PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
135
A " ratio " method for estimating assimilation efficiency, described
by Conover (1 966a), has the great advantage that the quantitative
collection of faecal pellets is unnecessary. The method depends on the
assumption that only the organic component of the food is significantly
altered by the digestion process ; hence, it is only necessary to obtain
the ratio ash-free dry weight : dry weight for a sample of the food and
a sample of the faeces to calculate the percentage assimilation of the
organic fraction. Conover used the method to estimate assimilation
efficiency in terms of the total amounts of organic material in the diet.
However, it could also be used to measure assimilation in terms of
particular dietary components, such as phosphorus, for percentage
assimilation can be expressed in terms of the ratio [organic P : total PI
in the food and in the faecal pellets. Total P could be determined
by digesting the sample with concentrated sulphuric acid and estimating the inorganic phosphate so formed using a standard method (e.g.
that of Murphy and Riley, 1962). Organic P would then be estimated
as the difference between this total value and the quantity of soluble
inorganic phosphate alone.
Conover (1966b) used the " ratio " method to study the assimilation efficiency of Calanus hyperboreus K r ~ y e r feeding on Thulassiosira
jluviatilis, and found that there was no significant correlation between
percentage assimilation and either the food level or the quantity of
food ingested. Thus, the average value for assimilation remained close
to 70% for a range of food concentrations representing 100 to 1 900 pg
C/1, the latter level being about three times the maximum concentration found at the peak of the spring bloom in the Gulf of
Maine. Conover also applied the method in further experiments,
carried out at sea, and found that mixed zooplankton feeding on
natural particulate material assimilated, on average, 67% of the
ingested food.
Conover (1966a, b) makes clear that the accurate determination of
assimilation by the method depends upon organic matter and ash being
ingested by zooplankton in the same proportions as they occur in the
natural food of the animals. This will not happen, however, if the food
is destroyed during capture and the contents spilled ; or if the animal,
when feeding on particulate material, selects the organic fraction of the
diet in preference to the inorganic fraction. Evidence that such selection may occur has been obtained in experiments with Calanus helgolandicus by Corner (1961). However, these animals were collected
from a coastal area where much of the unselected inorganic material
could have been terrigenous. Conover (1966a) found that in areas away
from the coast, estimations of assimilation with natural particulate
135
A " ratio " method for estimating assimilation efficiency, described
by Conover (1 966a), has the great advantage that the quantitative
collection of faecal pellets is unnecessary. The method depends on the
assumption that only the organic component of the food is significantly
altered by the digestion process ; hence, it is only necessary to obtain
the ratio ash-free dry weight : dry weight for a sample of the food and
a sample of the faeces to calculate the percentage assimilation of the
organic fraction. Conover used the method to estimate assimilation
efficiency in terms of the total amounts of organic material in the diet.
However, it could also be used to measure assimilation in terms of
particular dietary components, such as phosphorus, for percentage
assimilation can be expressed in terms of the ratio [organic P : total PI
in the food and in the faecal pellets. Total P could be determined
by digesting the sample with concentrated sulphuric acid and estimating the inorganic phosphate so formed using a standard method (e.g.
that of Murphy and Riley, 1962). Organic P would then be estimated
as the difference between this total value and the quantity of soluble
inorganic phosphate alone.
Conover (1966b) used the " ratio " method to study the assimilation efficiency of Calanus hyperboreus K r ~ y e r feeding on Thulassiosira
jluviatilis, and found that there was no significant correlation between
percentage assimilation and either the food level or the quantity of
food ingested. Thus, the average value for assimilation remained close
to 70% for a range of food concentrations representing 100 to 1 900 pg
C/1, the latter level being about three times the maximum concentration found at the peak of the spring bloom in the Gulf of
Maine. Conover also applied the method in further experiments,
carried out at sea, and found that mixed zooplankton feeding on
natural particulate material assimilated, on average, 67% of the
ingested food.
Conover (1966a, b) makes clear that the accurate determination of
assimilation by the method depends upon organic matter and ash being
ingested by zooplankton in the same proportions as they occur in the
natural food of the animals. This will not happen, however, if the food
is destroyed during capture and the contents spilled ; or if the animal,
when feeding on particulate material, selects the organic fraction of the
diet in preference to the inorganic fraction. Evidence that such selection may occur has been obtained in experiments with Calanus helgolandicus by Corner (1961). However, these animals were collected
from a coastal area where much of the unselected inorganic material
could have been terrigenous. Conover (1966a) found that in areas away
from the coast, estimations of assimilation with natural particulate