CARBON IN FRESHWATER SYSTEMS 



ROBERT G. WETZEL* and PETER H. RICHt 



*W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan; 



tBiology Department, Brookhaven National Laboratory, Upton, New York 



ABSTRACT 



Among the great diversity of concentrations and states of inorganic carbon of freshwaters, a 

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

 inadequate for metabolism, or indirectly inadequate as a result of chemical losses from the 

 system. Serious limitations to photosynthesis by inorganic carbon per se are unlikely, 

 although shifts to assimilation of bicarbonate at metabolic expense are common. 



The origins, rates of production, and fates of detrital dissolved and particulate organic 

 carbon of a small temperate-zone lake are summarized to exemplify the importance of 

 detrital— dynamic aspects of freshwater systems. The concept of net ecosystem productivity 

 is reviewed and applied to the example. Major contributions by the littoral producers and 

 from allochthonous sources are demonstrated and discussed in relation to increasing 

 fertility. Within this spectrum of autochthonous and allochthonous inputs of organic 

 carbon, the relative contribution by differing modes of metabolism to dissolved and 

 particulate carbon pools is considered. 



A majority of the carbon present in freshwater systems occurs as equilibrium 

 products of carbonic acid. A smaller amount of carbon occurs in organic 

 compounds as dissolved and particulate detrital carbon, and a small fraction 

 occurs as carbon of living biota. The complex equilibrium reactions of inorganic 

 carbon and the distribution of species of total C0 2 (2CC>2 = C0 2 + 

 HC0 3 + CO3 ) are understood in considerable detail. Similarly, rather sophisti- 

 cated knowledge of the structure and function of lacustrine and lotic biota is 

 available. Much less well understood are the structure and functional inter- 

 relationships of the large fraction of dissolved and dead particulate detrital 

 carbon in aquatic ecosystems. Separation of inorganic, detrital organic, and 

 biotic carbon is arbitrary, though essential for analytical purposes. True insight 



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