15.5 Phytoplankton Concentration in Stratified Ocean
461
periodic square with side of 256 km long. Although the model dynamics are
relatively simple, the simulation showed very complex resulting phytoplankton and zooplankton patchy-like patterns. Phytoplankton have distributions
similar to those of physical quantities, such as, for example sea surface temperature. On the other hand, zooplankton populations have marked a more
fine scale structure. In general, observed spatial patterns of phytoplankton and
zooplankton are the consequence of the time required to respond to changes of
the environment due to turbulent advection.
Detailed simulations, incorporating fluid-dynamics models of turbulence and
multi-component ecosystem dynamics (for example Smith et at., 1996; Gallagher et at., 1996) yield a similar conclusion, where zooplankton have a larger
variety of spatial pattern scales than phytoplankton.
15.5 Phytoplankton Concentration in Stratified Ocean
In the previous section we considered the patchiness of plankton on the sea
surface. However, the key to high biological productivity is the upwelling of
nutrients from deep waters into the euphotic zone and the retention of phytoplankton by stratification in well lit waters. Phytoplankton cells are suspended
motionless in the water and use the nutrients from the surrounding water. The
rate of nutrient uptake depends on the rate at which the nutrients can diffuse
towards them. Most phytoplankton cells are more dense than water and they
sink passively or generate their own locomotion. In particular, the density of
cytoplasm ranges from 1030 kg/m 3 to llOO kg/m 3 and diatoms have a cell wall
of hydrated silicon dioxide with density of about 2600 kg/m 3 . Other elements
of phytoplankton, such as coccolithophorids, have plates of calcite or aragonite with density as high as 2700-2950 kg/m 3 . Thus, except where an upward
movement of the sea water prevents it, the phytoplankton cells sink with velocity, w, which can be approximately estimated from the Stokes solution. Thus,
Eq. (2.94) gives:
w = (pp _ 1) gD 2
p
Pw
18v'
(15.6)
in which Pp and Pw are the phytoplankton and water densites, respectively, D
is the diameter of a plankton particle, v is the coefficient of kinematic viscosity.
Let us now assume that the horizontal gradient of the flow velocity, as well
as the gradients of phytoplankton and nutrient concentrations, are negligible.
Therefore, the basic equations for vertical dynamics take the form (Druet, 1994;
Druet and Zielinski, 1994):
acp +
a (wpc p ) _ ~ ( ac p ) -II),
at
az
az K z az - pCp
aCn
a ( ac n )
_
(15.7)
at
az Kzaz = IIncn
461
periodic square with side of 256 km long. Although the model dynamics are
relatively simple, the simulation showed very complex resulting phytoplankton and zooplankton patchy-like patterns. Phytoplankton have distributions
similar to those of physical quantities, such as, for example sea surface temperature. On the other hand, zooplankton populations have marked a more
fine scale structure. In general, observed spatial patterns of phytoplankton and
zooplankton are the consequence of the time required to respond to changes of
the environment due to turbulent advection.
Detailed simulations, incorporating fluid-dynamics models of turbulence and
multi-component ecosystem dynamics (for example Smith et at., 1996; Gallagher et at., 1996) yield a similar conclusion, where zooplankton have a larger
variety of spatial pattern scales than phytoplankton.
15.5 Phytoplankton Concentration in Stratified Ocean
In the previous section we considered the patchiness of plankton on the sea
surface. However, the key to high biological productivity is the upwelling of
nutrients from deep waters into the euphotic zone and the retention of phytoplankton by stratification in well lit waters. Phytoplankton cells are suspended
motionless in the water and use the nutrients from the surrounding water. The
rate of nutrient uptake depends on the rate at which the nutrients can diffuse
towards them. Most phytoplankton cells are more dense than water and they
sink passively or generate their own locomotion. In particular, the density of
cytoplasm ranges from 1030 kg/m 3 to llOO kg/m 3 and diatoms have a cell wall
of hydrated silicon dioxide with density of about 2600 kg/m 3 . Other elements
of phytoplankton, such as coccolithophorids, have plates of calcite or aragonite with density as high as 2700-2950 kg/m 3 . Thus, except where an upward
movement of the sea water prevents it, the phytoplankton cells sink with velocity, w, which can be approximately estimated from the Stokes solution. Thus,
Eq. (2.94) gives:
w = (pp _ 1) gD 2
p
Pw
18v'
(15.6)
in which Pp and Pw are the phytoplankton and water densites, respectively, D
is the diameter of a plankton particle, v is the coefficient of kinematic viscosity.
Let us now assume that the horizontal gradient of the flow velocity, as well
as the gradients of phytoplankton and nutrient concentrations, are negligible.
Therefore, the basic equations for vertical dynamics take the form (Druet, 1994;
Druet and Zielinski, 1994):
acp +
a (wpc p ) _ ~ ( ac p ) -II),
at
az
az K z az - pCp
aCn
a ( ac n )
_
(15.7)
at
az Kzaz = IIncn