PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
187
with the water outside the bay and only a weak surface current, SO that
transfer of plant cells from the area is small ; secondly, the shallowness
of the bay means that phytoplankton cells taken from the euphotic
zone by water moving downwards are quickly returned to it by complementary upward movements. Such a circulation of the plant cells
extends the period of plant production, for only a fraction of the total
population can photosynthesize at any given time.
Pratt (1965) pointed out that even before the main outburst of
growth in the winter, the nutrient concentrations had, for several
weeks, steadily decreased from the annual summer maximum although
the phytoplankton population remained low during the same period.
This was because of grazing of the growing plant material by the zooplankton which remained present in large numbers until the winter
(Martin, 1965) : the temperatures then became too low for the dcvelopment of the principal zooplankton species in the bay (Acartia tonsa,
Aeartia clausi and Oithona spp.) resulting in a reduction in grazing
pressure which allowed the phytoplankton bloom to proceed. Martin
considered that grazing was, in fact, the main factor controlling the
standing crop of phytoplankton throughout the year in this area, as
the increasing zooplankton numbers consequent upon the rising temperatures of spring ended the plant outburst. The plant population was
then gradually consumed and remained low from early summer until
the following winter. Martin also suggested that the build up of nutrients
which began in late summer was due t o their excretion by the zooplankton, the heavily grazed phytoplankton population being insufficient to
utilize the regenerated phosphate and ammonia.
In Long Island Sound, the seasonal cycles of the nutricnts are more
like those found in open temperate sea areas, although here also, the
inception of the main phytoplankton bloom is somewhat earlier (midJanuary t o the start of February) than usual. Riley and Conover
(1956) studied the changes in nutrient levels and the concomitant
variation in phytoplankton and zooplankton in the Sound over a
period of two years (Spring 1952-Spring 1954): some of their data are
illustrated in Fig. 16. Surface concentrations of phosphate-phosphorus
varied from about 2.3 pg-atoms11 in the winter to about 0.5 pg-atoms/l
in the spring. Surface levels of nitrate-nitrogen were reduced to almost
zero by plant growth, and remained low until the end of the summer,
after which they increased steadily back t o their winter maxima of
between 15 and 20 pg-atomsll. There were slight increases in nutrient
levels between the surface and the bottom during the first half of the
year while, occasionally in the autumn, lower concentrations existed
at the bottom than at the surface. Large rises in chlorophyll levels
187
with the water outside the bay and only a weak surface current, SO that
transfer of plant cells from the area is small ; secondly, the shallowness
of the bay means that phytoplankton cells taken from the euphotic
zone by water moving downwards are quickly returned to it by complementary upward movements. Such a circulation of the plant cells
extends the period of plant production, for only a fraction of the total
population can photosynthesize at any given time.
Pratt (1965) pointed out that even before the main outburst of
growth in the winter, the nutrient concentrations had, for several
weeks, steadily decreased from the annual summer maximum although
the phytoplankton population remained low during the same period.
This was because of grazing of the growing plant material by the zooplankton which remained present in large numbers until the winter
(Martin, 1965) : the temperatures then became too low for the dcvelopment of the principal zooplankton species in the bay (Acartia tonsa,
Aeartia clausi and Oithona spp.) resulting in a reduction in grazing
pressure which allowed the phytoplankton bloom to proceed. Martin
considered that grazing was, in fact, the main factor controlling the
standing crop of phytoplankton throughout the year in this area, as
the increasing zooplankton numbers consequent upon the rising temperatures of spring ended the plant outburst. The plant population was
then gradually consumed and remained low from early summer until
the following winter. Martin also suggested that the build up of nutrients
which began in late summer was due t o their excretion by the zooplankton, the heavily grazed phytoplankton population being insufficient to
utilize the regenerated phosphate and ammonia.
In Long Island Sound, the seasonal cycles of the nutricnts are more
like those found in open temperate sea areas, although here also, the
inception of the main phytoplankton bloom is somewhat earlier (midJanuary t o the start of February) than usual. Riley and Conover
(1956) studied the changes in nutrient levels and the concomitant
variation in phytoplankton and zooplankton in the Sound over a
period of two years (Spring 1952-Spring 1954): some of their data are
illustrated in Fig. 16. Surface concentrations of phosphate-phosphorus
varied from about 2.3 pg-atoms11 in the winter to about 0.5 pg-atoms/l
in the spring. Surface levels of nitrate-nitrogen were reduced to almost
zero by plant growth, and remained low until the end of the summer,
after which they increased steadily back t o their winter maxima of
between 15 and 20 pg-atomsll. There were slight increases in nutrient
levels between the surface and the bottom during the first half of the
year while, occasionally in the autumn, lower concentrations existed
at the bottom than at the surface. Large rises in chlorophyll levels