PLANKTON IN NITROGEN AND PHOSPHORUS OYOLES
189
in the zooplankton population which followed upon the spring outburst
consisted largely of the copepods Acartia clausi and Acartia toma
(Deevey, 1956).
Water movement in the Sound has been shown to take place on
two levels, nutrient-rich water flowing east to west into the bottom
while the surface layers leave the area in the opposite direction
(Riley, 1956a). In times of active plant growth, nutrients originally
present in the surface layers are carried downwards in the organic
material of sinking plant cells and the nutrient-depleted water moves
away. The nutrients regenerated in the deeper waters are later
carried back into the Sound. A nutrient conservation mechanism,
analogous to that occurring in coastal upwelling situations, is thus
in operation. Riley and Conover (1956) estimated that in 1953 this
led to an average increase of 1-56 pg-atoms PO:--P/l and 0.96
pg-atoms NO;-N/l in the Sound.
S. A. M. Conover (1956) confirmed, by enrichment experiments, that
nitrogen depletion was the main factor limiting growth during the postflowering period. After addition of nitrate to the water, growth occurred
in test cultures to give normal population levels, but addition of
phosphate caused only small increases in cell numbers. Harris (1959)
investigated the nitrogen cycle in more detail and found that although
nitrate was the main nitrogen source available during the spring
outburst, the bloom was followed by considerable production of regenerated ammonia which, in the spring and summer, was present at
concentrations at least as large as the combined nitrite and nitrate.
A fall in the total nitrogen in the water from the original 19.5 pg-atoms11
by mid-April was followed by a slight increase to 6 pg-atoms/l in June.
These changes probably represented the sedimentation of particulate
nitrogen to the bottom where, after some delay, nitrogen regeneration
began to increase the level in the water. Harris also showed, using
culture techniques, that ammonia was superior to nitrite and nitrate
as a nitrogen source for the natural phytoplankton ; but, surprisingly,
cell division did not occur in most of his experiments and the results
were based on chlorophyll increases which were thought to indicate an
improvement in the physiological condition of the plant cells.
Riley (1956b), by assuming as a first approximation that horizontal
diffusion and advection in Long Island Sound could be disregarded, has
been able t o estimate the rate of change (R) of phosphate-phosphorus
concentration caused by biological activity at various depths in the
Sound throughout the year. The total rate of change of concentration
at any point, on the basis of the above assumption, was set equal to the
sum of the biological effects and vertical eddy diffusion, i.e.
189
in the zooplankton population which followed upon the spring outburst
consisted largely of the copepods Acartia clausi and Acartia toma
(Deevey, 1956).
Water movement in the Sound has been shown to take place on
two levels, nutrient-rich water flowing east to west into the bottom
while the surface layers leave the area in the opposite direction
(Riley, 1956a). In times of active plant growth, nutrients originally
present in the surface layers are carried downwards in the organic
material of sinking plant cells and the nutrient-depleted water moves
away. The nutrients regenerated in the deeper waters are later
carried back into the Sound. A nutrient conservation mechanism,
analogous to that occurring in coastal upwelling situations, is thus
in operation. Riley and Conover (1956) estimated that in 1953 this
led to an average increase of 1-56 pg-atoms PO:--P/l and 0.96
pg-atoms NO;-N/l in the Sound.
S. A. M. Conover (1956) confirmed, by enrichment experiments, that
nitrogen depletion was the main factor limiting growth during the postflowering period. After addition of nitrate to the water, growth occurred
in test cultures to give normal population levels, but addition of
phosphate caused only small increases in cell numbers. Harris (1959)
investigated the nitrogen cycle in more detail and found that although
nitrate was the main nitrogen source available during the spring
outburst, the bloom was followed by considerable production of regenerated ammonia which, in the spring and summer, was present at
concentrations at least as large as the combined nitrite and nitrate.
A fall in the total nitrogen in the water from the original 19.5 pg-atoms11
by mid-April was followed by a slight increase to 6 pg-atoms/l in June.
These changes probably represented the sedimentation of particulate
nitrogen to the bottom where, after some delay, nitrogen regeneration
began to increase the level in the water. Harris also showed, using
culture techniques, that ammonia was superior to nitrite and nitrate
as a nitrogen source for the natural phytoplankton ; but, surprisingly,
cell division did not occur in most of his experiments and the results
were based on chlorophyll increases which were thought to indicate an
improvement in the physiological condition of the plant cells.
Riley (1956b), by assuming as a first approximation that horizontal
diffusion and advection in Long Island Sound could be disregarded, has
been able t o estimate the rate of change (R) of phosphate-phosphorus
concentration caused by biological activity at various depths in the
Sound throughout the year. The total rate of change of concentration
at any point, on the basis of the above assumption, was set equal to the
sum of the biological effects and vertical eddy diffusion, i.e.