Use of the Radiocarbon Method to Determine Primary Production
85
mg C cpm- l . This is the basic value needed for calculation of photosynthesis
rates according to the radioactivity measured in plant tissues (see Sect. 2.5.1).
2.5.4 Incuhation Procedures
During investigation of primary production of benthic hydrophytes and their
associations, two basic approaches have prevailed. One can be named the
transferred-in situ approach. Experimental vessels with specimens of
hydrophytes or isolated associations of periphyton and microphytobenthos
are incubated on the underwater platforms or at other sites at the bottom. The
alternative approach, which can be named native-in situ, means real in situ
incubations of the plant object listed above isolated within special enclosures
in their habitat at the bottom. The third possible approach, the incubation of
plant objects in the laboratory under artificial or simulated in situ light, is
employed mostly for physiological research in general, and not only for
primary production determinations.
Each of the two above field approaches has its own advantages and disadvantages. In the first approach, experiments are carried out in closed vessels,
which guarantees the stability of Cr during incubation. The vessels containing
experimental objects, being kept on an underwater platform, are very convenient for the various manipulations which are usually needed during the experiments, like injection and mixing of radioisotopes, stirring or pumping of water.
Finally, in this case, the radioisotope solution may be added and well mixed
within the vessel before the plant object is put in. A definite disadvantage is
that the plant objects needed for experiments must be taken out from their
habitat, and may be damaged or disturbed during this procedure. So when
using this approach it is necessary to minimize possible disturbance for the
experimental plants. It is important that only with this approach is it possible
to measure photosynthesis in benthic hydrophytes on the depth profiles along
the slopes. The alternative native-in situ approach provides minimal disturbing impact on plant objects and maximal coincidence with the in situ parameters of light and temperature. It does not even stop nutrient exchange on
the water-sediment boundary; but the main manipulations like stirring, injection of radioisotopes, subsequent rapid mixing, and also the high possibility of
losses of radioactivity (R;) due to exposure, make this technique much more
complicated and subject to errors. The best decision is to employ the first
approach (transferred-in situ) as routine, and the second (native-in situ) as
experimental, to control the data obtained during the routine estimations.
a) Transferred-in situ experiments
The plant specimens needed for the experiments are best collected in the afternoon of the previous day, in order not to lose the morning hours, which are
best for incubation. To collect the plant samples in shallows, a special raft may
85
mg C cpm- l . This is the basic value needed for calculation of photosynthesis
rates according to the radioactivity measured in plant tissues (see Sect. 2.5.1).
2.5.4 Incuhation Procedures
During investigation of primary production of benthic hydrophytes and their
associations, two basic approaches have prevailed. One can be named the
transferred-in situ approach. Experimental vessels with specimens of
hydrophytes or isolated associations of periphyton and microphytobenthos
are incubated on the underwater platforms or at other sites at the bottom. The
alternative approach, which can be named native-in situ, means real in situ
incubations of the plant object listed above isolated within special enclosures
in their habitat at the bottom. The third possible approach, the incubation of
plant objects in the laboratory under artificial or simulated in situ light, is
employed mostly for physiological research in general, and not only for
primary production determinations.
Each of the two above field approaches has its own advantages and disadvantages. In the first approach, experiments are carried out in closed vessels,
which guarantees the stability of Cr during incubation. The vessels containing
experimental objects, being kept on an underwater platform, are very convenient for the various manipulations which are usually needed during the experiments, like injection and mixing of radioisotopes, stirring or pumping of water.
Finally, in this case, the radioisotope solution may be added and well mixed
within the vessel before the plant object is put in. A definite disadvantage is
that the plant objects needed for experiments must be taken out from their
habitat, and may be damaged or disturbed during this procedure. So when
using this approach it is necessary to minimize possible disturbance for the
experimental plants. It is important that only with this approach is it possible
to measure photosynthesis in benthic hydrophytes on the depth profiles along
the slopes. The alternative native-in situ approach provides minimal disturbing impact on plant objects and maximal coincidence with the in situ parameters of light and temperature. It does not even stop nutrient exchange on
the water-sediment boundary; but the main manipulations like stirring, injection of radioisotopes, subsequent rapid mixing, and also the high possibility of
losses of radioactivity (R;) due to exposure, make this technique much more
complicated and subject to errors. The best decision is to employ the first
approach (transferred-in situ) as routine, and the second (native-in situ) as
experimental, to control the data obtained during the routine estimations.
a) Transferred-in situ experiments
The plant specimens needed for the experiments are best collected in the afternoon of the previous day, in order not to lose the morning hours, which are
best for incubation. To collect the plant samples in shallows, a special raft may