38 Artificial Aquatic Habitats 



toward the surface.-^ Cold, turbid waters entering Norris Reservoir 

 (Tennessee) often formed a wedge of water between surface and bot- 

 tom.^-' ^^ Water above and below this wedge was relatively clear, so that 

 the vertical extent of the wedge could be defined on the basis of turbidity 

 alone. 



In small ponds thermal stratification may be affected by blooms of 

 plankton algae which form a near-surface blanket insulating the lower 

 waters from light and the warmth of the sun's rays. For example, a farm 

 pond adjacent to a barn lot near Illiopolis, Illinois, sampled in July 1939, 

 showed an epilimnion 10 inches in thickness, containing a very dense 

 "bloom" of plankton algae. The temperature throughout the epilimnion 

 was 27.2°C (81°F), but at 13 inches below the surface the temperature 

 was 21.6°C (71°F), a drop of 5.6°C (10°F) within 3 inches. Also, the 

 dissolved oxygen was entirely gone at 13 inches below the surface. In 

 spite of these extreme conditions, this pond contained bluegills, some of 

 which were caught in traps set at the surface level. 



BIOLOGICAL PRODUCTIVITY OF WATER 



The biological productivity of water is a function of the nutrient ma- 

 terials (organic and inorganic salts) dissolved in it and available from 

 other sources. 



Although many experiments designed to test the value of inorganic 

 fertilizers in pond fish production will be reviewed in Chapter 6, now it 

 is important to stress that the addition of organic or inorganic plant 

 nutrients to a body of water facilitates an increase in the production of 

 phytoplankton, which, in turn, causes an increase in the production of 

 zooplankton and insect larvae and, somewhat later, of fish. Furdiermore, 

 increase in fish production is more pronounced among species that make 

 direct use of the available phyto- and zooplankton organisms and insects 

 than among those species of fishes that are piscivorous or have more 

 specific food requirements. 



It can be demonstrated that, in natural waters, the chemistry of soils 

 in the lake basin and its watershed are related to the water of the lake in 

 question. 



The amounts of certain chemical compounds dissolved in natural waters 

 are indicators of relative productivity. Several investigators have shown 

 a positive relationship between alkalinitx' and fish production in lakes 

 grouped as soft water (less than 50 ppm Methyl Orange alkalinity), 

 medium (50 to 150 ppm) and hard (above 150 ppm). However, the 

 greatest interruption of this relationship appears at about 40 ppm,-^ for 

 above 40 ppm there seems to be no concise relationship between car- 

 bonate content and fish yield. Also, none could be found between fish 



