PLANKTON IN NITROOEN AND PHOSPHORUS CYCLES
107
suggested that any deviations from this valuc should bc tcrmcd the
I ‘ anomaly of the nitratc : phosphatc ratio ”.
More recsnt work has indicated that, especially in coastal rsgions,
the nitrogen : phosphorus ratio is frequently anomalous in this sense,
and values of 5-1O:l are not uncommon, even during the autumn/
winter period when nutrient concentrations are maximal (e.6. Riley and
Conover, 1956; Ketchum et al., 1958; McAllister et al., 19GO). Pratt
(1965) found that, in Narragansett Bay, the ratio rarely exceeded 4:l
and was more usually less than 1:l. Nevertheless, it is still generally
true that nitrogen and phosphorus concentrations are linearly correlated
with regression coefficients of between 15:l and 16:l so that, upon
depletion of the nitrogen, some phosphorus remains. The slopes of these
regression lines, AN: AP, represent the relative changes in the nutrient
concentrations resulting from biological activity, and would be expected
to reflect more closely the ratios of the nutrients taken up by the phytoplankton. The value of AN: AP has, therefore, been termed the
I‘ assimilation ratio ”.
This ratio, AN: dP, can vary considerably from the usual valuc of
15 or 16:l. For example, Steftinsson (1968) has recently shown that
although a value of 16.6:l was obtained when data for the Irminger Sea
were treated collectively, therc was some variation with salinity, and
the nitrogen : phosphorus relationship was better described by the
expression [NO;-N] = 14-3 [PO:--P] + 0.845 (S%, - 35) - 0.5. The
AN: A P value of 14.3:l was rather lower than that usually found.
Evidence exists that phosphorus utilization by phytoplankton
continues after the nitrate-nitrogen concentration has fallen below the
detectable limit. McAllister et al. (1960) followed the growth of a large
scale culture of a natural population of phytoplankton and found that,
as expected from the anomalous initial nitrogen : phosphorus ratio of
9-2:1, the nitrate was depleted before the phosphate. Growth continued,
however, until the phosphorus had also disappeared, and assimilation
ratios varied between 14:l when nitrogen was plentiful to 8 4 : l during
the period of nitrogen deficiency. Indirect evidence that this also
occurs in the sea is provided by the nitrate-phosphate plot of Ketchum
et al. (1958) where the data for July and September (when the concentrations of nitrate were very low) indicate that nutrient assimilation
was occurring at ratios much lower than the 15:l value which applied
to the rest of the results. A similar finding has been reportcd by
Steftinsson and Richards (1963) for the north-eastern Pacific Ocean.
AN: AP values of 16:l were obtained at deep water (> 1 000 m) stations;
but in the euphotic zone, values approaching zero resulted from phosphate assimilation after the exhaustion of the nitrate. The authors
107
suggested that any deviations from this valuc should bc tcrmcd the
I ‘ anomaly of the nitratc : phosphatc ratio ”.
More recsnt work has indicated that, especially in coastal rsgions,
the nitrogen : phosphorus ratio is frequently anomalous in this sense,
and values of 5-1O:l are not uncommon, even during the autumn/
winter period when nutrient concentrations are maximal (e.6. Riley and
Conover, 1956; Ketchum et al., 1958; McAllister et al., 19GO). Pratt
(1965) found that, in Narragansett Bay, the ratio rarely exceeded 4:l
and was more usually less than 1:l. Nevertheless, it is still generally
true that nitrogen and phosphorus concentrations are linearly correlated
with regression coefficients of between 15:l and 16:l so that, upon
depletion of the nitrogen, some phosphorus remains. The slopes of these
regression lines, AN: AP, represent the relative changes in the nutrient
concentrations resulting from biological activity, and would be expected
to reflect more closely the ratios of the nutrients taken up by the phytoplankton. The value of AN: AP has, therefore, been termed the
I‘ assimilation ratio ”.
This ratio, AN: dP, can vary considerably from the usual valuc of
15 or 16:l. For example, Steftinsson (1968) has recently shown that
although a value of 16.6:l was obtained when data for the Irminger Sea
were treated collectively, therc was some variation with salinity, and
the nitrogen : phosphorus relationship was better described by the
expression [NO;-N] = 14-3 [PO:--P] + 0.845 (S%, - 35) - 0.5. The
AN: A P value of 14.3:l was rather lower than that usually found.
Evidence exists that phosphorus utilization by phytoplankton
continues after the nitrate-nitrogen concentration has fallen below the
detectable limit. McAllister et al. (1960) followed the growth of a large
scale culture of a natural population of phytoplankton and found that,
as expected from the anomalous initial nitrogen : phosphorus ratio of
9-2:1, the nitrate was depleted before the phosphate. Growth continued,
however, until the phosphorus had also disappeared, and assimilation
ratios varied between 14:l when nitrogen was plentiful to 8 4 : l during
the period of nitrogen deficiency. Indirect evidence that this also
occurs in the sea is provided by the nitrate-phosphate plot of Ketchum
et al. (1958) where the data for July and September (when the concentrations of nitrate were very low) indicate that nutrient assimilation
was occurring at ratios much lower than the 15:l value which applied
to the rest of the results. A similar finding has been reportcd by
Steftinsson and Richards (1963) for the north-eastern Pacific Ocean.
AN: AP values of 16:l were obtained at deep water (> 1 000 m) stations;
but in the euphotic zone, values approaching zero resulted from phosphate assimilation after the exhaustion of the nitrate. The authors