298 



BIOLOGY OF THE SEAS OF THE U.S.S.R. 



circulation, and the poor supply of nutrient salts do not provide favourable 

 conditions for plankton growth. Feeble development of plankton leads to an 

 almost complete disappearance of phosphates and nitrates in the surface 

 layer. The course of this process is shown in Fig. 141. The phosphates and 



Gonyaulax catenata 

 Melosira arctica 

 Dinobryon pellucidum 

 Thalassiosira baltica 

 Chaetoceras holseticus 

 Ch. wighami 

 Thalassiosira nana 

 Actinocyclus ehrenbergii 

 Peridinium pellucidum 

 Aphanizomenon flos-aquae 

 Nodularia spumigena 

 Chaetoceros danicus 

 Coscinodiscus grani 



Tintinnopsis tubulosa 

 Mesodinium rubrum 

 Coturnia maritima 

 Helicostomella subulala 

 Synchaeta baltica 

 Keratella quadrata 

 Pseudocalanus elongatus 

 Temora longicornis 

 Eurytemora hirundoides 

 Acarlia bifilosa 

 A. tonsa 



Evadne nordmanni 

 Podon polyphemoides 

 Bosmina coregoni maritima 

 Fritillaria borealis 

 Sagilta elegans 

 Pleurobrachia pileus 

 Mysis oculata relicta 

 Praunus flexuosus 



Fig. 140. Periods of intensive development of main plankton species in 

 Central Baltic (Nikolaev). 



nitrates are removed in March and April by an increase in the growth of 

 plankton (diatom). In May the dying plankton carries them to great depths ; 

 thus the surface layer of water loses both its plankton and its nutrient salts. 

 A partial regeneration of the phosphates and nitrates in June, July and August 

 results in a small new increase of plankton, when Cladocera is predominant 

 in the zooplankton. Plankton does not develop in winter when the tem- 

 perature is low and sunlight scarce, although the nutrient salts are more 



