264 Oxygen and Carbon Dioxide 



As a result of buffering, the surface waters of the open ocean rarely 

 depart from a pH of 8.0 to 8.4, the deeper waters being generally 

 confined to a pH of 7.4 to 7.9. In the littoral zone, and particularly 

 in salt ponds and tide pools, the range is greater. The pH in marine 

 muds may drop below 7.0 or rise above 8.5, and in dense growths of 

 sea weeds the pH may reach 9.0. Sea water and hard fresh water are 

 always on the basic side of neutrality. The pH of different fresh 

 waters varies from values below 3 to values higher than 10, although 

 the range of most streams and lakes is confined between pH 6.5 and 

 8.5. In soft waters with small amounts of combined carbon dioxide 

 and little buffering, the pH value may fluctuate widely within short 

 periods. 



The respiration of free-living aquatic plants and animals and the 

 decomposition of organic matter by microorganisms in the water add 

 to the carbon dioxide supply of the water medium. The photosyn- 

 thesis of the plants, on the other hand, tends to reduce the amount of 

 available carbon dioxide present. In poorly buffered ponds and lakes 

 variation in the relative rates of carbon dioxide production and con- 

 sumption produces significant changes in the abundance of free car- 

 bon dioxide and in the pH of the water. If these water bodies contain 

 a good growth of green plants, the fluctuation in carbon dioxide and 

 in pH displays a diurnal cycle as a result of the fact that photosyn- 

 thesis regularly exceeds respiration during daylight hours. This car- 

 bon dioxide and pH pulse is generally coordinated with, but recipro- 

 cal to, the oxygen pulse described above. In the Ohio lake observa- 

 tion illustrated in Fig. 7.1 the pH rose from 8 to 9 between sunrise 

 and sunset and dropped again during the night. 



A similar tendency for biological agents to change the pH takes 

 place in the ocean, but often the actual effect on pH is too small to 

 be measured because of the high buffering capacity of sea water. In 

 a study of the English Channel throughout a yearly period, during 

 which photosynthesis varied from its maximum in the spring and 

 summer to its minimum in the winter months, the pH of the upper 

 water layer varied only from 8.16 to 8.25. 



The relative constancy of pH values in the sea is one more instance 

 of the stability that characterizes the oceanic environment, and we 

 may pause for a moment to throw a backward glance over the topic. 

 We have noted the high degree of constancy in the open sea in re- 

 gard to salinity, temperatiu'e, and oxygen supply, and now in relation 

 to carbon dioxide, alkalinity, and pH. The ocean thus represents 

 the largest natural environment in which highly stable conditions exist 

 in respect to many ecological factors for weeks, months, or even years 



