METHODS OF PLANKTON RESEARCH. 



547 



but, owing to wave motion, it is better to make the haul 

 from a depth of five metres. This shallow surface layer 

 of the sea appears to be particularly rich in plankton, and 

 it is therefore conceivable from this how two tow-nets 

 pulled along the surface may differ in contents if one of 

 them is accidentally a little heavier than the other, or, 

 for some reason, has been towed a little deeper. 



The following figures from the German North Sea 

 catches will illustrate the differences in the volume from 

 different depths at the same stations : — 



Depth at which 

 catch was made. 



c.cm. under 1 sq. 

 metre area. 



c.cm. in 1 cub 

 metre. 



(35 — 5 metres 



t 5 -o » 





112 

 56 



3-7 

 11-3 



(44—5 

 \ 5—0 







72 

 72 



1-8 

 14-4 



(63—47 

 \ 47—5 

 I 5—0 



> > 





56 

 144 

 144 



3-5 

 3-4 



28-8 



In the Baltic, volume estimations have been made 

 and the catch also quantitatively examined. On one 

 expedition, for example, the volumes from Stations 1, 2 

 and 3 in the West Baltic, where the salt contents was 

 17 to 20 %o, were very large and above the average. 

 At Station 8, a point further east> there was also a large 

 catch, but the salt contents was only 8 to 10 % - 

 The constitution of the catch varies in the Baltic, probably 

 with the salt contents, which, unlike the North Sea, varies 

 within wide limits. Thus, Aphanizomenon flos-aquae 

 increases as one travels east. Chaetoceros decipiens and 

 C. didymum decrease and eventually drop out altogether. 

 Ceratium also decreases in the same way. At Station 8, 

 however, where there was a low salt contents, this decrease 

 for some reason was not present. In the North Sea the 

 simple hydrographical conditions of the Baltic do not 

 prevail, and the whole matter is rendered far more 



MM 



