Limnological Studies of Western Lake Erie: II. Light Penetration and Its Relation to Turbidity

Abstract

Changes in transparency of natural waters may be far more important in determining the quantity of light at a given depth than seasonal changes in the illumination reaching the surface of the water. Clarke ('38) has shown that in certain Atlantic coastal waters the seasonal variation in illumination at the surface resulted in a 2-fold difference in intensity of submarine light at 30 m., while a seasonal change in transparency caused a 1000-fold change at the same depth. If variations in transparency of water produce tremendous differences in the quantity of light reaching a given depth it is important that the causes and effects of these variations in transparency be investigated. This information is of particular value for bodies of water in which yearround ecological studies are being made, because a slight change in transparency may conceivably alter the depth of the photic zone and affect the activities of microcrustacea, fish and other aquatic organisms. The fact that the shallow water of western Lake Erie undergoes sudden and extreme changes in turbidity makes this particular study of considerable interest. The present report is concerned with variations in turbidity of the water of western Lake Erie and the effect of these variations on the depth to which one per cent of the surface light penetrates. In this investigation year-round observations were made of: (1) turbidity, (2) light penetration, (3) amount of organic and inorganic suspended matter, and (4) quantity of phytoplankton. The study extended from Sept., 1939, through Oct., 1940, during which time several observations were made each month. The writer is indebted to Leonard J. Bodenlos for field assistance and for the quantitative data pertaining to suspended materials, and to other associates who assisted from time to time.