CARBON IN FRESHWATER SYSTEMS 263 



DISCUSSION BY ATTENDEES 



Caplan: Your estimate of maximum gross production for Lawrence Lake is 

 8gC m year . Ryther predicts 6g for artificially enriched aquatic systems; 

 Japanese work has demonstrated a potential maximum of 16 g during short 

 periods of less than 3 months. How would you compare the potential gross 

 primary production of freshwater for agriculture to marine systems as described 

 by Ryther utilizing tertiary sewage effluent as a nutrient source? 



Wetzel: My general reaction would be that freshwater systems would have a 

 higher production. My conclusion is based on the higher organic concentrations 

 found there, as well as on higher nutrient inputs from surrounding land. 



Olson: The words we use to describe things in biology greatly affect how we 

 think about them. On the basis of the papers of both Riley and Wetzel, it seems 

 to me that the term "dissolved organic carbon" is highly unsatisfactory as 

 presently used. The term "colloidal organic carbon" should be introduced to 

 denote those forms of carbon which will pass through a filter but which will not 

 pass through a cell membrane under any circumstances. 



Wetzel: I agree. 



Baylor: It is obvious that the speakers here today are all aware of the 

 problem, but I think that the arbitrary use of "filtration" as a definition for 

 what is dissolved and what is not dissolved organic carbon is a kind of 

 procrustean bed upon which many investigators have been led astray. 



Livingstone: Richardson, Melack, and Kilham have been measuring produc- 

 tion in some rather unusual closed-basin lakes in East Africa. The country rock 

 around the lakes consists of readily soluble volcanics rich in nepheline, and the 

 sump lakes at the downstream end of the groundwater flow have high 

 concentrations of all the elements of which planktonic organisms are composed. 

 The productivity is very high, apparently between 3000 and 4000 g C m" 2 

 year , and the lakes are dominated by phytoplankton. The 99% extinction level 

 for light lies at a depth between 1 and 10 cm, and even germinating aquatics 

 with large seeds seem unable to grow up to the surface before they run out of 

 stored resources. Taking into account the whole spectrum of lake productivities 

 then, one finds a third phytoplankton-dominated peak. 



This is an example of the sort of difficulty one gets into by using Liebig's 

 Law of the Minimum in biology — it is not operationally definable, and there is 

 no feasible experimental method by which Liebig's sort of "limiting factor" can 

 be identified. In lakes with such a tremendous supply of nutrients as these 

 African ones, the rules of the productivity game are fundamentally altered, and 

 the phytoplankton comes very close to using all the available light of appropriate 

 wavelength in photosynthesis. 



