WEATHER IN CRANBERRY CULTURE 7 



CONDITIONS UNDER WHICH INJURY OCCURS 



The conditions under which injury from oxygen deficiency occurs may be con- 

 sidered with reference to: (1) the dissolved oxygen content of the water; (2) the 

 oxygen requirements of cranberry vines under winter-flooding conditions; and (3) 

 the ability of cranberry vines to withstand a lack of oxygen. Each of these has a 

 part in determining the severity of injury and must be known if the conditions 

 under which injury occurs are to be fully understood. 



Factors Affecting the Dissolved Oxygen Content of Water 



The factors which determine the dissolved oxygen content of the water on a 

 flooded bog (or of a natural body of wcter) may be placed in two groups according 

 to their nature and the efi'ect of their action. Those of one group are physical; 

 those of the other are biological since their effect is due to the activities of living 

 organisms. Physical factors tend to bring the water to a definite, uniform dis- 

 solved oxygen content and to keep it in that condition ; biological factors tend to 

 prevent this. Physical factors are fundamentally the controlling ones. 



Physical Factors 



Water in contact with the air normally contains oxygen in solution. The 

 amount depends upon its solubility in water and upon the proportion of oxygen 

 in the air. Since the latter does not change appreciably, the amount of dissolved 

 oxygen in water in contact with the air depends only on the solubility of oxygen 

 in water. This varies with the temperature of the water; it is greatest at 32° F. 

 and decreases as the temperature rises. If the water does not contain all the 

 oxygen that it is able to absorb at a given temperature, more is taken up from the 

 air; if it has more than it can hold in solution, some is given off until an equilib- 

 rium with the oxygen of the air is reached. The greatest amount of oxygen that 

 water exposed to the air, and at a given temperature, can hold in solution is 

 known as its saturation capacity. At 32° F. this is about 10 cc, at 40° about 

 8.7 cc. and at 50° about 7.8 cc. per liter (61 cu. in.). 



The temperature of the water of a flooded bog, however, usually changes from 

 hour to hour during the day and from day to day according to weather conditions. 

 This changes the capacity of the water to hold oxygen in solution and the equilib- 

 rium between the dissolved oxygen of the water and the oxygen of the air must 

 be reestablished under the new conditions. Other factors also often disturb the 

 balance between the oxygen in solution and that of the air. Whenever this 

 happens, a transfer of oxygen from the air to the water, or from the water to the 

 air, is necessary to restore the equilibrium. The transfer takes place by diffusion, 

 a process of migration by the spontaneous movement of molecules, through the 

 surface layer of water in contact with the air. Oxygen diffuses into or out of 

 this surface layer very rapidly but, because its rate of diffusion through water is 

 extremely slow, only a very thin layer at the surface is thus brought to an equilib- 

 rium, and if the oxygen were distributed b> diffusion alone it would take a very 

 long time to bring the water of a flooded bog to an equilibrium with the oxygen 

 of the air. There are other methods, however, by which oxygen is distributed 

 more rapidly. These are by convection currents and by circulation of the water 

 by the wind. 



Convection currents are those caused by changes in the temperature of the 

 water. Water has its greatest density at 39° F. and becomes lighter as it becomes 

 warmer or colder. Convection currents are set up if the surface water becomes 

 denser than the underlying water. The surface water then sinks and water from 

 below rises. This tends to equalize the distribution of oxygen in solution in the 



