534 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



ring distributions lie close to or just under the cold edge. The 

 explanation of these concentrations is probably in terms of a 

 balance between the tendency of the fish to enter the cold water 

 for food and an opposing reaction against the low temperatures, 

 corresponding to the experimental result that herring tend to 

 stop feeding below 4° C. This type of extreme situation shows 

 how one dimension of randomness is removed since, although the 

 fish are probably "patchy" along the edge, they are nonrandom 

 across it, making it easier to study some of the relations between 

 productivity and herring distributions. 



As a final example, there is the problem of the adequacy of 

 chlorophyll data in studying detailed vertical distributions of the 

 plants. Saijo and Ichimura (1959) and also Menzel and Ryther 



(1959) describe chlorophyll maxima at the foot of the euphotic 

 zone and attribute this to the effects of sinking. Steele and Yentsch 



(1960) have shown that the sinking rate would have to decrease 

 in the region of these maxima, and we have found some experi- 

 mental evidence for this in terms of a decreasing sinking rate 

 when nitrogen-starved cultures are enriched. This would corre- 

 spond to the higher nutrient concentrations found at the foot of 

 the euphotic zone. In the sea, however, there will generally be 

 vertical variations in species composition. The dominance of 

 different species at increasing depths may be due to the effects 

 of sinking on a succession of species growing near the surface, but 

 it may be the result of differential adaptation for growth at dif- 

 ferent depths. Thus as Menzel and Ryther point out, an under- 

 standing of these changes is necessary before an explanation in 

 terms of sinking can finally be accepted. Some of the complexities 

 of the vertical distributions of individual species or groups is to 

 be found in the work of Bernard (1956, 1959). 



In these various examples I have tried to show the kind of diffi- 

 culties which arise in interpreting ecological data relating to pri- 

 mary production. The theoretical pictures which are sometimes 

 dignified by the title "mathematical models" are very idealized 

 descriptions of what may happen. They have to use such simplifi- 

 cations as the concept of a two-layered sea or of equal grazing 

 at all depths by the zooplankton. Yet, because of the complex 



