The Liberation and Utilisation of Oxygen By the Population of Rock-Pools

The present paper shows that in rock-pools containing a good growth of seaweed the oxygen content rises markedly during low water in the daytime, as a result of photosynthesis, but falls equally definitely at night in the absence of the latter. Our highest daytime record, in a pool containing a very dense growth of algae, was 26·2 mg. per litre; the lowest night record in the same pool being 1·2 mg. (The value for the open sea at the same place may be taken as about 8·7 mg.) In a pool containing animals but no appreciable growth of plants, the oxygen content falls both in daylight and in darkness; our lowest record for such a pool being 1·1 mg. per litre (at night). No appreciable difference was observed between the oxygen values obtained on moonless and on moonlight nights. The pH in the pools behaves in accordance with the amount of CO2 liberated in respiration or utilised in photosynthesis (though these are not the only factors which determine it), and rises to 9·0 or higher during strong photosynthesis, falling well below 8·0 in the absence of the latter. The populations of three of the pools used are described quantitatively, for correlation with the oxygen data.