Light Penetration into Fresh Water Ii. Light Penetration and Changes in Vegetation Limits in Windermere by W. H. Pearsall and T. Hewitt

DURING measurements of the penetration of light into water (4), evidence was obtained which suggested that the water of Windermere was less transparent in 1932 than it had formerly been. There were also indications that the depth distribution of vegetation in this lake had changed. It was, therefore, decided to repeat some measurements of light penetration and vegetation distribution which had been made in 1919-20 and in this way to obtain evidence of the apparent changes. The earlier measurements of light penetration were made by the iodide-sulphuric acid method, in which the light intensity is measured by estimating the amount of iodine liberated when a solution of potassium iodide mixed with sulphuric acid is exposed to the light. The same procedure was employed in 1932 as in the earlier experiments. Twenty cubic centimetres of 2 per cent, potassium iodide were mixed with an equal volume of 1-15 per cent, sulphuric acid (by weight) and were then exposed to light. After exposure, the iodine liberated was titrated with freshly prepared and standardised potassium thiosulphate, Njioo or JV/1000 depending on the amount of iodine liberated. The solutions were exposed in conical flasks fastened by wire to a weighted line at fixed depths. The line was suspended from a small buoy, which had little or no shading effect at 1 m. The measurements were normally made in duplicate or triplicate at each depth, so that an average could be taken. Normally the solutions were exposed from 6 to 8 hours. The full light intensity was measured by exposing a series of successive similar solutions for from 1 to 2 hours each, during the whole time the submerged solutions were exposed. Two samples were kept in the dark, since the solution degenerates slightly in the dark during warm weather. The values for these dark controls were subtracted from all light readings. The results for sub-aqueous intensities were then expressed as percentages of the values for full daylight.