THE BALTIC SEA 295 



A. cochlearis, A. eichwaldi, A. tecta, A. quadrata, Collotheca pelagica, C. muta- 

 bilis, Notolca striata, Triarthra Iongiseta, Polyarthra trigla, Asplanchna 

 brightwellii) or the brackish-water ones (Arntrea cruciformis var. eichwaldi, 

 Synchaeta ba/tica, S. monopus, S.fennica and S. littoralis). 



The copepod crustaceans are presented mainly by Eurytemora hirundoides, 

 E. affinis, E. hirundo, Acartia bifilosa (on some sites A. tonsa), Pseudocalanus 

 elongatus, Temora longicornis and Eurytemora hirundoides, and in the coldest 

 parts of the Sea Limnocalanus grimaldi (Fig. 138), among the daphnid Bos- 

 mina maritima, Evadne nordmanni and some species of Podon, and in the 

 parts of the Gulf with the lowest salinity Daphnia cucullata, Chidorus sphaeri- 

 cus, Leptodora kindti and other fresh-water forms. In the deeper layers of the 

 western part of the Sea Calanus finmarchicus, Oithona similis and Sagitta 

 elegans f. ba/tica are frequently encountered. The mysid M. oculata is 

 widely distributed throughout the Baltic Sea. The other mysids — Gastrosac- 

 cus spinifer, Praunus inermis and P.flexuosus — are found in the Baltic Sea in 

 smaller numbers. The larvae of the bottom-living animals and especially 

 Macoma, Hydrobia, Balanus, Membranipora and the polychaetes form a 

 considerable constituent of the plankton. Among the tunicates Oikopleura 

 dioica and Fritillaria are encountered. Among the large plankton forms the 

 medusa Aurelia aurita is found at times in large numbers throughout the Sea, 

 and in the southern part of the Sea Cyanea capillata, Pleurobrachia pileus, 

 Hyperia gal/a, Sagitta elegans baltica (I. Markovsky, 1950). 



An interesting phenomenon was noted by J. Valikangas (1926) for the 

 Baltic Sea, namely that a large number of fresh-water forms develop most 

 rapidly not in fresh water but at a salinity of 3-45 to 5-4% . Examples are 

 Tintinnidium fluviatile, Floscularia sp., Asplanchna brightwellii, Triarthra 

 Iongiseta, T. brachiata and others. 



C. Brandes (1939) distinguishes three groups of forms in the plankton of the 

 Baltic Sea: the 'marine', the brackish-water and the fresh- water. Although 

 many of the marine forms penetrate deep into the Sea, they are fairly rare 

 there and do not have a mass development. 



The Darss ridge forms a marked boundary as regards both hydrography 

 and biology. This is particularly clear in the case of plankton. Brandes has 

 noted that the 'marine' forms are preponderant to the west and the brackish- 

 water ones to the east. At a salinity of more than 9% the marine Ceratium tripos, 

 Melosira, Rhizosolenia and the ciliates Parafavella are markedly preponderant. 

 With further loss of salinity the brackish-water form Chaetoceros danicus and 

 the ciliates Helicostomella and Aphanizomenon are no less markedly pre- 

 ponderant. At a salinity below 6-5% the fresh-water forms Chlorophyceae, 

 Chroococcacea and some Rotifera (Brachionus, Ratulus and others) become 

 abundant. The change of some plankton in a cross section from the Fehmarn 

 Belt to deep inside the Baltic Sea is shown in Fig. 139. 



Two biogeographical communities stand out clearly in the Baltic Sea 

 phytoplankton (I. Nikolaev, 1951): the Arctic and the Boreal Arctic of the 

 spring period {Table 123). 



The two communities partly overlap one another, but broadly speaking 

 the Arctic community is more marked in the spring at a temperature of 



