JOHN H. RYTHER 401 



concentrations. They appear to require the presence of some one or 

 more unknown organic substances, and there is some indication that 

 they benefit from the previous existence of a large popuhition of 

 diatoms. It is doubtful if all these conditions are usually satisfied at 

 any one time, and unlikely that they are all necessary for the domi- 

 nance of dinoflagellates. 



An examination of Table II reveals one factor which is almost 

 universal in red water outbreaks, the occurrence of a high water 

 temperature. In temperate or boreal regions of the ocean, red water 

 appears to be restricted to the summer months, and the notation is 

 frequently made that it is preceded by periods of unusually hot, calm 

 weather. Along the Indian coast, red water occurs during the clear, 

 hot periods between the southwest and the northeast monsoons 

 (Hornell and Nayudu, 1923; Menon, 1945; Bhimachar and George, 

 1950). Off the Peruvian and Southwest African coasts it appears 

 during the southern summer when the upwelling of cold water is at a 

 minimum and water temperatures are the highest of the year (Bron- 

 gersma-Sanders, 1948 ) . Allen ( 1946 ) described a number of occur- 

 rences of red or yellow water along the California coast which have 

 always occurred in mid-summer during periods of hot, calm weather 

 and smooth seas. 



There is less evidence that low salinity is an important factor in 

 red water outbreaks. Slobodkin (1953) has shown a close correlation 

 of red tides along the west coast of Florida with previous periods of 

 exceptionally heavy rainfall. He has proposed that the organisms 

 develop in small, discrete masses of relatively low-salinity water which 

 result from the increased land drainage during heavy rains and are 

 maintained in the ocean by density gradients. Table II gives several 

 other instances in which red water has been preceded by heavy 

 rainfall ( Whitelegge, 1891; Hornell and Nayudu, 1923; Lund, 1936; 

 Menon, 1945; Connell and Cross, 1950). However, this situation is 

 by no means universal. On the South African, Peruvian, and California 

 coasts, red water occurs during periods when precipitation is at a 

 minimum. Furthermore, where such measurements have been made, 

 the salinity in patches of red water does not appear to be significantly 

 lower than that of the surrounding, clear ocean water (Ketchum and 

 Keen, 1948; Chew, 1953; Torrey, 1902). 



