T H E PRODUCTION O F MARINE PLANKTON
137
supposed. Equally Cushing ( 1959a and b) has demonstrated for tropical
waters that rates of production may be higher than were earlier envisaged, and that our ideas on tropical production have been unduly
influenced by the usually low value of the phytoplankton standing crop.
Nevertheless, rates of production are limitled by nutrient levels, although there have been relatively few laboratory experiments which
have demonstrated conclusively this precise dependence for any particular phytoplankton species. The work of Ketchum (1939) demonstrated
that the growth of Nitzschia was unaffected as long as about 16 mg of
phosphate-phosphorus/m3 were present. Growth could continue at
concentrations below that figure although on a somewhat reduced level,
but at concentrations below 5 mg/m3 the rske of division fell off very
sharply. More recent investigations such as those of Curl and McLeod
(1961) also demonstrate the importance of nutrient level on photosynthetic rate. It is almost certain that just as different species of
phytoplankton may have different light cha,racteristics so the precise
nutrient levels demanded by different species also varies. Barker (1935)
showed that some flagellates could grow effectively at greater dilutions
of both nitrogen and phosphorus than typical diatoms. On the other
hand, the investigations of Ryther (1954) suggest that the green algae
Nannochloris and Stichococcus flourish only at relatively high nitrogen
concentrations. Again Kain and Fogg (1958) ;suggest that Isochrysis has
higher phosphate needs than most diatoms. Precise phosphorus requirements are particularly difficult to establish since many phytoplankton cells apparently absorb phosphate extremely rapidly, though they
may later release a considerable proportion of the nutrient either as
phosphate or in labile organic forms which :me rapidly converted into
orthophosphate. Unfortunately even less is known of the precise nitrate
requirements for single species of phytoplankton. Field observations
suggest that the diatom Thalassiosira node wkioldii possibly has very
high nutrient requirements as also does the neritic coccolithophore,
Cricosphaera carterae. Although therefore we may know but little concerning the precise concentrations of nitrate and phosphate required,
the history of the culture of phytoplankton diatoms and flagellates
focuses attention on the absolute need for i;hese nutrients. From the
classical work of Allen and Nelson (1910) to the highly specialized
techniques of Provasoli and his school, the successful culture of phytoplankton organisms depends inter alia upon a relatively high concentration of nitrate and phosphate. Moreover the mass culture experiments carried out by Ketchum and Redfield (1938) and by Ketchum
(1939), as well as the field fertilization e~periinents~of Gross et al. (1947,
1950), Raymont and Adams (1958) and Anriell et al. (1963, 1964), all
point to the importance of nitrate and phosphate in primary production.
137
supposed. Equally Cushing ( 1959a and b) has demonstrated for tropical
waters that rates of production may be higher than were earlier envisaged, and that our ideas on tropical production have been unduly
influenced by the usually low value of the phytoplankton standing crop.
Nevertheless, rates of production are limitled by nutrient levels, although there have been relatively few laboratory experiments which
have demonstrated conclusively this precise dependence for any particular phytoplankton species. The work of Ketchum (1939) demonstrated
that the growth of Nitzschia was unaffected as long as about 16 mg of
phosphate-phosphorus/m3 were present. Growth could continue at
concentrations below that figure although on a somewhat reduced level,
but at concentrations below 5 mg/m3 the rske of division fell off very
sharply. More recent investigations such as those of Curl and McLeod
(1961) also demonstrate the importance of nutrient level on photosynthetic rate. It is almost certain that just as different species of
phytoplankton may have different light cha,racteristics so the precise
nutrient levels demanded by different species also varies. Barker (1935)
showed that some flagellates could grow effectively at greater dilutions
of both nitrogen and phosphorus than typical diatoms. On the other
hand, the investigations of Ryther (1954) suggest that the green algae
Nannochloris and Stichococcus flourish only at relatively high nitrogen
concentrations. Again Kain and Fogg (1958) ;suggest that Isochrysis has
higher phosphate needs than most diatoms. Precise phosphorus requirements are particularly difficult to establish since many phytoplankton cells apparently absorb phosphate extremely rapidly, though they
may later release a considerable proportion of the nutrient either as
phosphate or in labile organic forms which :me rapidly converted into
orthophosphate. Unfortunately even less is known of the precise nitrate
requirements for single species of phytoplankton. Field observations
suggest that the diatom Thalassiosira node wkioldii possibly has very
high nutrient requirements as also does the neritic coccolithophore,
Cricosphaera carterae. Although therefore we may know but little concerning the precise concentrations of nitrate and phosphate required,
the history of the culture of phytoplankton diatoms and flagellates
focuses attention on the absolute need for i;hese nutrients. From the
classical work of Allen and Nelson (1910) to the highly specialized
techniques of Provasoli and his school, the successful culture of phytoplankton organisms depends inter alia upon a relatively high concentration of nitrate and phosphate. Moreover the mass culture experiments carried out by Ketchum and Redfield (1938) and by Ketchum
(1939), as well as the field fertilization e~periinents~of Gross et al. (1947,
1950), Raymont and Adams (1958) and Anriell et al. (1963, 1964), all
point to the importance of nitrate and phosphate in primary production.