two brine flies, Ephydra gracilis and E. hians. There 

 are no fish in the Great Salt Lake of Utah (Wood- 

 bury 1936). 



By the law of Archimedes, the buoyancy of an 

 object is equal to the weight of the water it dis- 

 places. Buoyancy varies with the density of water, 

 and is influenced by the factors that affect density. 

 Viscosity, the measure of the internal friction of 

 water, varies inversely with temperature and also in- 

 fluences buoyancy. 



An organism will sink unless it keeps station by 

 swimming movements, or unless it has special adap- 

 tations to decrease the specific gravity of the body 

 and take advantage of any turbulence in the water. 

 Such adaptations take several forms : absorption of 

 large amounts of water to form jelly-like tissues; 

 storage of gas or air bubbles within the body : forma- 

 tion of lightweight fat deposits within the body, or 

 oil droplets within the cell ; increase of surface area 

 in proportion to body mass, which increases frictional 

 resistance (Davis 1955). When an organism so 

 equipped dies, the special mechanisms quickly cease 

 to function, and it sinks to the bottom. If dead or- 

 ganisms did not sink to the bottom, living organisms, 

 with the exception of some bacteria, could not exist in 

 an aquatic habitat. 



An interesting phenomenon is cyclomorphosis. a 

 seasonal change in body form that develops in many 

 plankton organisms, both plant and animal, including 

 protozoans, cladocerans, and rotifers. In general, the 

 summer generations have higher crests, longer spines, 

 longer beaks, or longer stalks, than do the winter gen- 

 erations. It is believed that the increased surface area 

 provided in the summer forms is induced by the 

 higher water temperatures obtaining then, and may 



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 1 5 per cent 

 during the autumn (Juday 1940). In the unusually 

 clear waters of Crystal Lake, Wisconsin, measure- 

 ments with a pyrlimnometer indicated only a small 

 surface reflection light loss, a penetration of 67 per 

 cent of full intensity to a depth of one meter, and 

 10.5 per cent of full intensity at 10 meters (Birge 

 and Juday 1929). In pure water, red light is ab- 

 sorbed most rapidly, at a rate of 64.5 per cent per 

 meter : orange, at 23.5 per cent per meter ; yellow, at 

 3.9 per cent ; green, at 1.1 per cent ; blue at only 0.52 

 per cent. Blue penetrates the farthest. Violet is ab- 

 sorbed at 1.63 per cent per meter. Very little ultra- 

 violet penetrates the water, and nearly all the infra- 

 red is absorbed in the first meter (Clarke 1939, 

 Ruttner 1953). 



Suspended material in water produces turbidity, 

 and reduces light penetration. In western Lake Erie, 



60 Habitats, communities, succession 



