244 WETZEL AND RICH 



systems which affects the chemical milieu without clearly defined energetic 

 transformations (Wetzel, Rich, Miller, and Allen, 1972). The organic coatings 

 also inhibit dissolution of sedimenting CaCOs in lakes and form a major sink for 

 inorganic and organic detrital carbon (Wetzel, 1970, 1971). 



It is evident that estimates of theoretical dissociation constants and kinetics 

 of the 2C0 2 complex derived from pure solutions differ significantly from 

 in situ results in natural waters. However, in most freshwater systems, 

 dissociation rates of ionic C0 2 species and the maintenance of near-equilibrium 

 conditions between atmospheric C0 2 and the water are sufficiently rapid that 

 severe inorganic-carbon limitation to photosynthesis is unlikely, even under 

 conditions of low 2C0 2 . Nonetheless, experimental evidence on freshwater 

 photosynthetic utilization of inorganic carbon indicates a strong relationship 

 between physiological availability and the forms of 2C0 2 (Wetzel and Hough, 

 1972). The ability to assimilate bicarbonate ions is highly variable among 

 planktonic algae, macroalgae, and submersed angiosperms (see a most compre- 

 hensive review by Raven, 1970). Where this ability does occur, additional 

 reactions are needed for bicarbonate assimilation which are not required for C0 2 

 assimilation. Active bicarbonate transport with dehydration in the cytoplasm 

 apparently is required and is coupled to a similarly active stoichiometric 

 hydroxyl efflux. Where aquatic plants have similar affinities for C0 2 and 

 bicarbonate, utilization of bicarbonate generally occurs when bicarbonate 

 concentration exceeds C0 2 by more than 10 times (Raven, 1970). Free C0 2 

 concentrations (about 10 micromoles) of most freshwaters and the sea are in 

 equilibrium with the atmosphere; however, many freshwaters contain bicar- 

 bonate concentrations far in excess of 10 times that quantity. Equilibrium free 

 C0 2 , particularly in the common alkaline hard waters with a pH >8, is 

 inadequate to saturate photosynthesis in plants adapted to utilize bicarbonate. 

 As these waters become more productive, and in densely populated littoral zones 

 of less productive lakes, pH is rapidly modified by metabolism on a diurnal basis 

 (pH range from 6 to 10 per 24 hr) and can be associated with reduced carbon 

 fixation and bicarbonate assimilation. There is no question that under stagnant 

 conditions the shift to bicarbonate metabolism, as well as the increased pH, is 

 associated with depletion of C0 2 . 



Bicarbonate assimilation in media of high pH assumes greater significance in 

 larger aquatic macrophytes that morphologically have long diffusion paths. 

 Moreover, many angiosperms with large intercellular gas lacunae refix C0 2 of 

 respiration and photorespiration rather efficiently (Hough and Wetzel, 1972). 

 The efficiency of refixation and photosynthetic efficiency of carbon fixation 

 must be highly plastic, related in part to induced shifts to bicarbonate 

 assimilation, and can significantly affect rates of net primary production 

 (Wetzel, 1969; Wetzel and Hough, 1972). 



In summary, evidence is available that, among the enormous diversity of 

 concentrations and states of inorganic carbon in freshwaters, there exists a large 

 number of situations where free C0 2 in equilibrium with the atmosphere may be 



