CARBON IN FRESHWATER SYSTEMS 243 



The diffusion of C0 2 from the atmosphere and the dissociation kinetics of 

 dissolved carbonates are obviously of major importance to photosynthetic 

 organisms dependent on inorganic-carbon availability. The magnitude of C0 2 

 exchange between the atmosphere and water cannot be determined by 

 partial-pressure differences alone. Many lakes near neutrality are slightly 

 supersaturated with C0 2 relative to the atmospheric pressure of C0 2 . Many 

 other waters are not in equilibrium with the C0 2 of the atmosphere (although 

 they may be with other gases, e.g., oxygen) because of low turbulence and 

 slowness of gas-transfer reactions, especially in alkaline bicarbonate waters 

 containing carbonate in large amounts. Diffusion of atmospheric C0 2 has been 

 elaborated by Broecker and coworkers (1965, 1968, 1971, 1972) where 

 techniques of determining gas transfer between the atmosphere and water have 

 been measured by 226 Ra decay and flux of 222 Rn. Where applied to a 

 soft -water lake of the Canadian Shield of very low 2C0 2 , atmospheric invasion 

 of C0 2 was adequate (0.12 ± 0.06 g C irf 2 day" 1 ) to account for 30 to 90% of 

 the carbon fixation by phytoplankton (Schindler et al., 1972). How universal 

 such diffusion rates are, how they shift seasonally during ice-free periods, and 

 how they are affected within an array of dynamic chemical and biotic C0 2 

 demands on, and fluxes by, the system remain obscure. 



In addition to highly dynamic biotic demands for C0 2 and inputs of C0 2 to 

 freshwaters, complex shifts in precipitation and dissolution reactions of 

 carbonate occur spatially and temporally. In alkaline hard-water lakes, much 

 larger (2X) concentrations of calcium and bicarbonate are commonly found than 

 would be expected on the basis of equilibrium with pressures normally found in 

 the atmosphere (Ohle, 1952; Wetzel, 1966, 1971). The solubility product of 

 CaC0 3 is low (0.48 X 10" 8 ), and CaC0 3 can start precipitating from calcareous 

 waters when the pH is sufficiently high (uniformly in a buffered system or in 

 microzones associated with active photosynthesis). However, very large amounts 

 of inorganic carbon may exist as carbonate and CaC0 3 in metastable conditions, 

 and there is strong evidence that considerable CaC0 3 occurs in a stable colloidal 

 form. The importance of colloidal CaC0 3 , in addition to larger particulate 

 CaC0 3 , is just beginning to be appreciated in relation to indirect effects upon 

 metabolism and flux rates of organic carbon. 



Labile organic compounds (amino acids, fatty acids) adsorb strongly to 

 particulate and colloidal CaC0 3 (Chave, 1965; Chave and Suess, 1970; Suess, 

 1968, 1970; Meyers and Quinn, 1971a ; Wetzel and Allen, 1972). Although such 

 adsorption could be viewed as scavenging and concentrating labile dissolved 

 organic carbon from dilute solution for a more ready utilization by bacteria, 

 empirical evidence indicates, rather, a chemical competition with the bacteria for 

 the substrates. A large fraction of CaC0 3 is precipitated during photosynthetic 

 removal of C0 2 by algae and macrophytic vegetation. Frequently the plant cells 

 serve as a nucleus for particulate-CaC0 3 formation, which occurs at the site of 

 simultaneous secretion of organic compounds. This association of dissolved 

 organic detrital carbon with CaC0 3 is a component of certain freshwater 



