180
E. D. 9. CORNER AND ANTHONY 0 . DAVIES
levels. During the first four months of 1958, for example, net carbon
fixation within the upper 400 m amounted to 3.83 g-atoms C/ma.
Assuming a carbon : nitrogen : phosphorus ratio of 100 :15 :1 in the
resulting phytoplankton, this sbould have corresponded to a depletion
of 0.57 g-atoms N/m2 and O.Oh8 g-atoms P/m2. In fact, in the same
period, the nitrogen decreased by only 0.25 g-atoms/m2 and the phosphorus increased by 0.035 g-atoms/m2, indicating that regeneration
of the nutrients was occurring at rates comparable t o or greater than
their rates of assimilation during photosynthesis. It was pointed out,
however, that the northern Sargasso Sea was not representative of the
Sea as a whole: further south, the thermocline at 100-150 m was
probably permanent so that phytoplankton production was always
nutrient limited.
Ammonium-nitrogen represents an important nitrogen source in this
region also. Menzel and Spaeth (1962) found that in the euphotic zone
concentrations of this form of the element were 2-4 times greater than
the combined nitrite- and nitrate-nitrogen, and could be directly
correlated with the amount of rainfall. Beers and Herman (1969)
similarly showed that at stations close to the Bermuda islands,
ammonium-nitrogen (which here also included labile amino-acid
nitrogen) was more abundant than the anionic forms throughout the
summer and early autumn, though nitrate concentrations rose very
quickly at the start of the winter and soon reached their maximal
levels.
An interesting difference was found in the timing of the main
phytoplankton bloom in these inshore waters of the Sargasso Sea as
compared with the oceanic station used by Menzel and Ryther (1 960).
Beers and Herman (1969) discovered that phytoplankton levels-as
measured by chlorophyll a concentrations-underwent a marked
increase in late summer (Fig. 12). This was found t o be due to an
increase in nutrients in the euphotic zone resulting from the breakdown
of the thermocline at this time of the year (Fig. 12), though, here again,
the outburst was short-lived and chlorophyll a levels soon returned to
their normal values. The lateness of the plant bloom meant that the
zooplankton population did not reach its maximum until the start of
the following year (Fig. 12) (Herman and Beers, 1969).
Excretion of phosphate by zooplankton may have contributed to
the increase in concentration of this nutrient at night, for the animal
population in the upper 200 m began t o increase in the afternoon and
reached its maximal density at 21.00 h (Ryther et al., 1961). Ammonia
levels in the euphotic zone seem at first glance to be unrelated to
zooplankton excretion, for Beers and Kelly (1965) found that these
E. D. 9. CORNER AND ANTHONY 0 . DAVIES
levels. During the first four months of 1958, for example, net carbon
fixation within the upper 400 m amounted to 3.83 g-atoms C/ma.
Assuming a carbon : nitrogen : phosphorus ratio of 100 :15 :1 in the
resulting phytoplankton, this sbould have corresponded to a depletion
of 0.57 g-atoms N/m2 and O.Oh8 g-atoms P/m2. In fact, in the same
period, the nitrogen decreased by only 0.25 g-atoms/m2 and the phosphorus increased by 0.035 g-atoms/m2, indicating that regeneration
of the nutrients was occurring at rates comparable t o or greater than
their rates of assimilation during photosynthesis. It was pointed out,
however, that the northern Sargasso Sea was not representative of the
Sea as a whole: further south, the thermocline at 100-150 m was
probably permanent so that phytoplankton production was always
nutrient limited.
Ammonium-nitrogen represents an important nitrogen source in this
region also. Menzel and Spaeth (1962) found that in the euphotic zone
concentrations of this form of the element were 2-4 times greater than
the combined nitrite- and nitrate-nitrogen, and could be directly
correlated with the amount of rainfall. Beers and Herman (1969)
similarly showed that at stations close to the Bermuda islands,
ammonium-nitrogen (which here also included labile amino-acid
nitrogen) was more abundant than the anionic forms throughout the
summer and early autumn, though nitrate concentrations rose very
quickly at the start of the winter and soon reached their maximal
levels.
An interesting difference was found in the timing of the main
phytoplankton bloom in these inshore waters of the Sargasso Sea as
compared with the oceanic station used by Menzel and Ryther (1 960).
Beers and Herman (1969) discovered that phytoplankton levels-as
measured by chlorophyll a concentrations-underwent a marked
increase in late summer (Fig. 12). This was found t o be due to an
increase in nutrients in the euphotic zone resulting from the breakdown
of the thermocline at this time of the year (Fig. 12), though, here again,
the outburst was short-lived and chlorophyll a levels soon returned to
their normal values. The lateness of the plant bloom meant that the
zooplankton population did not reach its maximum until the start of
the following year (Fig. 12) (Herman and Beers, 1969).
Excretion of phosphate by zooplankton may have contributed to
the increase in concentration of this nutrient at night, for the animal
population in the upper 200 m began t o increase in the afternoon and
reached its maximal density at 21.00 h (Ryther et al., 1961). Ammonia
levels in the euphotic zone seem at first glance to be unrelated to
zooplankton excretion, for Beers and Kelly (1965) found that these