Figure 12. Biomass of plankton (in mg/jj^3) in the 0-50 m. layer in May, June and July 1951. 



The boundary of the feeding area of the herring in June 1951 is indicated by the 

 continuous line. 



During the early biological spring (April-May) the water temperature is the same throughout 

 almost its entire depth and scarcely any warming of the upper layers can be observed. Plankton is 

 evenly distributed. Its biomass is very low - 20-30 mg/^S (graph I). Then, from June onwards, 

 the temperature of the upper water strata rises, and there is one layer (thermocline) where the 

 abrupt temperature change is very marked (graph II). The mass development of plankton begins 

 during this season. Its biomass increases noticeably and the plankton collects above the thermo- 

 cline, which, as it were, creates a kind of "floor" which contains the plankton. 



During the summer months, the vertical distribution's dependence on the temperature is 

 somewhat different . This is partly due to the biology of the organisms themselves . 



95 



