130 NIELSEN [UHAP. 7 



algae, we find that the losses are due partly to the respiration in the herbivores, 

 partly to incomplete digestion of the algae used as food. Due to incomplete 

 digestion a greater or smaller part of the organic matter is thus not transferred 

 to the next ordinary trophic level but instead to a ''side track" consisting of 

 heterotrophic plants which assimilate the organic matter. In all probability 

 some of these heterotrophic plants are eaten by certain herbivores. Some of 

 the organic matter transferred into the "side track" may thus be returned to 

 the "main track". In shallow water, incompletely digested algae in the form of 

 "pellets" may sink to the bottom where they are used as food by herbivorous 

 bottom animals (see Harvey, 1945, p. 151). 



A part of the respiration in the herbivores supplies the necessary energy for 

 the synthesis of various organic substances on the basis of the organic matter 

 assimilated. These substances are used for growth and reproduction. Another 

 part of the energy activated by respiration is used for work — in particular for 

 the work involved in the catching of food. Our present knowledge of this subject 

 is but slight ; probably the work which the herbivores must necessarily carry 

 out in order to collect food increases with decreasing concentrations of the 

 algae. It is to be expected that the animals in the plankton-poor parts of the 

 oceans must work hard in order to collect enough food. Thus a very con- 

 siderable part of the organic matter assimilated by the herbivores in oligotrophic 

 water is used to sustain the respiration operating the mechanisms necessary 

 for the catching of food. Only a relatively small part of the assimilated matter 

 can be used here for growth and reproduction. Accordingly, the lifetime of the 

 herbivores in such areas must be relatively longer than in eutrophic areas. This 

 is a very important point to be considered if we want to use the size of the 

 standing stock of the herbivores for estimating their production. In all proba- 

 bility the relation between standing stock and production in the herbivores and 

 carnivores varies to a considerable extent, being highest in oligotrophic water. 



The rates of the biochemical processes taking place in the organisms need not 

 be especially high just because the temperature is high. These processes are 

 enzymatic and their rates are dependent on the combined effects of temperature 

 and enzyme concentration. A low temperature may be counteracted by a high 

 concentration of the enzymes (see Steemann Nielsen and Hansen, 1959). In 

 fact, it has been known for a long time that in species of animals found both 

 in the Arctic and in temperate waters, the rate of respiration at a certain 

 temperature is highest by far in the Arctic (see Sparck, 1936). Finally, a selec- 

 tion takes place in nature. In oligotrophic waters herbivores able to tolerate a 

 slow growth rate will presumably become predominant. 



Few oceanic organisms are left to die from natural causes. Usually nearly all 

 of them are devoured. However, in very eutrophic shallow coastal waters this 

 seems not always to be true for the planktonic algae. The same is the case in 

 areas where the spring bloom starts as an "explosion". In contrast to the 

 open ocean, where the main bulk of planktonic algae is usually found in the 

 photic zone (see Table I), most of the planktonic algae in very eutrophic 

 shallow freshwater lakes are often located below the lower boundary of the 



