A study is described of the factors affecting measurcmcnts of phytoplankton photosynthesis by the much used field method, based upon changes of oxygen concentration in clear and darken&l bottles as dctcrmined by the Winkler method. Unialgal cultures of a marine plankton diatom, Chndtoceras nffinis, were used with short exposllres of 1-3 hours in both laboratory and sea. Indications of ,i limitation of photosynthetic rate by inadcquatc CO2 supply were only found for culture media with pH raised by prolonged photosynthesis. Otherwise photosynthetic rates wcrc unaltcrcd by the bicarbonate concentration over a wide range, including that of uncnrichcd sea-water, or by ccl1 sedimentation in unshaken cultures. A comparison of the Winkler and manometric methods of measuring oxygen production gave very similar results. High rates of photosynthesis were well maintained over the main period of each exposure, despite the development of oxygen supersaturation.
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