76
The Radiocarbon Method to Estimate Primary Production
synthesis of glycollate and its subsequent respiratory oxidation. Photorespiration generally inhibits photosynthesis during midday, being also stimulated by
the afternoon oxygen oversaturation. It obviously distorts the results of gross
photosynthesis measurements made with the aid of the dark-light oxygen
method. Indeed, in this case, respiration proceeds differently in the light and
III the dark vessels, being accelerateJ in the light une~ under thc influencc of
photorespiration. Thus, the dark respiration control during daytime exposures,
and especially during afternoon incubation, appears to be invalid. The use of
oxygen methodology to estimate photosynthesis rates in the benthic association isolated in situ within the enclosure together with the bottom area, also
gives hardly interpretable results because a larger share in the oxygen consumption (or CO2 production) in this case belongs to processes other than
plant metabolism, e.g., respiration of heterotrophs or its chemical consumption by sulfide-containing sediments.
Definite problems are created by the formation of gas bubbles on the walls
of experimental vessels under conditions of oxygen oversaturation at high
summer water temperatures. Finally, mistakes in estimating photosynthesis of
individual benthic vascular plants by the oxygen bottle method can be caused
by the permanent presence on their thalli or leaves of epiphytic and periphytonic overgrowth. The photosynthesis rate of epiphytes can often be even more
than that of their hosts (Cattaneo and Kalff 1980; Littler and Arnold 1985).
2.5.1 General Principles in the Use of 14C-Methods
The above-mentioned obstacles to using oxygen methods for measuring
primary production by benthic hydrophytes decidedly supported the introduction of the alternative radiocarbon method after it appeared in 1952. This
method avoids all these problems. It measures a real rate of external CO2
uptake by benthic plants. The experiments in this case may be performed
without problems of prefiltration of water, oxygen oversaturation, presence of
air bubbles in the enclosures, sulfides in sediments, or epiphytes. The experiments can be carried out even in open aquaria or in situ in open enclosures
without upper walls or lids. The possible exchange of 14C02 with air in the
water contained in an open aquarium, and the corresponding decrease in T
CO2 specific radioactivity most often is usually undetectable for the several
hours of the experiment, and in any case, can be easily controlled. Epiphytes
can be avoided mechanically or washed out from the surface of thalli at the
end of incubation with 14C02 (Hatcher 1977; Arnold 1980). The radiocarbon
method has its intrinsic problems connected with correct radio assay of plant
tissue samples, and photorespiratory and exudation losses of 14C-assimilates
by benthic plants. The problems in using the 14C-technique for measuring
primary production in benthic plants and their associations were reviewed and
discussed by Wetzel (1964a,b, 1965, 1974), Gargas (1970); Wetzel and Hough
(1973), Marshall et al (1973), and Arnold and Littler (1985).
The Radiocarbon Method to Estimate Primary Production
synthesis of glycollate and its subsequent respiratory oxidation. Photorespiration generally inhibits photosynthesis during midday, being also stimulated by
the afternoon oxygen oversaturation. It obviously distorts the results of gross
photosynthesis measurements made with the aid of the dark-light oxygen
method. Indeed, in this case, respiration proceeds differently in the light and
III the dark vessels, being accelerateJ in the light une~ under thc influencc of
photorespiration. Thus, the dark respiration control during daytime exposures,
and especially during afternoon incubation, appears to be invalid. The use of
oxygen methodology to estimate photosynthesis rates in the benthic association isolated in situ within the enclosure together with the bottom area, also
gives hardly interpretable results because a larger share in the oxygen consumption (or CO2 production) in this case belongs to processes other than
plant metabolism, e.g., respiration of heterotrophs or its chemical consumption by sulfide-containing sediments.
Definite problems are created by the formation of gas bubbles on the walls
of experimental vessels under conditions of oxygen oversaturation at high
summer water temperatures. Finally, mistakes in estimating photosynthesis of
individual benthic vascular plants by the oxygen bottle method can be caused
by the permanent presence on their thalli or leaves of epiphytic and periphytonic overgrowth. The photosynthesis rate of epiphytes can often be even more
than that of their hosts (Cattaneo and Kalff 1980; Littler and Arnold 1985).
2.5.1 General Principles in the Use of 14C-Methods
The above-mentioned obstacles to using oxygen methods for measuring
primary production by benthic hydrophytes decidedly supported the introduction of the alternative radiocarbon method after it appeared in 1952. This
method avoids all these problems. It measures a real rate of external CO2
uptake by benthic plants. The experiments in this case may be performed
without problems of prefiltration of water, oxygen oversaturation, presence of
air bubbles in the enclosures, sulfides in sediments, or epiphytes. The experiments can be carried out even in open aquaria or in situ in open enclosures
without upper walls or lids. The possible exchange of 14C02 with air in the
water contained in an open aquarium, and the corresponding decrease in T
CO2 specific radioactivity most often is usually undetectable for the several
hours of the experiment, and in any case, can be easily controlled. Epiphytes
can be avoided mechanically or washed out from the surface of thalli at the
end of incubation with 14C02 (Hatcher 1977; Arnold 1980). The radiocarbon
method has its intrinsic problems connected with correct radio assay of plant
tissue samples, and photorespiratory and exudation losses of 14C-assimilates
by benthic plants. The problems in using the 14C-technique for measuring
primary production in benthic plants and their associations were reviewed and
discussed by Wetzel (1964a,b, 1965, 1974), Gargas (1970); Wetzel and Hough
(1973), Marshall et al (1973), and Arnold and Littler (1985).