Use of the Radiocarbon Method to Determine Primary Production
75
and end of incubation. Then the corresponding rates of net and gross photosynthesis are calculated. The rates of photosynthesis production in the benthic
plants appeared to be dependent on the water motion (Westlake 1967; Littler
and Arnold 1985). Therefore various technical accessories are often employed
in the experimental vessels to induce mixing of water in them, such as stirrers
or peristaltic pumps (Kanwisher 1966; Westlake 1967; Littler 1979; Heine 1983;
Propp et al. 1983; Chabardji et al. 1988). The problems occurring with the use
of the oxygen method to measure the photosynthesis rate in benthic plants
have been reviewed (Winberg 1960; Wetzel 1964a,b; 1965; Vollenweider 1974;
Littler and Littler 1985).
Another approach to estimate primary production in whole-plant communities is to calculate if the measured ranges of diurnal O 2 or total CO2
fluctuations in open water with or without accounting for its exchange with
the atmosphere (Odum and Hoskin 1958; Verduin 1960; Winberg 1960; Teal
and Kanwisher 1966; Kinsey and Domm 1974; Vollenveider 1974; Littler and
Littler 1985; Sorokin 1993). This approach gives only a very rough approximation of the daytime photosynthesis values and of nocturnal respiration in
benthic plant communities, and it was rightly criticized (Wetzel 1965); but its
use is strongly recommended in shallow basins or biotopes, I-2m deep, dominated by benthic plant associations, in conjunction with parallel experimental estimation of photosynthesis rates in basic elements of these associations.
This methodology gives quite safe results in biotopes characterized by a high
density of benthic plants and by large ranges of diurnal oxygen or CO2 fluctuations, ca. 50-150% oxygen saturation or 10 to 30% T CO2 content in the
water. In this case, even the gas exchange with the atmosphere becomes a
factor of secondary importance during calculations of the in situ metabolic
rates.
Returning to the evaluation of the experimental dark-light enclosures
method, it is important to know that it also has specific problems which
decrease its reliability (Wetzel 1964a,b; 1965). One of the most probable
sources of error with this method is the ability of most vascular hydrophytes
to store a significant stock of oxygen within their lacunar system, not excreting it directly to the surrounding aquatic medium (Hartman and Brown 1967).
So the rate of oxygen diffusion from thalli of hydrophytes out to the water
column is not obviously proportional to the photosynthesis production in them
(Wetzel 1964a,b; Littler and Arnold 1985). Some amounts of the oxygen stored
in the lacunes inside the thalli are also used by plants for respiration. Both
these features in hydrophytes may entail large underestimations of photosynthesis and respiration when using the oxygen bottle method. Another possible source of error with this method is the acceleration of respiratory
consumption of oxygen in the transparent vessels (in comparison with the dark
ones) under the influence of strong light due to photorespiration, which is
peculiar to hydrophytes (Hough 1974; Tolbert and Osmond 1976; Lloyd 1977).
As a result of this process, the oxygen is consumed and CO2 excreted by the
plant under the influence of strong light, being a consequence of the excess
75
and end of incubation. Then the corresponding rates of net and gross photosynthesis are calculated. The rates of photosynthesis production in the benthic
plants appeared to be dependent on the water motion (Westlake 1967; Littler
and Arnold 1985). Therefore various technical accessories are often employed
in the experimental vessels to induce mixing of water in them, such as stirrers
or peristaltic pumps (Kanwisher 1966; Westlake 1967; Littler 1979; Heine 1983;
Propp et al. 1983; Chabardji et al. 1988). The problems occurring with the use
of the oxygen method to measure the photosynthesis rate in benthic plants
have been reviewed (Winberg 1960; Wetzel 1964a,b; 1965; Vollenweider 1974;
Littler and Littler 1985).
Another approach to estimate primary production in whole-plant communities is to calculate if the measured ranges of diurnal O 2 or total CO2
fluctuations in open water with or without accounting for its exchange with
the atmosphere (Odum and Hoskin 1958; Verduin 1960; Winberg 1960; Teal
and Kanwisher 1966; Kinsey and Domm 1974; Vollenveider 1974; Littler and
Littler 1985; Sorokin 1993). This approach gives only a very rough approximation of the daytime photosynthesis values and of nocturnal respiration in
benthic plant communities, and it was rightly criticized (Wetzel 1965); but its
use is strongly recommended in shallow basins or biotopes, I-2m deep, dominated by benthic plant associations, in conjunction with parallel experimental estimation of photosynthesis rates in basic elements of these associations.
This methodology gives quite safe results in biotopes characterized by a high
density of benthic plants and by large ranges of diurnal oxygen or CO2 fluctuations, ca. 50-150% oxygen saturation or 10 to 30% T CO2 content in the
water. In this case, even the gas exchange with the atmosphere becomes a
factor of secondary importance during calculations of the in situ metabolic
rates.
Returning to the evaluation of the experimental dark-light enclosures
method, it is important to know that it also has specific problems which
decrease its reliability (Wetzel 1964a,b; 1965). One of the most probable
sources of error with this method is the ability of most vascular hydrophytes
to store a significant stock of oxygen within their lacunar system, not excreting it directly to the surrounding aquatic medium (Hartman and Brown 1967).
So the rate of oxygen diffusion from thalli of hydrophytes out to the water
column is not obviously proportional to the photosynthesis production in them
(Wetzel 1964a,b; Littler and Arnold 1985). Some amounts of the oxygen stored
in the lacunes inside the thalli are also used by plants for respiration. Both
these features in hydrophytes may entail large underestimations of photosynthesis and respiration when using the oxygen bottle method. Another possible source of error with this method is the acceleration of respiratory
consumption of oxygen in the transparent vessels (in comparison with the dark
ones) under the influence of strong light due to photorespiration, which is
peculiar to hydrophytes (Hough 1974; Tolbert and Osmond 1976; Lloyd 1977).
As a result of this process, the oxygen is consumed and CO2 excreted by the
plant under the influence of strong light, being a consequence of the excess