178
E. D. 9. CORNER AND ANTHONY Q. DAVJES
about eight times faster than its remineralization. Accordingly,
Ketchum and Corwin suggested that the regeneration rate may be the
factor which ultimately limits phytoplankton production.
B. Tropical and sub-tropical regions
In regions nearer to the equator, persistently higher light energies
and temperatures allow phytoplankton production to proceed throughout
the year and the regenerative processes are much more rapid. This
causes faster cycling of thenutrients, and seasonal maxima and minima
in their concentrations are therefore much less evident. For example,
at a station off Bermuda in the sub-tropical northern Sargasso sea,
Menzel and Ryther (1960) found that nitrate and phosphate levels
remained low throughout the year (Fig. 11). In the euphotic zone
(down to 100 m), nitrate plus nitrite-nitrogen varied from zero to
1.8 pg-atoms/l and phosphate-phosphorus between 0.02 and 0.16
pg-atoms/l. During the winter, when mixing of the water column down
to the permanent thermocline at 400 m occurred, the nutrient levels at
the surface reached their maximal values of only 1-2 pg-atoms N/1 and
0.1-0.2 pg-atoms P/I, indicating that only very low concentrations of
regenerated nutrients were present in the waters below the seasonal
thermocline. However, as in more temperate regions, use of the comparatively higher nutrient levels available in the euphotic zone in the
winter was delayed until reformation of the thermocline prevented
dispersion of the phytoplankton throughout the mixed layer. A shortlived bloom was then quicltly restricted by the exhaustion of the
nutrients.
Chlorophyll a levels (Fig. 11) showed that much of the resulting
phytoplankton gradually sank below the euphotic zone and accumulated at 100-150 m where the nutrients regenerated from the plant
material remained until the following winter. During the period of
thermal stratification, when nutrient levels in the euphotic zone were
very low, plant production still continued, probably because small
amounts of nutrients were being supplied by vertical mixing with
deeper waters and regeneration by zooplankton. The zooplankton
population reached its maximal level at about the same time or just
after the spring bloom (Fig. 11). During the remainder of the year their
numbers were low and varied little. Menzel and Ryther (1961) thought
that as the animal population was limited by the plant supply, any
available phytoplankton were rapidly grazed and the nutrients regenerated, thus maintaining a " highly efficient biological system ".
Menzel and Ryther (1960) found that there was no correlation
between primary productivity in the ares and changes in nutrient
E. D. 9. CORNER AND ANTHONY Q. DAVJES
about eight times faster than its remineralization. Accordingly,
Ketchum and Corwin suggested that the regeneration rate may be the
factor which ultimately limits phytoplankton production.
B. Tropical and sub-tropical regions
In regions nearer to the equator, persistently higher light energies
and temperatures allow phytoplankton production to proceed throughout
the year and the regenerative processes are much more rapid. This
causes faster cycling of thenutrients, and seasonal maxima and minima
in their concentrations are therefore much less evident. For example,
at a station off Bermuda in the sub-tropical northern Sargasso sea,
Menzel and Ryther (1960) found that nitrate and phosphate levels
remained low throughout the year (Fig. 11). In the euphotic zone
(down to 100 m), nitrate plus nitrite-nitrogen varied from zero to
1.8 pg-atoms/l and phosphate-phosphorus between 0.02 and 0.16
pg-atoms/l. During the winter, when mixing of the water column down
to the permanent thermocline at 400 m occurred, the nutrient levels at
the surface reached their maximal values of only 1-2 pg-atoms N/1 and
0.1-0.2 pg-atoms P/I, indicating that only very low concentrations of
regenerated nutrients were present in the waters below the seasonal
thermocline. However, as in more temperate regions, use of the comparatively higher nutrient levels available in the euphotic zone in the
winter was delayed until reformation of the thermocline prevented
dispersion of the phytoplankton throughout the mixed layer. A shortlived bloom was then quicltly restricted by the exhaustion of the
nutrients.
Chlorophyll a levels (Fig. 11) showed that much of the resulting
phytoplankton gradually sank below the euphotic zone and accumulated at 100-150 m where the nutrients regenerated from the plant
material remained until the following winter. During the period of
thermal stratification, when nutrient levels in the euphotic zone were
very low, plant production still continued, probably because small
amounts of nutrients were being supplied by vertical mixing with
deeper waters and regeneration by zooplankton. The zooplankton
population reached its maximal level at about the same time or just
after the spring bloom (Fig. 11). During the remainder of the year their
numbers were low and varied little. Menzel and Ryther (1961) thought
that as the animal population was limited by the plant supply, any
available phytoplankton were rapidly grazed and the nutrients regenerated, thus maintaining a " highly efficient biological system ".
Menzel and Ryther (1960) found that there was no correlation
between primary productivity in the ares and changes in nutrient