecological significance
that are associated with it may also be predicted, especially nutrients, light, and the
biomass of both phytoplankton and zooplankton. The interactions between nutrients,
light, turbulence, and the biota in the mixed layer and pycnocline have been a central
theme of biological oceanography in recent decades: for a good discussion of all this, see
Mann and Lazier (2006) or Banse and English (1994).
The simplest expression of the feature is the typical tropical profile (TTP) of Herbland
and Voituriez (1977); this is a description of an oligotrophic profile that is either the end
member of plankton succession in midlatitudes from spring to summer, or the permanent
condition in tropical seas. Under these conditions, the depths of nutricline, pycnocline,
deep chlorophyll maximum (DCM), and productivity maximum do not differ by more
than a few meters, and the vertical distribution of zooplankton and micronekton also
conforms in a predictable manner (Longhurst and Harrison, 1989). Below the pycnocline,
in regions where the TTP is a permanent regional feature of the water column, bacterial
oxidation of sinking organic material commonly leads to the development of an oxygen
minimum zone and a characteristic anomaly in the usual vertical distribution of plankton
profiles (Longhurst, 1967; Saltzman and Wishner, 1997a,b).
It is now generally accepted that in many situations the appropriate model for an
oligotrophic profile has a two-layered euphotic zone. Where the mixed layer water is
sufficiently clear (few algal cells and little suspended particulates) that light attenuation
is dominated by seawater absorption, the 1% isolume often lies within the pycnocline.
In this case, the upper (mixed layer) zone is well lit and nitrate poor, while the deeper
(pycnocline) zone is poorly lit and nutrient rich. Here we have a sufficient model for
the steady-state DCM in which algae are larger, shade-adapted, and receive a constant
vertical flux of fresh nitrate so that new production is high relative to production based on
regenerated ammonium. On the contrary, the well-lit and nitrate-poor upper mixed layer
has low rates of new production relative to ammonium-based, regenerated production.
Production in the DCM relative to the production in the mixed layer is complex to
compute but usually lies within the range 5–50%. Though the shade-adapted cells of the
DCM may have access to adequate nitrate, their P max (photosynthetic rate per unit of
chlorophyll at light saturation) may be 10 times lower than for near-surface cells. The
plots of seasonal cycles for individual provinces (see Chapters 9–12) show that whereas an
illuminated pycnocline is the normal condition in low latitudes, it is ephemeral in polar
seas. These graphs thus illustrate the potential for increase in the absolute rate of primary
production in the deep chlorophyll maximum during seasons when the pycnocline lies
shoaler than the photic depth.
In short, all this describes an ecosystem vertically ordered about a discontinuity in
the density, nutrient, and biotic gradients, as shall be discussed later. Nitrate diminishes
to very low values upward across the pycnocline as both chlorophyll biomass and the