174
E. D. 5. CORNER AND ANTHONY a. DAVIES
The data of Ketchum et al. (1958) illustrate particularly well these
seasonal changes in nutrient concentrations off the New England coast
(Figs. 9A, B). In July 1957, the concentrations of nitrate-nitrogen in
the upper 30 m were less than 1 pg-atom11 while inorganic phosphatephosphorus levels were 0-3-0-4 pg-atomll. We have already seen
(Section 111) that phytoplankton tend to utilize nitrogen and phosphorus in the ratio 16 :1, so that here nitrate was the limiting nutrient.
Below the euphotic zone, concentrations of both nutrients increaaed
gradually with depth. By September, the phosphate in the upper layers
was also depleted, indicating that, despite the almost complete absence
of nitrate, phytoplankton growth had continued since July. The
mixing of the water column, which had started in November, was
complete by January so that from the surface to the depth of the
continental shelf, nutrient concentrations were almost constant.
Vaccaro (1963) found similar variations in the nitrate and phosphate
levels in the same region, but also showed that, throughout the year, up
t o 2 pg-atomsll of ammonium-nitrogen was present in the top 30 m,
with occasional increases with depth. Vaccaro thought that thia
ammonium-nitrogen provided an alternative nitrogen source enabling
plant production to continue after the nitrate-nitrogen had been
depleted. The origin of the ammonium-nitrogen was not investigated,
but its excretion by zooplankton and presence in substantial quantities
in rainwater (about 5 pg-atom NHt-N/l were found inrain collected in
Bermuda by Menzel and Spaeth, 1962) were possible sources.
The relationship between nitrite and nitrate concentrations in thia
region has also been investigated (Vaccaro and Ryther, 1960). It was
found that as nitrate-nitrogen increased to a level of 5.5 pg-atoms/l,
nitrite-nitrogen increased in proportion in the ratio of 0.05 :l. Above
this level, however, the ratio decreased. Concentrations of nitritenitrogen, which were always low, reached their highest values (about
0-3 pg-atomsll) in December. It was shown that the maxima innitritenitrogen concentrations, which were found just below the euphotic zone,
might have been caused by phytoplankton releasing nitrite at low
light energies.
Planktonic organisms were also involved in the cycle of phosphom
in the nearby Gulf of Maine, examined in detail by Ketchum and Corwin
(1965). Concentrations of inorganic phosphate, particulate and dissolved organic phosphorus were measured on two occasions separated
by ten days in April, 1964. A parachute drogue set at 10 m was used aa
a marker for the surface water so that it would be possible to examine
the same water mass at both times. Because of changes in the water
column caused by movement of the water to a different location, the
E. D. 5. CORNER AND ANTHONY a. DAVIES
The data of Ketchum et al. (1958) illustrate particularly well these
seasonal changes in nutrient concentrations off the New England coast
(Figs. 9A, B). In July 1957, the concentrations of nitrate-nitrogen in
the upper 30 m were less than 1 pg-atom11 while inorganic phosphatephosphorus levels were 0-3-0-4 pg-atomll. We have already seen
(Section 111) that phytoplankton tend to utilize nitrogen and phosphorus in the ratio 16 :1, so that here nitrate was the limiting nutrient.
Below the euphotic zone, concentrations of both nutrients increaaed
gradually with depth. By September, the phosphate in the upper layers
was also depleted, indicating that, despite the almost complete absence
of nitrate, phytoplankton growth had continued since July. The
mixing of the water column, which had started in November, was
complete by January so that from the surface to the depth of the
continental shelf, nutrient concentrations were almost constant.
Vaccaro (1963) found similar variations in the nitrate and phosphate
levels in the same region, but also showed that, throughout the year, up
t o 2 pg-atomsll of ammonium-nitrogen was present in the top 30 m,
with occasional increases with depth. Vaccaro thought that thia
ammonium-nitrogen provided an alternative nitrogen source enabling
plant production to continue after the nitrate-nitrogen had been
depleted. The origin of the ammonium-nitrogen was not investigated,
but its excretion by zooplankton and presence in substantial quantities
in rainwater (about 5 pg-atom NHt-N/l were found inrain collected in
Bermuda by Menzel and Spaeth, 1962) were possible sources.
The relationship between nitrite and nitrate concentrations in thia
region has also been investigated (Vaccaro and Ryther, 1960). It was
found that as nitrate-nitrogen increased to a level of 5.5 pg-atoms/l,
nitrite-nitrogen increased in proportion in the ratio of 0.05 :l. Above
this level, however, the ratio decreased. Concentrations of nitritenitrogen, which were always low, reached their highest values (about
0-3 pg-atomsll) in December. It was shown that the maxima innitritenitrogen concentrations, which were found just below the euphotic zone,
might have been caused by phytoplankton releasing nitrite at low
light energies.
Planktonic organisms were also involved in the cycle of phosphom
in the nearby Gulf of Maine, examined in detail by Ketchum and Corwin
(1965). Concentrations of inorganic phosphate, particulate and dissolved organic phosphorus were measured on two occasions separated
by ten days in April, 1964. A parachute drogue set at 10 m was used aa
a marker for the surface water so that it would be possible to examine
the same water mass at both times. Because of changes in the water
column caused by movement of the water to a different location, the