Microcystis aeruginosa Kuetz.; emend. Elenkin 1924, p. 14 



[M. aeruginosa var. major G. M. Smith; M. flos-aquae 



( Wittr. ) Kirchner; M . ichthyoblabe Kuetzing] 



PI. 102, Figs. 1-4 



An ovate, spherical, or irregularly lobed, saccate and clathrate 

 colony of numerous spherical cells which are much crowded within 

 a gelatinous matrix (several colonies sometimes invested by a com- 

 mon tegument); colonial mucilage hyaline and homogeneous, re- 

 taining a definite shape; cell contents blue-green, highly granular 

 and with conspicuous pseudovacuoles; cells 3-4.5/x in diameter. 



Very common in hard water lakes, becoming especially abundant 

 during late summer periods and appearing in such dense growths 

 in favorable habitats as to color the water. Mich., Wis. 



This species is a frequent component of water blooms, especially 

 in lakes with eutrophic characteristics, although it is common to a 

 great variety of aquatic habitats. The tendency to float high in the 

 water results in the formation of large, macroscopic clots and floating 

 crusts which develop as the plants push each other above the 

 surface. Like Aphanizomenon flos-aquae, this species is notorious as 

 a spoiler of water for domestic uses, swimming, and recreation and 

 often causes the death of fish in heavily infested lakes. 



In assigning species of Microcystis collected in this region, I have 

 followed the nomenclature recommended by students who have 

 given special attention to the genus. There long have been recog- 

 nized two species names for the very common planktonic forms of 

 Microcystis, which show considerable variation in colony form. 

 The name M. aeruginosa Kuetz. has been used for the plant which 

 has a perforated or clathrate and much lobed colony. The name 

 M. flos-aquae (Wittr.) Kirch, has been applied to the form with 

 globose, non-perforate or non-clathrate colonies. Elenkin (1924, p. 

 14), after a critical study, reduced this latter species to synonymy 

 with M. aeruginosa Kuetz. At the same time he recognized two 

 forms: fa. minor Elenkin, cells 3-5/^ in diameter, and fa. major 

 ( Wittr. ) Elenkin, cells 4-7/x in diameter. It is held by some that the 

 perforate condition of the colony is merely a character accompany- 

 ing age and that the many intergrading variations are related to 

 physical conditions in the habitat such as water currents, or to 

 methods of collecting. Virieux (1916) and others (Drouet and 

 Daily, 1939) also have advocated uniting M. aeruginosa and M. 

 flos-aquae. Drouet and Daily (I.e.) have published on the synonymy 

 of Microcystis aeruginosa and have discussed the fresh-water species 



[456] 



