[Chap. XLVI UNDER- WATER ENVIRONMENTS 593 



plants and animals, the oxvgen content mav become so reduced during 

 hot weather that fish and other submerged animals suffocate. The death 

 of submerged plants in shallow water may likewise occur from suffo- 

 cation. 



Carbon dioxide. The sources of CO2 in the water of ponds and lakes 

 are diffusion from the atmosphere, oxidation of organic matter by bac- 

 teria and also the respiration of all other organisms, inflowing streams, 

 and the release of CO- when the soluble bicarbonates of calcium and 

 magnesium are changed to insoluble carbonates and accumulate as marl 

 (Fig. 228). 



When carbon dioxide dissolves in natural bodies of water, more than 

 half of it combines with the water, forming carbonic acid: COi- + H-O 

 -^ HlCO:^. The remainder exists as free carbon dioxide. Submerged 

 green plants utilize in photosynthesis not only the free CO- but also 

 the CO- in carbonic acid and the so-called "bound" CO- in bicarbonates, 

 such as those of calcium, magnesium, sodium, and iron.^ 



There is about one-third of a cubic centimeter of CO- in a liter of air, 

 and it is constantly available at the surface of land plants because of 

 continuous air currents. In lakes the COl> concentration is highly varia- 

 ble. It may be unavailable at times, particularlv in bog lakes, and at 

 other times it may accumulate to the equivalent of 20 cubic centi- 

 meters per liter of water. Apparently the death of submerged vegetation 

 in warm shallow water in midsummer mav result from starvation, from 

 suffocation, or from some other indirect effect of high temperature. In 

 spring and autumn carbon dioxide is quite unifomily distributed from 

 the top to the bottom of deep lakes, but in summer and winter it may 

 be either abundant or nearlv absent at different depths. 



Nitrogen. The nitrogen (N2) in solution in water is no more usable 

 by aquatic plants than is the nitrogen of the atmosphere by land plants. 

 However, as on land, nitrogen may be combined by microorganisms 

 into such usable substances as ammonia, nitrates, and organic nitrogen 



.HCO3 /HCO3 yHCOj 



^ The bicarbonates are metal hydrogen carbonates: Ca\ - Mg<( - Fe<( > 



' ^HCOa \HCO3 ^HCOa 



and NaHCOa. When carbon dioxide is released from these bicarbonates, the carbonates 

 CaCOa, MgCOri, and FeCOs are formed. These carbonates, with the exception of sodium, 

 are relatixely insoluble and accumulate on plant surfaces; eventually they form layers of 

 marl on lake bottoms in limestone regions. Some of the limestones of the Middle Western 

 States originated in fresh-water lakes. 



