The results are plotted from an experiment performed in March 1983 on Lago di
Mergozzo waters to evaluate the possibility of directly measuring the variations in DOC
concentration caused by the autotrophic - heterotrophic coupling. A 20 1 lake water
sample was taken in the morning (10 a.m.) at a depth of 2 m and immediately filtered
through a 126 µm mesh plankton net to remove larger filter feeders whilst not noticeably
altering the majority of the algal population. The sample was then incubated at lake
temperature in a 12 hours light-dark cycle. An on-line filter (Whatman GF/ C precombusted) placed between the incubation bottle and the analytical instrument made it possible
to continuously measure the DOC concentration in the sample during the incubation
time, running 10 analyses per hour. Although only the results from the first 4 days of
incubation are shown here, the experiment lasted for 7 days. The microcosm collapsed on
the 6th day and no significant variations in DOC concentration occurred after this time. It
is not the purpose of this paper to undertake a detailed discussion about autotrophheterotroph coupling. Here I want only to stress that, at least in a mesotrophic water
body, the influence of such coupling on DOC concentration is directly measurable.
CONCLUSIONS AND OUTLOOK
The OC consumption rates estimated from the direct measurements which were performed in Lago Maggiore turned out to be higher by a factor of 10 compared to the rates
measured in the same water body with labelled organic substrate uptake method (Melchiorri et al., 1975 ; Bertoni, 1983). The rates obtained by the direct measurements are of
the same order as those measured in German lakes with the dark assimilation method
(Overbeck, 1981) and from sedimentation trap data (Ohle, 1976). Furthermore a direct
estimation of heterotrophic uptake of natural DOC at three stations in the North Atlantic
revealed an average rate of 9.8 µgC/l.h (Sieburth et al., 1977). These findings would
support the consideration (Overbeck, 1983) that the heterotrophic activity evaluated by
labelled organic substrate uptake method is an underestimation of the whole heterotrophic activity in an ecosystem. If this is true, the heterotrophic utilization of the pool of
natural organic substrate may be large enough to produce, in a few hours, variations of
the substrate concentration which are large enough to be measured by the more recent
analytical instrument both in lake and sea water.
The direct measurement of the heterotrophic activity by microanalytical methods, of
course, also suffers from some drawbacks. In particular I would like to mention the longer
incubation time required compared to the radiochemical methods. This can be overcome
by avoiding the incubation itself by using an automatic sampler that, during a preset
period of time, takes samples at hourly intervals during the night, fixing them immediately (Bertoni, 1985). If previous knowledge of the hydrodynamics or the use of a free
floating buoy guarantees that always the same water mass is sampled, the series of
samples collected by such an instrument can permit the construction of an OC concentration variation curve from which the heterotrophic activity in the water mass can be
estimated. If that instrument is suspended in the euphotic layer to take samples during a
night-day cycle, the variations in OC concentration owing to the autotroph-heterotroph
coupling can also be studied. Sampling an open system by using the above mentioned
instrument presents obvious advantages compared to the closed incubation systems.
ACS Committee and Subcommittee, 1980. Guidelines for data acquisition and data quality evaluation in
environmental chemistry. Anal. Chem., 52: 2242-2249.
107
Mergozzo waters to evaluate the possibility of directly measuring the variations in DOC
concentration caused by the autotrophic - heterotrophic coupling. A 20 1 lake water
sample was taken in the morning (10 a.m.) at a depth of 2 m and immediately filtered
through a 126 µm mesh plankton net to remove larger filter feeders whilst not noticeably
altering the majority of the algal population. The sample was then incubated at lake
temperature in a 12 hours light-dark cycle. An on-line filter (Whatman GF/ C precombusted) placed between the incubation bottle and the analytical instrument made it possible
to continuously measure the DOC concentration in the sample during the incubation
time, running 10 analyses per hour. Although only the results from the first 4 days of
incubation are shown here, the experiment lasted for 7 days. The microcosm collapsed on
the 6th day and no significant variations in DOC concentration occurred after this time. It
is not the purpose of this paper to undertake a detailed discussion about autotrophheterotroph coupling. Here I want only to stress that, at least in a mesotrophic water
body, the influence of such coupling on DOC concentration is directly measurable.
CONCLUSIONS AND OUTLOOK
The OC consumption rates estimated from the direct measurements which were performed in Lago Maggiore turned out to be higher by a factor of 10 compared to the rates
measured in the same water body with labelled organic substrate uptake method (Melchiorri et al., 1975 ; Bertoni, 1983). The rates obtained by the direct measurements are of
the same order as those measured in German lakes with the dark assimilation method
(Overbeck, 1981) and from sedimentation trap data (Ohle, 1976). Furthermore a direct
estimation of heterotrophic uptake of natural DOC at three stations in the North Atlantic
revealed an average rate of 9.8 µgC/l.h (Sieburth et al., 1977). These findings would
support the consideration (Overbeck, 1983) that the heterotrophic activity evaluated by
labelled organic substrate uptake method is an underestimation of the whole heterotrophic activity in an ecosystem. If this is true, the heterotrophic utilization of the pool of
natural organic substrate may be large enough to produce, in a few hours, variations of
the substrate concentration which are large enough to be measured by the more recent
analytical instrument both in lake and sea water.
The direct measurement of the heterotrophic activity by microanalytical methods, of
course, also suffers from some drawbacks. In particular I would like to mention the longer
incubation time required compared to the radiochemical methods. This can be overcome
by avoiding the incubation itself by using an automatic sampler that, during a preset
period of time, takes samples at hourly intervals during the night, fixing them immediately (Bertoni, 1985). If previous knowledge of the hydrodynamics or the use of a free
floating buoy guarantees that always the same water mass is sampled, the series of
samples collected by such an instrument can permit the construction of an OC concentration variation curve from which the heterotrophic activity in the water mass can be
estimated. If that instrument is suspended in the euphotic layer to take samples during a
night-day cycle, the variations in OC concentration owing to the autotroph-heterotroph
coupling can also be studied. Sampling an open system by using the above mentioned
instrument presents obvious advantages compared to the closed incubation systems.
ACS Committee and Subcommittee, 1980. Guidelines for data acquisition and data quality evaluation in
environmental chemistry. Anal. Chem., 52: 2242-2249.
107