242 WETZEL AND RICH 



into causality and control necessitates an understanding of their functional 

 relationships. 



INORGANIC CARBON 



In most freshwaters, inorganic carbon dominates over organic forms and 

 exists almost totally as equilibrium products of C0 2 and carbonic acid. The 

 quantitative diversity of inorganic carbon (2C0 2 ) of lakes is extreme. Inland 

 waters range from solutions chemically similar to distilled water to highly saline 

 carbonate brines in which 2C0 2 exceeds several moles per liter. A more typical 

 range for 2C0 2 is 50 micromoles to 10 millimoles, the latter not uncommon in 

 hard-water lakes of glaciated morainal regions of the temperate zone. 



Dissociation equilibria and kinetics of the hydration and dehydration of C0 2 

 are very well delineated in pure-solution chemistry [e.g., see extensive reviews of 

 Kern (1960), Raven (1970), and Stumm and Morgan (1970)] . Free C0 2 , HC0 3 , 

 or pH can be computed from activity theory if two variables and the pK are 

 known, yielding concentration (or activity of C0 2 as H 2 C0 3 ) in solution rather 

 than analytical free C0 2 (Hutchinson, 1957). Practically all determinations of 

 [2C0 2 ] are made from empirical measurements of pH and [HC0 3 ] . Although 

 calculated and analytical C0 2 are identical at pH 7.6, large discrepancies occur 

 when calculated 2C0 2 is compared to analytically determined C0 2 over a 

 spectrum of pH values (Hutchinson, 1957; Wood, 1970). At pH >8, calculated 

 values greatly exceed analytical values; at low pH, calculated SC0 2 is larger than 

 analytical C0 2 , in part a result of interferences by organic acids. 



Atmospheric C0 2 (about 0.033 vol.%) dissolves in water to yield an 

 unhydrated C0 2 concentration similar to that in air (about 10 micromoles). The 

 hydration of C0 2 to H 2 C0 3 is relatively slow chemically (e.g., 15 sec), which 

 results, at equilibrium, in a concentration of H 2 C0 3 0.0025 times that of C0 2 . 

 The dissociation of H 2 C0 3 to HC0 3 and C0 3 " is essentially instantaneous. If 

 C0 2 is in equilibrium in a solution buffered to constant pH, the [C0 2 ] and 

 [H 2 C0 3 ] are independent of pH, whereas [HC0 3 ] and [C0 3 ~] increase with 

 pH until saturation kinetics are achieved. These equilibria are strongly modified 

 by temperature and salinity. At the salinity of seawater, pK x (HC0 3 ) is about 

 0.5 unit, and the pK 2 (C0 3 ~) about 1 unit, lower than freshwater (Raven, 1970). 

 Both oceanic and freshwaters are close to equilibrium with atmospheric C0 2 . In 

 the marine habitat the inorganic-carbon pool contains about 2 millimoles C, 

 largely as HC0 3 , a reservoir some 50 times that of the atmosphere. In 

 freshwaters, 2C0 2 is much more pH dependent in relation to [HC0 3 ] and 

 [C0 2 3 ]. 



In marine systems, inorganic-carbon fluxes are, for all practical purposes, 

 associated with these inorganic ionization equilibria. In freshwaters, biotic 

 respiratory sources and fluxes of C0 2 can be significant to overall carbon 

 metabolism. We will return to this point later. 



