THE SEA OF AZOV 485 



The blue-green algae (chiefly in the Gulf of Taganrog) constitute even at the 

 time of their maximum bloom (summer) barely 13 per cent of all the phyto- 

 plankton, and on the average only 4-2 per cent. 



Diatoms have two maxima : a larger spring one (March-April) {Skeletonema 

 costatum and Coscinodiscus spp.), when the amount of diatoms reaches 

 7 g/m 3 , and a late autumn one (October-November) {Rhizosolenia calcar-avis, 

 Leptocylindrus danicus, Ditylium brightwelli, Skeletonema costatum, Thalas- 

 sionema nitzschioides, Thalassiosira nana and different species of Chaetoceros 

 and Coscinodiscus), with a maximum of up to 2 g/m 3 . During the rest of the 

 year the amount of diatoms decreases sharply (May, June, July, December, 

 January, February). During the bloom periods the diatoms form 90 to 98 per 

 cent of the total phytoplankton biomass. At times the biomass of the Sea of 

 Azov reaches a colossal amount, one which has been found in no other sea. 

 For August 1925 P. Usachev gives a plankton biomass of 270 g/m 3 (385 g/m 3 

 in other years) and for October 1924 — 106 g/m 3 ; moreover, the plankton 

 consisted almost entirely of the diatom Rhizosolenia calcar-avis. As G. Pitzik 

 has shown (1951), such a huge phytoplankton biomass has not been found since 

 1934 owing to a great reduction of the number of diatoms of Rhizosolenia 

 calcar-avis during the periods of bloom, when it was no higher than 13 g/m 3 , 

 and usually about 2 to 4 g/m 3 . 



The peridineans form an almost equally important component of the Sea 

 of Azov plankton. Their maximum multiplication takes place in the summer. 

 Their summer bloom is preceded by a small early-spring bloom in March 

 (about 0-85 g/m 3 ). At its maximum development, in August, the peridinean 

 biomass is on the average about 3-5 g/m 3 , consisting mainly of Exuviella cor- 

 data, Goniaulax polyedra, G. triacantha, Proter о centrum micans, Peridineum 

 knipowitschi, and others; owing to their mass development the water is 

 coloured reddish-brown. 



Among the Cyanophyceae mainly Nodularia spumigena and, to a lesser 

 extent, Aphanizomenon fios-aquae, Microcystis aeruginosa and some species of 

 Anabaena develop in large masses. Volvocales and the green algae are mostly 

 concentrated in the parts of the Sea with a lower salinity. 



Blooming in patches is at times caused by a mass development of Flagellates 

 of the orders Chrysomonadina and Cryptomonadina. Among the Silico- 

 flagellates Ebria tripertita is predominant. 



Phytoplankton development in the Gulf of Taganrog has some features 

 peculiar to itself (Fig. 230a and в). In the first place, phytoplankton is con- 

 siderably more developed here, and the blue-green algae (Microcystis and 

 Aphanizomenon) are predominant during the whole warm season (June to 

 November), reaching in the autumn a phytoplankton biomass of 85 to 93 

 per cent; the diatoms become markedly preponderant only in spring (Fig. 

 231). 



The number of Aphanizomenon filaments may be more than 5-5 mil- 

 liards/m 3 , each of the filaments consisting of 100 to 150 cells. The number of 

 peridineans and diatoms at one station also reached 4-5 millions of 

 specimens/m 3 . 



All these quantitative data, which are mainly taken from the work of 



