The Benthic Algae of Some Streams in Southern England: II. The Primary Production of the Epilithon in a Small Chalk-Stream

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Abstract
The primary production of epilithic algae in a small hard-water stream was investigated. Natural flints and gravel were placed in photosynthesis chambers and rates of photosynthesis estimated by measuring changes in dissolved O2. The construction of the chamber and its use for routine in situ field experiments are described. An inverse relationship exists between the specific rate of gross photosynthesis [mg O2 (mg chlorophyll a)-h-1] and biomass (mg m-2 chlorophyll a). The highest rates were observed in May and June when the specific rates varied from 2.72-5.65 mg O2 (mg chlorophyll a)-1h-1 between algal densities of 79.8 and 28.2 mg m-2 chlorophyll a, respectively. Maximum rates of photosynthesis were found in April 1970 (about 500 mg m-2h-1O2), corresponding to the observed maximum biomass. Minimum rates occurred between Nov. and Jan. (< 30 mg m-2hO2). Experiments indicated that irradiance may have been limiting photosynthetic rates during winter and spring. Diurnal variations in photosynthetic activity were investigated. No evidence was obtained for a post-noon suppression of photosynthesis. Annual net production of epilithic algae was calculated from gross photosynthetic rates and amounted to 76 g organic matter/m2 of river bed in 1969 and 124 g in 1970. The problems arising from the difficulties of measuring algal respiration are discussed.

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