V. The photo-electric measurement of the penetration of light of various wave-lengths into the sea and the physiological bearing of the results

Abstract

The chemical examination of sea water had shown that, as the spring advanced, the surface water became more alkaline and its phosphate content much reduced. These changes were shown to arise from the action of the phytoplankton, and the fact that they were, in these latitudes, mainly limited to the upper fifteen or twenty metres suggested that lack of light hindered growth at greater depths. It was natural to seek for quantitative information on this subject, and as alternatives the photographic and photo-electric methods presented themselves. The former had been used to a considerable extent, notably by Grein (1913, 1914). It is specially suited for detecting very feeble illumination and for showing the depths to which light of various colours can penetrate. The difficulties and limitations of the method are obvious when quantitative results are required at lesser depths with relatively intense light, such as is wanted for photosynthetic processes to be carried on at a rate great enough to preponderate over respiration. Not the least of such difficulties is imposed by the condition that the light of the sky, including the sun if uncovered, is normally very variable, and that the surface of the sea is rarely at rest. Furthermore, the obtaining of a large number of measurements necessitates the exposure and development of many plates, and becomes very tedious. Moreover, the transmissive exponents calculated from Grein’s results are highly irregular.