148
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
control of the possible presence of labeled bacteria. These aliquots are filtered
through 0.2-llm pore size membrane filters. The filters are washed in the funnel
three times with 5 ml of water and immediately radioassayed. Another control
should also be made to estimate a possible absorption of DOM by animal
surfaces. To achieve this, a parallel incubation of the animals first killed by
formalin, then washed and placed into the experimental vessel with the labeled
DOM added is carried out. The possible appearance of bacteria makes the
whole experiment invalid.
When the incubation is finished, the labeled food absorbed on the animal
surfaces is carefully washed off and then they are radioassayed (see Sect.
3.4.2). The rate of DOM uptake thus measured is actually equal to the rate
of its assimilation (Ad), because no feces are formed. The value of Ad is be
calculated using the equation: Ad = RaCr 1.15 mg C Sp.-l h-l, where: Ra is
the radioactivity of animals, cpmsp.-l; (with correction on control with
dead animals), Cr is the inverse specific radioactivity of labeled DOM, Ilg C
cpm-', t is the time of incubation, h, and 1.15 is the correction coefficient
for the losses of assimilated 14C as the respiratory 14C02 due to the incubation of animals in the presence of labeled DOM. Experiments showed that
these losses could be about 10 to 15% during 1-2h of incubation (Lampert
1977; Sorokin 1977a, 1978a). In order to make the data on the rates of
the labeled DOM uptake by aquatic animals applicable for evaluation of the
real role of DOM as a source of nutrition and for characterization of the
participation of DOM pool in the trophodynamics of aquatic ecosystems, it is
necessary:
1. To compare the efficiency of their nutrition with the DOM at its concentrations close to the ambient contents of the labile DOM in water.
The latter may be evaluated within the range of 20 to lOOllg (depending
on the trophicallevel of water basin). As criterion for such a comparison,
the CalC assimilation index (see Sect. 3.5.2) could be used, or the percentage ratio of the DOM assimilation rate Ad to the rate of respiration
(M), both expressed in the same carbon units (Aj M, %). An example of
such a comparison is shown in Fig. 3.20. From this Figure it is clearly
seen that in the animals with developed ciliary epithelium the nutrition
with DOM at its concentration close to that in natural waters (~100 Ilg
CI- 1 ) may compensate from 15 to 60% of total metabolic losses. Moreover,
it appeared to be that the DOM as the source of nutrition for this kind
of aquatic invertebrate might even be comparable with the particulated
food.
2. Another important criterion of the trophical importance of DOM is the
comparison of the optimal DOM concentrations, found from the rate
shown by concentration curves, with the integrated ambient contents in
water of low and medium molecular weight organic compounds, such as
amino acids, peptides, polysaccharides, and lipids. Their basic source in
water basins is their exudation by plants (Khailov 1971 b).
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
control of the possible presence of labeled bacteria. These aliquots are filtered
through 0.2-llm pore size membrane filters. The filters are washed in the funnel
three times with 5 ml of water and immediately radioassayed. Another control
should also be made to estimate a possible absorption of DOM by animal
surfaces. To achieve this, a parallel incubation of the animals first killed by
formalin, then washed and placed into the experimental vessel with the labeled
DOM added is carried out. The possible appearance of bacteria makes the
whole experiment invalid.
When the incubation is finished, the labeled food absorbed on the animal
surfaces is carefully washed off and then they are radioassayed (see Sect.
3.4.2). The rate of DOM uptake thus measured is actually equal to the rate
of its assimilation (Ad), because no feces are formed. The value of Ad is be
calculated using the equation: Ad = RaCr 1.15 mg C Sp.-l h-l, where: Ra is
the radioactivity of animals, cpmsp.-l; (with correction on control with
dead animals), Cr is the inverse specific radioactivity of labeled DOM, Ilg C
cpm-', t is the time of incubation, h, and 1.15 is the correction coefficient
for the losses of assimilated 14C as the respiratory 14C02 due to the incubation of animals in the presence of labeled DOM. Experiments showed that
these losses could be about 10 to 15% during 1-2h of incubation (Lampert
1977; Sorokin 1977a, 1978a). In order to make the data on the rates of
the labeled DOM uptake by aquatic animals applicable for evaluation of the
real role of DOM as a source of nutrition and for characterization of the
participation of DOM pool in the trophodynamics of aquatic ecosystems, it is
necessary:
1. To compare the efficiency of their nutrition with the DOM at its concentrations close to the ambient contents of the labile DOM in water.
The latter may be evaluated within the range of 20 to lOOllg (depending
on the trophicallevel of water basin). As criterion for such a comparison,
the CalC assimilation index (see Sect. 3.5.2) could be used, or the percentage ratio of the DOM assimilation rate Ad to the rate of respiration
(M), both expressed in the same carbon units (Aj M, %). An example of
such a comparison is shown in Fig. 3.20. From this Figure it is clearly
seen that in the animals with developed ciliary epithelium the nutrition
with DOM at its concentration close to that in natural waters (~100 Ilg
CI- 1 ) may compensate from 15 to 60% of total metabolic losses. Moreover,
it appeared to be that the DOM as the source of nutrition for this kind
of aquatic invertebrate might even be comparable with the particulated
food.
2. Another important criterion of the trophical importance of DOM is the
comparison of the optimal DOM concentrations, found from the rate
shown by concentration curves, with the integrated ambient contents in
water of low and medium molecular weight organic compounds, such as
amino acids, peptides, polysaccharides, and lipids. Their basic source in
water basins is their exudation by plants (Khailov 1971 b).