Determination of the in Situ Decomposition Rates
173
In relation to ordinary organic substances like amino acids or sugars, there is
little reason to measure their absolute turnover time or decomposition rate,
simply because these data cannot be used for evaluation of overall or even of
microbial decomposition rates. Native microbial popUlations deal mostly with
various polymers like peptide phospholipids, polysaccharides, etc. rather than
with individual sugars or amino acids, the turnover time of which is many times
less than that of the total stock of the labile organic matter accessible to microbial uptake. The first is measured by hours, the second by weeks (5 to 20 days
even in summer). An attempt to calculate the overall decomposition rates
from the data on decomposition rates of mixtures of amino acids even taking
their pool in natural water into account gave values of about an order less than
direct measurement by the oxygen method (Sepers et al. 1982). Therefore for
the use of radiolabeled substances in this direction of research, the only practically sound reason remains to employ radiolabeled pollutants and organic
polymers, which are decomposed in aquatic environments by a specific
microflora, in order to measure the individual rates of their decomposition and
their turnover time in natural waters. These studies are important for the evaluation of their self-purification capacity. Hydrocarbons, phenol detergents,
pesticides, lignocellulose, cellulose, and humic acids were used among such
specific labeled substrates (Kirk et al. 1975; Romanenko and Kuznetsov 1976;
Hachett et a1. 1977; Lee and Takahashi 1977; Benner et a1. 1984; Namsaraev
and Ivanov 1992). All these substances are subjected to gradual oxidative
degradation in aquatic environments with the participation of specific microbial populations, which is accompanied by the evolving of 14C02.
Another prospective approach is the use of radio labeled plant material
for studying its decomposition in aquatic biotopes. The evolving of 14C02 has
evidenced in this case the intensity of this process in time (Romanenko and
Romanenko 1969; Kudryavtzev and Romanenko 1973; Romanenko and
Dobrynin 1973). Kelly et al. (1986) had sequenced the decomposition process
of 14C-labeled organic matter after whole-lake labeling with 14C-bicarbonate
and the formation in it of the stock of labeled dissolved and particulated
organic material. Finally, Romanenko (1965) offered a very useful practically
indirect method for the estimation of microbial respiration (Mb) in natural
water, based on the measuring of 14C02 dark uptake (Cd), which appeared to
be well correlated with the uptake of dissolved oxygen by plankton in dark
bottles (Mt ): M t = 140Cdmg02l-1 day-I, if Cd is expressed in mgCl- 1 day-I. Such
a correlation is a sequence of the correlation between Cd and microbial production Ph (Pb = 15Cd ) and the coupling of the latter with microbial respiration (Mb = 2.125 P). In accordance with these ratios, the relationship between
Mb and Cd must be as follows: Mb = 15 x 2.67 x 2.125 = 85Cd. The direct comparisons of Cd and M t values in natural waters after Romanenko (1985) gave
their ratio as over 1.5 times as much, but the correlation between them in
experiments was quite good. The above difference might be explained by the
difference between M t and M b • It makes this method quite reliable for widescale monitoring of overall decomposition rates in water columns, because the
173
In relation to ordinary organic substances like amino acids or sugars, there is
little reason to measure their absolute turnover time or decomposition rate,
simply because these data cannot be used for evaluation of overall or even of
microbial decomposition rates. Native microbial popUlations deal mostly with
various polymers like peptide phospholipids, polysaccharides, etc. rather than
with individual sugars or amino acids, the turnover time of which is many times
less than that of the total stock of the labile organic matter accessible to microbial uptake. The first is measured by hours, the second by weeks (5 to 20 days
even in summer). An attempt to calculate the overall decomposition rates
from the data on decomposition rates of mixtures of amino acids even taking
their pool in natural water into account gave values of about an order less than
direct measurement by the oxygen method (Sepers et al. 1982). Therefore for
the use of radiolabeled substances in this direction of research, the only practically sound reason remains to employ radiolabeled pollutants and organic
polymers, which are decomposed in aquatic environments by a specific
microflora, in order to measure the individual rates of their decomposition and
their turnover time in natural waters. These studies are important for the evaluation of their self-purification capacity. Hydrocarbons, phenol detergents,
pesticides, lignocellulose, cellulose, and humic acids were used among such
specific labeled substrates (Kirk et al. 1975; Romanenko and Kuznetsov 1976;
Hachett et a1. 1977; Lee and Takahashi 1977; Benner et a1. 1984; Namsaraev
and Ivanov 1992). All these substances are subjected to gradual oxidative
degradation in aquatic environments with the participation of specific microbial populations, which is accompanied by the evolving of 14C02.
Another prospective approach is the use of radio labeled plant material
for studying its decomposition in aquatic biotopes. The evolving of 14C02 has
evidenced in this case the intensity of this process in time (Romanenko and
Romanenko 1969; Kudryavtzev and Romanenko 1973; Romanenko and
Dobrynin 1973). Kelly et al. (1986) had sequenced the decomposition process
of 14C-labeled organic matter after whole-lake labeling with 14C-bicarbonate
and the formation in it of the stock of labeled dissolved and particulated
organic material. Finally, Romanenko (1965) offered a very useful practically
indirect method for the estimation of microbial respiration (Mb) in natural
water, based on the measuring of 14C02 dark uptake (Cd), which appeared to
be well correlated with the uptake of dissolved oxygen by plankton in dark
bottles (Mt ): M t = 140Cdmg02l-1 day-I, if Cd is expressed in mgCl- 1 day-I. Such
a correlation is a sequence of the correlation between Cd and microbial production Ph (Pb = 15Cd ) and the coupling of the latter with microbial respiration (Mb = 2.125 P). In accordance with these ratios, the relationship between
Mb and Cd must be as follows: Mb = 15 x 2.67 x 2.125 = 85Cd. The direct comparisons of Cd and M t values in natural waters after Romanenko (1985) gave
their ratio as over 1.5 times as much, but the correlation between them in
experiments was quite good. The above difference might be explained by the
difference between M t and M b • It makes this method quite reliable for widescale monitoring of overall decomposition rates in water columns, because the