Article:  Claude E. Boyd, Ph.D.
 Professor, Department of Fisheries
 And Allied Aquaculture
 Auburn University
 Auburn, Alabama 36849
E-mail:  boydce1@auburn.edu
 
Article copied from: Global Aquaculture Advocate
 August 2004, Pg.59-60
 
Productivity increases in aquaculture ponds in response to nutrient inputs of fertilizer and feed. These inputs, however, increases the demand for oxygen by organisms, and low concentrations of dissolved oxygen can occur in pond water.
Usually, low dissolved oxygen concentrations occur during early morning hours, but also result from phytoplankton die offs and cloudy weather. Most species of shrimp are stressed by dissolved oxygen concentrations below 3-4 mg/l. In ponds with consistently low concentrations, culture organisms usually have diminished appetites, greater susceptibility to disease, and higher mortality. Feed conversion efficiency and production accordingly suffer.
Mechanical aeration is used widely in pond aquaculture to prevent low dissolved oxygen concentrations, especially at night; improve the efficiency of feed use; and increase production of the culture species.
 
Dissolved oxygen
 
The capacity of water to dissolved oxygen depends upon atmospheric pressure, oxygen percentage in the air, water temperature, and salinity. Representative concentrations of dissolved oxygen at saturation in waters of different salinities and temperatures are provided in Table 1.
The tabular values of dissolved oxygen are for water in contact with the atmosphere at 760 mm mercury, normal barometric pressure at sea level. The saturation concentration of dissolved oxygen (DO) for any other barometric pressure is estimated as follows:
 
                                            Barometric Pressure (mm)
DO saturation = DO table   x             760 mm
      
For example, if the water temperature is 25°C, salinity is 10ppt, and barometric pressure is 735 mm Hg, the dissolved oxygen concentrations at saturation is:
 
DO saturation = 7.79 x 735 = 7.53 mg/l
   760
 
Table1. Solubility of dissolved oxygen (mg/l) in water at standard atmospheric pressure (760 mm Hg)
 
Temperature (°C): 10
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 11.28  /10.58 /9.93   /9.32   /8.75
 
Temperature (°C): 15
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 10.07  / 9.47  /8.91   /8.38   /7.88
 
 
Temperature (°C): 20
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 9.08   /8.56   /8.06   /7.60   /7.17
 
Temperature (°C): 25
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 8.24   /7.79   /7.36   /6.95   /6.56
 
Temperature (°C): 30
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 7.54   / 7.14  /6.75   /6.39   /6.05
 
Temperature (°C): 35
Salinity (ppt)      : 0 ppt/10ppt/20ppt/30ppt/40ppt
DO (mg/l)            : 6.93   / 6.58  /6.24   /5.91   /5.61
 
Barometric pressure changes continuously, but these changes are minor. The main factor causing differences in barometric pressure is elevation. Barometric pressure declines with increasing elevation (Table2).
Dissolved oxygen diffuses between air and water in response to oxygen pressure. The oxygen pressure is roughly 21% of barometric pressure because 20.95% of normal air is oxygen. The oxygen pressure in air and water are equal when water is saturated with dissolved oxygen.
During daytime, pond water often has a dissolved oxygen concentration greater than saturation because of oxygen released by the photosynthesis of phytoplankton. In this situation, known as super saturation, oxygen diffuses from the water to the air because the pressure of the oxygen in water is greater than in air.
At night, when photosynthesis stop, respiration continues and dissolved oxygen concentrations often fall below saturation. Nevertheless, the dif

Posted Date: 27-Oct-2004
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