THE BALTIC SEA 297 



substances (Ice and Ancylus Lakes). At the same time in some sections of the 

 Baltic Sea, in bays and gulfs well supplied with organic matter from the main- 

 land, plankton development is vigorous. 



I. Nikolaev (1957) notes that the seasonal changes in the qualitative com- 

 position of the plankton are very marked owing to the fact that what he terms 

 the ' marine cold-water (Arctic) communities ' and the ' fresh- water brackish 

 (warm- water) ones' change places during the cold and warm periods in the 

 year. The accumulation of nutritive matter in the upper layers of the Sea and 

 the arrival of the sunny period result in a springtime 'blacking' of diatomous 

 phytoplankton in April. Intensive flowering is at that time observed in the 

 inlets. During the blooming the following forms develop in specially large 

 masses : the diatoms Sceletonema costatum, Achnanthes taeniata, Thalassio- 

 sira baltica, Chaetoceras holsaticus, and Melosira arctica; and among the 

 peridineans : Dinobryon pellucidum. Among the zooplankton the following 

 take part in the spring blooming : the ciliates Mesodinium rubrum, 3 or 4 

 species of Strombidium, Tintinnopsis tubulosa, T. brandti, Cothurnia maritima 

 and others; the Rotifera Synchaeta monopus and S. baltica; the Copepoda 

 Pseudocalanus, Acartia longiremis, A. bifilosa, Temora longicomis, Eurytemora 

 hirundoides ; and in the inlets Limnocalanus grimaldi, Acartia bifilosa, Euryte- 

 mora hirundoides, Sagitta elegans baltica, Fritillaria borealis ; the mysids My sis 

 oculata var. relicta and M. mixta. 



In summer these forms disappear gradually and the dominant position is 

 occupied by the blue-green algae Aphanizomenon Jlos-aquae and Nodularia 

 spigena ; the diatoms Chaetoceras wighami, Actinocyclus ehrenbergi, Thalassio- 

 sira nana, T. baltica ; and among the peridians Peridimum pellucumid, Dino- 

 physis baltica. In July and August the blue-green algae are in full bloom every- 

 where. By the end of the summer period the following animal forms reach 

 their maximum mass development : among the ciliates Helicostomella subu- 

 lata; the Rotifera Keratella cochlearis, K. aculeata; the Copepoda Acartis 

 bifilosa and Eurytemora hirundoides ; and in huge numbers the Cladocera Bos- 

 mina coregoni f. maritima and Evadne nordmanni. The fresh- water aspect of the 

 summer plankton is infringed only by the Medusa Cyanea capillata, Amelia 

 aurita, and the Ctenophore Pleurobrachius pileus. The larvae of the bottom- 

 dwelling invertebrates are also mixed with plankton in large masses at this 

 time of the year. 



In autumn (November, December) the plankton loses its summer forms. 

 The diatom Coscinodiscus grani begins to grow in large masses : the seasonal 

 changes described are clearly shown in Fig. 140. 



All plankton species are very poorly represented in the Gulf of Bothnia, 

 especially in the central parts of its northern half, which K. Levander called 

 in 1900 'practically sterile'. The plankton there does not bloom even at the 

 beginning of the summer when sunlight is abundant. 



Indices of plankton productivity. In the Arcona depression the very small 

 possibility of plankton development is evident from the vertical distribution 

 of the basic factors of the medium. The marked differences in the temperature 

 and salinity of the surface and deep-water layers, which restrict vertical 



