Bogorov (4) gives the following vertical graduation for plankton: "Surface", "middle-depth" 

 and "deep-water". These divisions, however, are very arbitrary. Although the individual species 

 are found in masses in specific strata ("surface" species and "deep-water" species), we cannot go 

 so far as to say that they are found only in a given layer , 



The organisms change their level according to the biological season, time of day, and onto- 

 genetic characteristics of their development . 



Table 1 shows how the individual plankton species in the North Atlantic are dependent on 

 specific depths, temperatures, and degrees of salinity. 



Table 3 shows the quantitative distribution of a small number of species of Copepoda in the 

 North Atlantic seas . 



QUANTITATIVE DISTRIBUTION OF PLANKTON 



Influence of Currents on the Quantitative Distribution of Plankton 



An increase in the quantity of warm-water or cold-water organisms indicates increased in- 

 flux of warm or cold waters . 



A variation in the intensity of the influx of warm or cold water exerts a considerable in- 

 fluence on the development of the biological processes and on the times of the biological seasons . 

 Mass development of plankton is characteristic of the biological spring. As a rule, however, an 

 increase or decrease in the plankton volume during the biological spring period is due to a varia- 

 tion in the intensity of the influxes of warm or cold waters . 



Development of plankton in 1952 and 1953 in the Norwegian Sea began at the same time, and 

 the plankton biomass in these years in the latitude of Andj^y was almost the same (Figures 11 and 

 12). We can conclude from this that the intensity of the influx of warm waters into the North 

 Atlantic seas was the same during the last two years, but in 1953 an increase in the quantity of 

 warmth-loving organisms was observed in the central and north-central areas of the Norwegian 

 Sea and in the south of the Greenland Sea (see Figure 1); this appears to be due to a certain strength- 

 ening of the western branch of the Norwegian current. 



During the summer months, there is a noticeable decline in the plankton biomass, which is 

 mainly due to its being eaten by planktonophages, especially the herring. In Figures 10, 11 and 12, 

 the places with the lowest plankton biomass are in the areas through which the herring have passed 

 or remained for a considerable period (according to the figures of the Herring Laboratory). This 

 distribution of the plankton biomass for the different years is for the same season Qune) and for the 

 same depth (0-50 m. layer). 



The observations of 1952 showed that the plankton biomass is smallest in the currents (Fig- 

 ure 13, graphs I, H, III) and at the places where warm and cold waters intermingle (see Figures 

 10, 11, and 12). In order to get an idea of the times at which the biological processes develop in 

 the current and beyond its boundaries, we used the figures for the mean biomass of plankton from 

 stations in and outside the paths of the currents . The illustrations show how the biomass increases 

 towards the centre of the current. 



In the spaces between the currents the biological processes become later in time, and their 

 development is slow and uniform (low biomass). This can be seen particularly clearly in graph II. 



92 



