SUMMARYMeasurements were made, throughout the year 1928, of the penetration of daylight into sea-water in the English Channel. One vacuum and two gas-filled potassium photo-electric cells were used below water. In all save the first series an efficient diffusing surface was used with the photometers so that the corrections for obliquity of illumination, necessary in our earlier work, were rendered almost negligible. From October onwards a more sensitive gas-filled cell was available for use, together with a more powerful amplifier and a more efficient insulation of the high-tension batteries; improvements in the switching of the photometers on and off were also incorporated.The following conclusions have been reached :—1. The conclusions put forward tentatively in 1928, concerning the agreement between the vacuum and gas-filled cells, have been substantiated in the main, although the vacuum photometer, which is relatively more sensitive to the blue end of the spectrum, generally indicated rather higher opacities.2. There are no regular seasonal changes in the opacity of the water. A very high opacity occurred in the autumn of 1927, but this was not repeated in 1928.3. On one occasion, in April 1928, an unusual degree of clearness of the water has been shown to have been associated with the influx of water of lesser salinity, from further out in the Channel. The clearness of the water at El in December, 1927, and September, 1928, is probably due to an influx of clearer water, though the salinity changes indicating this are not so well marked.
1. A description is given of some measurements of the penetration of light into sea-water carried out near Plymouth in the autumn of 1925 by means of photo-electric cells, the current through the cell being, measured by a method, previously described, which is not affected by the motion of the ship.2. The absorption coefficients found for off-shore waters down to 30 metres are smaller than the average of those found by several recent observers in various localities, but considerably larger than those found by Shelford and Gail for the deeper waters of Puget Sound. The coefficients tend.to increase with increase of depth, and are generally larger for in-shore waters. The clearest water, 20 miles out in the English Channel, gave an absorption coefficient 0-110 for the upper 10 metres, 0-117 for the second, and 0-133 for the third. This water, with glassy surface, transmitted 0-54% of the vertical illumination to 34-8 metres, 28-3% to 8-9 metres, at which depth a white disc was just visible, and 71-2% to 1-5 metres.
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.
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