Calcium Carbonate 111 



loss of cations needed as plant nutrients, as will be considered further 

 in the next chapter. Carbon dioxide is thus a vital part of a com- 

 plex web of physicochemical interdependencies in the soil involving 

 carbonic acid, pH, Ca+^ ions, and CaCOs, each of which has significant 

 ecological relations with the plants and animals living in the soil. 



Deposits of CaCOs in fresh water are referred to as marl, although 

 the term is also sometimes used for lime deposits in other situations. 

 Marl in fresh water is sometimes the result of the accumulation of 

 shells, but more often it is produced by the activity of plants. In 

 hard-water lakes which contain large amounts of Ca^^ ion, the removal 

 of carbon dioxide by photosynthesis readily causes the precipitation 

 of CaCOs— usually deposited on or in the tissues of the plants them- 

 selves. The pond weed Potamogeton often feels gritty because of the 

 lime present, and Chara was named "stonewort" for the same reason. 

 As much as 30 per cent of the dry weight of the latter plant is ac- 

 counted for by CaCOs in its tissues. Certain bacteria and various 

 types of algae, especially the Myxophyceae, also cause the formation 

 of marl on the bottom of lakes and ponds. The alga Claclopliora 

 forms a biscuit-shaped deposit of lime about its tissues, and these 

 "Cladoplwra balls," commonly attaining diameters of more than 8 

 cm, may accumulate like paving stones over the bottom or along the 

 shore. 



In soft-water lakes and particularly those with an acid reaction, 

 we find the reverse situation. Ca++ and C03= ions are not only less 

 abundant but they also tend to stay in solution, with the result in 

 extreme instances that the deposition of CaCOs by the activity of 

 organisms is dijfficult or impossible. The Ca++ ion plays an essential 

 role in membrane permeability and other physiological processes of 

 plants and animals. CaCOs is a principal component of the shells 

 of mollusks and worms, and is incorporated in the exoskeletons of 

 arthropods where it serves to add stiffness. The skeletons of verte- 

 brates are composed of about 80 per cent calcium phosphate and a 

 considerable portion of calcium carbonate and other calcium com- 

 pounds. It is not surprising, then, to find that some animals are 

 limited by subminimal supplies of Ca++ and C03= ions in the water. 

 Robertson (1941) reports that Gammarus piilex requires at least 5 

 mg Ca per liter before it can harden its exoskeleton. As a general 

 rule few mollusks are found in acid lakes; those occurring are specially 

 adapted to withstand the unfavorable conditions and characteristically 

 display thin shells, often protected by a covering of chitin. How- 

 ever, the bivalves Pisidiuin and Campeloma were recorded by Morri- 

 son ( 1932) in water of pH 5.1 to 6.1 containing 3 to 5 mg Ca per liter 



