. Animal Ecology. Animal ecology. FIG. 6-1 Cyclomorphosis of Daphnia retrocurva In a Connecticut lake (from Brooks 1946). be an adaptation to the decreased buoyancy of the water at this season, but factors other than temper- ature also appear to be involved (Brooks 1946). Light The daily alternation of light and darkness es- tablishes a rhythm in the activities of many aquatic organisms. Light is essential to plant photosynthesis ; some fish require light by which to feed. Many or- ganisms orient to light, and some are sensitive to light of particular wavelengths, notably ultraviolet. Small, s
. Animal Ecology. Animal ecology. FIG. 6-1 Cyclomorphosis of Daphnia retrocurva In a Connecticut lake (from Brooks 1946). be an adaptation to the decreased buoyancy of the water at this season, but factors other than temper- ature also appear to be involved (Brooks 1946). Light The daily alternation of light and darkness es- tablishes a rhythm in the activities of many aquatic organisms. Light is essential to plant photosynthesis ; some fish require light by which to feed. Many or- ganisms orient to light, and some are sensitive to light of particular wavelengths, notably ultraviolet. Small, soft-bodied, bottom-dwelling organisms are particularly sensitive to light, and it is thought that the evolution of pigmentation, chitinous exoskeletons, shells, cases, and similar structures may have helped certain otherwise photosensitive species to survive in shallow, well-lighted areas (Welch 1952). A common way to measure the relative transpar- ency of water is to lower a Secchi disk, a white plate 20 cm in diameter attached to a cord marked off in linear units, marking the depth at which the disk dis- appears from sight. The disk is lowered a bit farther, then raised until it reappears, and that depth marked. The two depths are averaged. The light intensity at the depth of disappearance of the disk is usually about 5 per cent of that at the surface (Hutchinson 1957). Other more exact procedures employ photographic methods, pyrlimnometers, or photoelectric cells (Shelford 1929). The depth to which light penetrates into water is affected by intensity of the light, angle of ray in- cidence, reflection at the surface, scattering within the water, and absorption. Penetration anywhere is re- duced when the sun is away from the zenith; is less in waters at high latitudes ; and is much less in winter compared with summer. About 10 per cent of the light falling on Lake Mendota, Wisconsin, during the spring and summer is reflected; about 15 per cent during the autumn (Juday 1940). I
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