FURTHER MEASUREMENTS OF PRIMARY PRODUCTION USING A LARGE‐VOLUME PLASTIC SPHERE

Abstract

The experiment described by McAllister, et al. (1961), in which a phytoplankton bloom was induced to occur in a free‐floating 20‐ft diameter thin transparent plastic sphere has been repeated. Daily measurements were made of nutrients, particulate matter, and photosynthetic rates with less frequent assays for vitamins and dissolved organic matter. In situ light was recorded by a bolometer. The experiment was prolonged to 100 days to study phytoplankton decay, most of this period being in the dark. The phytoplankton consisted mainly of 6 species of diatom and one large dinoflagellate. The mean composition of this crop at various stages of its development is reported by ratios involving chlorophyll a and particulate organic carbon. A detailed discussion is given of the findings of the experiment which, in general, confirmed those of the earlier work and yielded, in addition, valuable new information. The plant cells excreted 35–40% of their organic matter during growth. The C 14 method of measuring photosynthesis gave results agreeing well with the production of particulate carbon. The growth kinetics of the bloom were dominated by the constancy of the mean cell division rates which were relatively independent of temperature and light. The rate of photosynthesis per unit chlorophyll was also remarkably constant and not proportional to light intensity, cells developing a highly efficient photosynthetic mechanism with respect to available radiant energy. The mean chemical composition of a cell changed after depletion of nutrients from the surrounding water. The carbon, protein, chlorophyll a and phosphorus decreased. The silicon and lipid contents remained nearly constant and the carbohydrate increased. During the decay period over half the particulate phosphorus was remineralized in 2 weeks. Silicon returned to solution more slowly but at a constant rate. There was no significant nitrification even after 75 days. The consumption of oxygen occurred mainly from the oxidation of “dissolved” organic matter and not from the interaction of oxygen with particulate material. The latter may have been important as a surface for bacteria. The evolution rate of carbon dioxide was constant with time but proceeded with a variable RQ, which was around 0.5 immediately after the bloom but increased to nearly 2 in the “old” water present at the beginning and end of the experiment.