202 Marine Microbiology 



water in the lake "Malo Jezero" (4) and, successively, in 1954, 

 in the lake "\'eliko Jezero" (5). Both brackish lakes, situated in 

 the island of Mljet in Jugoslavia, present morphological and 

 chemical-physical characteristics quite similar to those ascertained 

 in the lake of Faro. 



Such a pecuHar sulphur-oxidizing bacterial flora, providing 

 the above colour, is to be expected in environments showing the 

 same ecological characteristics as the lake of Faro. However, it 

 must be pointed out that, in order to describe the phenomenon, 

 I carried out a careful and detailed exploration of whole column 

 of water, meter by meter, wliile, for any other hydrobiological 

 research, samples of more distant levels would satisfy. It may 

 be supposed that such environments more frequently would 

 show the occurrence of "red water," if examined in more detailed 

 way, considering the limited thickness that such a layer of water 

 sometimes has. 



It must also be pointed out that in such environments the 

 phenomenon is in relation to the existence of deteniiined ideal 

 conditions such as an adequate temperature and a noiTnal strat- 

 ification of the water, causing especially a rather constant 

 quantity of H-S at the layer immediately underneath the one 

 where the "red water" is found. 



Under such conditions, even pH would maintain within 

 lower limits, although according to Van Niel (10) and Baas 

 Becking and Ferguson Wood (2), the pui-ple bacteria can de- 

 velop within much higher limits of pH. 



If such environmental stability which is the one condition- 

 ing the presence of the "red water" should fail, then as a conse- 

 quence the fundamental basis for the occurrence of the phenom- 

 enon in nature would fail. 



These conditions, in reality, occurred in the lake of Faro 

 during the course of these investigations. The exceptional situ- 

 ation which arose in August, followed a first period of normal 

 stratification of the water which lasted until July and during 

 whicli time the phenomenon of "red water" was observed. In 

 fact, in the sampling carried out on 4 August, tlie hydrogen 

 sulphide, present at a short distance from the surface, produced 

 a distrophic crisis, causing a remarkable damage to mussel farm- 



