THE FRKSH-WATKR CYANOPHYCEÆ OF ICKLAND 297 



were killed at the eruption in 1729, and that the present fauna and 

 flora must have immigrated since that time. It is tlien conceivable that 

 new forms are still iminigrating and tinding good conditions, and that 

 the Anabæna thus has arrivcd some time in the period between 1904 

 and 1914. 



The other explanation, which also has some probability. concerns 

 the temperature. Ostenfeld and Wesenberg-Lund (1906 p. 1141) give the 

 following statements regarding the temperature of the lake; ice-bound 

 until May 28. 



July 15, I2V2** C. (highest temp.). 



Aug. 1, 12V2*^ - - 



— 15, 8« C. - 



Sept. 18, 6« - — 



In July 1914 I measured 16*'— 17*^ C. at various piaces in the lake. 

 The measurements were undertaken from a boat out on the lake, and 

 they can thus hardly have been influenced by hot springs (Ostf. and 

 W.-L., 1906 p. 1142). This summer temperature, which undoubtedly is 

 exceptionally high for Myvatn, may perhaps also serve as an explanation 

 of the occurrence of the Anabæna, at any rate it is certainly the cause 

 of the "water-bloom" formation. It is not improbable, that the Anabæna 

 spores, in case the temperature does not rise to a certain degree, may 

 remain through a series of years without germinating in order to awaken 

 to new growth during an exceptionally warm summer (cf. Bristol 1919 

 p. 29 tf.). Wesenberg-Lund mentions 16 — 18 ** C. as the most favourable 

 to the development of the species, and a similar statement (14,5— 18*' C.) 

 exists from Finland (Wahlberg 1913 p. 29). As seen above, the tempe- 

 rature in Myvatn did not in 1904 reach the optimum temperature of 

 the species as it did in 1914. Unfortunately the lowest temperature, at 

 which the spores may germinate, is unknown; but it is not improbable 

 that the low summer temperature in 1904 may have been the cause of 

 the non-appearance of the Anabæna that 3'ear. During the summer 1922 

 Poul Larsen visited Myvatn and observed great quantities of Anabæna 

 in the water. The summer 1922 was not particularly warm; it is to 

 be regretted that he did not measure the temp. of the water. 



Anabæna flos-aqnæ is a pronounced plankton-form thriving by pre- 

 ference in fresh water, but it may also occur in brackish water. It 

 has been found in lake- as well as river-plankton. 



Anabæna inæqualis (Kiitz.) Born. et Flab. 



Bornet et Flahault, Bevision IV p. 231. Lemmermann 1910 p. 181. 



S. Icel. I^ingvellir, pool, temp. 4 *' ^^/s 1914. Geitaberg, pool, temp. 

 IS'* ^/8 1914. — E. Icel. Arnkilsgeråi ^2/^ i894 (H. J.)! 

 Area: Eur., Afr., N. Am., Austr. 



This species, which generally is found epiphytic on other algae, 

 especialh" larger Chloro])hyceæ, sometimes in stagnant water, at other 

 times in but slowly moved water, occurs in a similar way in Iceland. 

 B. Muriel Bristol (1920 p. 62) has found it as a common occurrence in 

 samples of cultivated soil from England. 



The Botany of Iceland. Vol. II. 20 



