172
E. D. 8. CORNER AND ANTHONY 0 . DAVIES
species of zooplankton in the sea, once excretion rates and K , values
(or, preferably K 1 values, sincc these allow for the fact that not all the
captured food is assimilated) for the species are known, and the neccssary field data are available.
XII. PLANKTON PRODUCTION AND NUTRIENT LEVELS
IN CERTAIN SEA AREAS
In the previous sections we have been mainly concerned with
laboratory studies of the uptake and release of nitrogen and phosphorus
compounds by phytoplankton and zooplankton. In this final section we
show how fluctuations in the sizes of plankton populations are related
to overall changes in nutrient levels in certain sea areas and, in our
discussion of partly enclosed regions, we deal with studies in which
both physical and biological factors have been quantitatively assessed.
A. Temperate regions
In temperate regions, nitrate and phosphate concentrations undergo
marked seasonal cycles. During the autumn and winter the cooling of
the sea surface and turbulence caused by stormsresult in the breakdown
of the temperature stratification of the previous summer, and concentrations of nitrate and phosphate in the nutrient-depleted euphotic zone
rise as mixing occurs with the deeper nutrient-rich water. In the relatively shallow seas over continental shelf areas the whole of the water
column may become homogeneous so that nutrient concentrations vary
little with depth ; but in deeper water there is usually a marked rise in
concentrations below the mixed layer. The increasing insolation of
spring and early summer warms again the upper layers of the water
promoting the formation of the thermocline ; and the combination of
higher temperature, greater illumination and stability of the water
column favours the multiplication of the phytoplankton. Rapid
growth ensues and the nutrients are quickly diminished. The bloom is
finally arrested when either one of the nutrients (usually nitrate) is
depleted, or grazing by zooplankton limits the plant population. Some
of the cells slowly sink below the euphotic zone where, in the absence
of light, they cease to be viable and regeneration processes gradually
return the nutrients contained in the organic matter to the water.
Because of the thermocline, however, these reformed nutrients are
largely prevented from being transported back into the euphotic zone
and so remain unavailable until the following winter, although
occasionally some vertical movement of the nutrients does occur to
produce from time to time, resurgences of phytoplankton growth.
E. D. 8. CORNER AND ANTHONY 0 . DAVIES
species of zooplankton in the sea, once excretion rates and K , values
(or, preferably K 1 values, sincc these allow for the fact that not all the
captured food is assimilated) for the species are known, and the neccssary field data are available.
XII. PLANKTON PRODUCTION AND NUTRIENT LEVELS
IN CERTAIN SEA AREAS
In the previous sections we have been mainly concerned with
laboratory studies of the uptake and release of nitrogen and phosphorus
compounds by phytoplankton and zooplankton. In this final section we
show how fluctuations in the sizes of plankton populations are related
to overall changes in nutrient levels in certain sea areas and, in our
discussion of partly enclosed regions, we deal with studies in which
both physical and biological factors have been quantitatively assessed.
A. Temperate regions
In temperate regions, nitrate and phosphate concentrations undergo
marked seasonal cycles. During the autumn and winter the cooling of
the sea surface and turbulence caused by stormsresult in the breakdown
of the temperature stratification of the previous summer, and concentrations of nitrate and phosphate in the nutrient-depleted euphotic zone
rise as mixing occurs with the deeper nutrient-rich water. In the relatively shallow seas over continental shelf areas the whole of the water
column may become homogeneous so that nutrient concentrations vary
little with depth ; but in deeper water there is usually a marked rise in
concentrations below the mixed layer. The increasing insolation of
spring and early summer warms again the upper layers of the water
promoting the formation of the thermocline ; and the combination of
higher temperature, greater illumination and stability of the water
column favours the multiplication of the phytoplankton. Rapid
growth ensues and the nutrients are quickly diminished. The bloom is
finally arrested when either one of the nutrients (usually nitrate) is
depleted, or grazing by zooplankton limits the plant population. Some
of the cells slowly sink below the euphotic zone where, in the absence
of light, they cease to be viable and regeneration processes gradually
return the nutrients contained in the organic matter to the water.
Because of the thermocline, however, these reformed nutrients are
largely prevented from being transported back into the euphotic zone
and so remain unavailable until the following winter, although
occasionally some vertical movement of the nutrients does occur to
produce from time to time, resurgences of phytoplankton growth.