considered to be estuarine or coastal, while 

 the naked forms are found more in the plankton 

 of the open seas. 



Gymnodinium breve is apparently not estu- 

 arine, but neritic. In controlled experiments, 

 Aldrich and Wilson (1960) found the best 

 growth at salinities between 27 and 37 p.p.t. 

 (parts per thousand). Field observations indi- 

 cate ranges of salinity for good growth be- 

 tween 31 and 37 p.p.t. From computer analysis 

 of the 1954-61 field data we find no real 

 abundance of the red-tide organism below 

 30.5 p.p.t. salinity (fig. 2). The upper limit for 

 abundance of organisms appears to vary with 

 temperature. The upper limit is 37 p.p.t.; 

 growth is poor above 36 p.p.t, when tempera- 

 tures exceed about 23° C. 



Rainfall 



Heavy rainfall is often given as a likely 

 factor in producing red tides. Thus, Hornell 

 (1917) mentioned the annual occurrence of 

 red tide "after the passing of the rainy season" 

 along the Malabar Coast. A translation of 

 Niimann (1957) follows: "According to all 

 reports, outbreaks of red water most obviously 

 occur in coastal areas a short time after 

 precipitation. The bloom of Exuviella occurred 

 in the coastal area of Angola after heavy rain- 

 fall in the hills of Biniienland. The Congo and 

 Cuanza Rivers brought nnuch water which was 

 spread throughout the surface layers of 

 coastal waters." On the other hand, red tide 

 occurs in many regions, expecially in areas 

 of upwelling, without abnormal rainfall. 



For the red tide in Florida the evidence 

 is somewhat contradictory. Thus, the heavy 

 red tide of 1946-47 commenced in November 

 1946 after a comparatively dry spell and 

 ceased in September 1947 during very heavy 

 rains. Because all eight recent outbreaks 

 commenced in the fall (table 2), we have 

 plotted the accumulated inches of rainfall for 

 22 south Florida weather stations for March 

 to September, inclusive. Tabulated data were 

 available for 51 years, from 1910 to 1960 

 (see app. table). 



The chance for an outbreak of red tide 

 appears to be much better in the autumn of 

 years with heavy spring and sumnner rainfall 

 than in other years (fig. 3). Only two red-tide 

 outbreaks occurred in years with less than 

 40 inches. The 1916 outbreak, although appar- 

 ently heavy, lasted only 2 months (October 

 and November) and was confined to the area 

 from Boca Grande Pass to Big Marco Pass. 

 Likewise, the 1952 outbreak commenced in 

 November, stopped in January, and was con- 

 fined to the areas adjacent to Sanibel Island, 

 The other outbreaks were all more wide- 

 spread, occurring at various points between 

 Tampa Bay and Cape Romano. The innplication 

 is that rainfall alone has a rather low predic- 

 tive value, but, at the same time, the chances 



Figure 3. — Red-tide years in relation to rainfall In south 

 Florida, March-September, 1910-60. 



that an outbreak will occur are enhanced by 

 high rainfall. Thus, high rainfall must be 

 considered as one of the factors favoring 

 outbreaks. 



Wind 



The effect of wind, or lack of wind, is often 

 mentioned in discussion of factors favorable 

 for blooms. Thus, the spectacular blooms of 

 the diatom Aulacodiscus kittoni at Copalis 

 Beach (Becking, Tolman, McMillin, Field, 

 and Hashimoto, 1927) occur when a heavy- 

 rainstorm is followed by gentle westerly 

 (onshore) winds. Blooms of Noctiluca nniliaris 

 on the Malabar Coast occur between monsoons 

 during calm weather (Bhimachar and George, 

 1950; Hornell, 1917; Hornell and Nayudu, 

 1923). Blooms of Mesodinium rubrum in British 

 Columbia occurred also during calm weather 

 (Clemens, 1935). Lackey and Hynes (1955) 

 commented on the effect of heavy weather 

 in breaking up dense concentrations of G^. breve , 



Pomeroy, Haskin, and Ragotskie (1956) com- 

 mented on blooms of Amphidinium fusiforme 

 and Gymnodinium splendens , "All the Delaware 

 Bay blooms occurred during periods of light 

 winds, not exceeding Beaufort force 2 [4 to 6 

 knots]. On one occasion a patch disappeared 

 when the wind rose from force 2 to 3 [7 to 10 

 knots]." 



Light 



Since most of the overblooming plankton 

 that causes red tide is autotrophic, or depends 

 on organisms that are, light intensity must 

 be considered. Brongersma-Sanders (1957) 

 stated, for instance, that plenty of sunshine 

 seems to be another requirement for red tide, 

 Clemens (1935) noted that the bloom of Mesodi- 

 nium in British Columbia was preceded by 



