with sea water, although I have not tested such samples, I believe that it would be
impractical to use the high temperature oxidation unit with sea water because the salt
content strongly reduces the life span of oxidation catalysts and quickly cloggs the sample
injection system. The U.V. oxidation unit, on the other hand, appears to be more useful
for salt water, although a lack of precision was observed (Chioetto, pers. comm.).
MEASUREMENT OF HETEROTROPHIC ACTIVITY
The direct measurements of heterotrophic consumption of naturally occurring OC have
been performed on samples of 10 1 always collected between 9 and 10 a.m. with a plastic
Van Dorn type bottle. They were immediately filtered through 10µm mesh plankton net
to remove larger heterotrophs and larger particles and were incubated in the dark at lake
temperature in glass bottles under gentle stirring. The OC concentration in the samples
was measured continuously, running 10 analyses per hour.
From previous experiments, performed in the above described way on lake water samples
both untreated and treated with mercuric chloride, it was possible to conclude that the
consumption of naturally occurring organic substrate (particulate OC with size < 10 µm
and dissolved OC) can be ascribed exclusively to the metabolic activity of heterotrophs
while no measurable physico-chemical losses of organic matter took place (Bertoni et al.,
1982). Such experiments lasted for three days or more, to follow the pattern of heterotrophic OC consumption. However, in this paper I will consider only the OC decrease
measured over 12 hours of incubation. This period of time is generally long enough to
obtain a significant OC decrease and it is short enough not to change natural conditions
for microbial life and activity. Thus results of OC consumption may be directly related to
naturally occurring events.
The experiments chosen to exemplify this kind of measurement were performed on
samples taken from the euphotic layer of Lago Maggiore and of Lago di Mergozzo.
General information on these lakes is given by de Bernardi et al. (1984) ; a study on the OC
cycle in Lago Maggiore has been published by Bertoni and Callieri (1982). The results of
the experiments are presented in Figures 2 and 3. The average values of 10 analyses for
each hour of incubation are plotted with their 95 % confidence limits. The overall decrease
of OC is 140 µgC/1 in Lago Maggiore and 603 µgC/1 in Lago di Mergozzo. As one might
M9 C/I
Figure 2 : Decrease in Organic Carbon concentration during the dark incubation of a lake water
sample from Lago Maggiore.
105
impractical to use the high temperature oxidation unit with sea water because the salt
content strongly reduces the life span of oxidation catalysts and quickly cloggs the sample
injection system. The U.V. oxidation unit, on the other hand, appears to be more useful
for salt water, although a lack of precision was observed (Chioetto, pers. comm.).
MEASUREMENT OF HETEROTROPHIC ACTIVITY
The direct measurements of heterotrophic consumption of naturally occurring OC have
been performed on samples of 10 1 always collected between 9 and 10 a.m. with a plastic
Van Dorn type bottle. They were immediately filtered through 10µm mesh plankton net
to remove larger heterotrophs and larger particles and were incubated in the dark at lake
temperature in glass bottles under gentle stirring. The OC concentration in the samples
was measured continuously, running 10 analyses per hour.
From previous experiments, performed in the above described way on lake water samples
both untreated and treated with mercuric chloride, it was possible to conclude that the
consumption of naturally occurring organic substrate (particulate OC with size < 10 µm
and dissolved OC) can be ascribed exclusively to the metabolic activity of heterotrophs
while no measurable physico-chemical losses of organic matter took place (Bertoni et al.,
1982). Such experiments lasted for three days or more, to follow the pattern of heterotrophic OC consumption. However, in this paper I will consider only the OC decrease
measured over 12 hours of incubation. This period of time is generally long enough to
obtain a significant OC decrease and it is short enough not to change natural conditions
for microbial life and activity. Thus results of OC consumption may be directly related to
naturally occurring events.
The experiments chosen to exemplify this kind of measurement were performed on
samples taken from the euphotic layer of Lago Maggiore and of Lago di Mergozzo.
General information on these lakes is given by de Bernardi et al. (1984) ; a study on the OC
cycle in Lago Maggiore has been published by Bertoni and Callieri (1982). The results of
the experiments are presented in Figures 2 and 3. The average values of 10 analyses for
each hour of incubation are plotted with their 95 % confidence limits. The overall decrease
of OC is 140 µgC/1 in Lago Maggiore and 603 µgC/1 in Lago di Mergozzo. As one might
M9 C/I
Figure 2 : Decrease in Organic Carbon concentration during the dark incubation of a lake water
sample from Lago Maggiore.
105