seedlings are critical stages in the plant-salinity 

 relationship; plants become more tolerant to salin- 

 ity with age. Adult plants of cattail, hard-stem 

 bulrush, and alkali bulrush can withstand saline 

 solutions of 10, 15, and 18 mmhos conductivity, 

 respectively. 



There is an increasing amount of cultivation of 

 agricultural crops for waterfowl feeding purposes 

 (barley and Bermuda pasture grass) on waterfowl 

 management areas along the lower Colorado River 

 (Land, personal communication). An increasing 

 level of salinity in the river, if the crops are irri- 

 gated with river water, may have a detrimental 

 effect upon this practice. It has been stated that 

 alkali bulrush grows in soil having salinity levels 

 well above the survival range of agricultural crops 

 (Nelson, 1953). Therefore, though natural marsh 

 growth along the lower Colorado River should not 

 be affected by an increase in salinity, artificially 

 developed waterfowl feeding areas on wildlife 

 management areas may be detrimentally affected. 



Factors Associated with Increased Salinity 



There are three other aspects of water quality 

 that are normally associated with irrigation return 

 water. They are toxic residues, turbidity, and high 

 temperature. 



These waters may contain toxic residues of 

 insecticides and herbicides used as a part of agri- 

 cultural practices, which may affect avian game 

 species (Rudd and Genelly, 1956). Turbidity has 

 a very definite effect on submerged aquatic plants, 

 limiting growth or even eliminating all submergent 

 vegetation. Another characteristic of irrigation 

 return water is high temperature. A rise in tem- 

 perature causes an amplification of the effects of 

 salinity upon vegetative growth (Ani and Powers, 

 1938). 



Assessment of any of these possible sources of 

 wildlife damage would necessitate thorough ex- 

 amination of existing conditions for correlation to 

 projected conditions. 



Light penetration 



Algae, turbidity from silts and clays, and color 

 of the water all affect one environmental factor of 

 major importance in the productivity of aquatic 

 wildlife habitat — light penetration of the water. 

 The results of many of man's activities, including 

 agriculture, industry, navigation, channelization, 

 dredging, land modification, and eutrophication 

 from sewage or fertilizers, often reduce light trans- 

 mission to the degree that aquatic angiosperms of 

 value to wildlife cannot grow. 



Bioassays and field studies by Bourn (1932) 

 and Sincock (unpublished data) demonstrated 

 that at least 5 percent of the total incident light at 

 the surface was required for growth of several 

 aquatic plants (as measured while the sun was 

 near its apex, between 10 a.m. and 2 p.m.). Opti- 

 mum production occurred where 10 to 15 percent 

 of the light reached the bottom. Most aquatic 

 plants will grow in water depths of 6 feet or more 

 if sufficient light is available. For optimum growth 

 in aquatic wildlife habitats the light at the 6-foot 

 depth should be 10 percent of incident light at the 

 surface; tolerable limits would be 5 percent of the 

 light at the surface to the same depth. In situ deter- 

 minations of light penetration, as measured with a 

 subsurface photometer, provide the best indication 

 of suitability for plant growth. 



Observations have indicated that prolonged ex- 

 clusion of adequate light results in the destruction 

 of submerged aquatic plants; the period during 

 which the plants must endure less than 5 percent 

 of the incident light at the surface should probably 

 not exceed 7 consecutive days if they are to 

 survive. 



Of course, light penetration and the factors 

 affecting it; e.g., turbidity, color, and algal con- 

 centrations, vary in intensity daily, seasonally, and 

 annually. In most areas, the submerged aquatic 

 plants die back in the fall and winter and the 

 quantity of light required becomes less critical as 

 a requirement. In the spring and summer, how- 

 ever, sufficient light is imperative to growth. 



Settleabie substances 



Accumulation of silt deposits is destructive to 

 aquatic plants, not only by the associated turbidity, 

 but by the creation of a soft, semiliquid sub- 

 stratum inadequate for anchoring the roots. Back 

 Bay, Va., and Currituck Sound, N.C., serve as 

 examples of the destructive nature of silt deposi- 

 tion. Approximately 40 square miles of bottom are 

 covered with soft, semiliquid silts up to 5 inches 

 deep; these areas, constituting one-fifth of the total 

 area, produce only 1 percent of the total aquatic 

 plant production. 



Waterbirds, muskrats, otters, and many other 

 wildlife species require water that is free of surface 

 oil. Studies by Hartung (1965) demonstrated that 

 egg laying was inhibited when mallards ingested 

 small quantities of oil. When oil from the plumage 

 was coated on mallard eggs, it reduced hatching 

 from 80 to 21 percent. The full significance of this 



96 



