432 J.L. Tiwarietal. 



detrital particles (RB103) and by utilizing particulate carbon by co- 

 metabolism (RB107) with usable DOC. The loss of bacterial biomass is a 

 result of respiration (RB104), death (RB105), and predation. 



Consumers 



Chironomids, ciliates, flagellates, and micrometazoans are the 

 predominant consumer species in the pond sediment and play a major role 

 in the dynamics of the benthic carbon. Among these, chironomids are the 

 dominant species and constitute about 80% of the total weight. The total 

 chironomid population consists of four cohorts which differ in their 

 feeding rate, mortality, and respiration; hence these cohorts are treated in 

 the model as four separate state variables. These organisms feed on algae, 

 bacteria, detritus, and even other consumer species. Ciliates and flagellates 

 live mostly on algae and bacteria, respectively. Microbenthos consume 

 algae, bacteria and ciliates. (For biological details see Chapter 8 and 

 Fenchel 1975.) 



The rate of ingestion of a particular food type by a consumer species 

 is a function of its biomass, the density of food, and temperature. As 

 discussed earlier, the effect of food density on the rate of ingestion follows 

 a Michaelis-Menten type response curve. To take a specific example, let us 

 consider the rate of ingestion of bacteria by flagellates. The functional 

 form of this process is 



RB217=X203A:207 [ Xioi/(Xioi+^209 ) ] T, (13) 



where 



RB217 = rate of ingestion of benthic bacteria by benthic flagellates, 



JCioi = bacterial biomass, 



JC203 = flagellate biomass, 



k 207 = specific V ^ax for ingestion of bacteria by flagellates 



at the reference temperature, 

 ^209 = half-saturation constant for ingestion of bacteria by 



flagellates, and 

 Tt = temperature effect. 



If a consumer species feeds on more than one food source, then it is 

 assumed that each such consumer has a total rate of ingestion that is a 

 function of the biomass of the consumer, of the total biomass of all food 

 sources, and of temperature. The expression for the food density is of the 

 Michaelis-Menten type. The actual rate of ingestion of a food source, S,, 

 is then the product of the total rate of ingestion and S, divided by the sum 

 of all food sources available to the consumer. In the second expression 



