"A couple of weeks of bright, sunny, calm 

 weather." Conover (1954) listed "high radia- 

 tion values" as one of the essentials for a 

 bloom of Gonyaulax . Hornell (1917) stated 

 that red tide on the Malabar Coast required 

 "A continuance of fine weather for a week or 

 ten days, with plenty of sunshine." 



There is evidence, however, that light is 

 seldom a limiting factor in G. breve abundance. 

 Wilson (1955) obtained good growth of cultures 

 by using 175 ft.-c. (foot-candles) for 15 hours 

 per day, and Aldrich (1960) stated that light 

 intensity is probably not a growth-limiting 

 factor above the 200 ft.-c. level. Aldrich (1962) 

 established, however, that G. breve is auto- 

 trophic, requiring light for survival. 



Upwelling 



In many of the regions where red tides 

 occur, nutrients are brought to the surface 

 by upwelling of nutrient- rich waters. The 

 nutrients do not of themselves appear to be 

 the cause of overblooming, however. All 

 authors seem to agree that red tides usually 

 occur after the cessation of upwelling. For 

 instance, Brongersma-Sanders (1948) stated 

 that blooms of Noctiluca in Walvis Bay occur 

 during periods of minimum upwelling; she 

 also said (1957) that "in areas where upwelling 

 occurs during part of the year only, red water 

 usually develops toward the end or directly 

 after the period of upwelling." 



In regard to the situation on the west coast 

 of Florida, Graham, Amison, and Marvin 

 (1954) showed that the surface waters of the 

 Gulf gradually decrease in phosphorus out to 

 a distance of about 85 miles, and that limited 

 upwelling of deep water at certain times had 

 no apparent effect on the phosphorus content 

 of water in the euphotic zone. 



Nutrient Levels 



It appears to be the concensus of many 

 authors that red tides caused by dinoflagellates 

 do not depend upon a high concentration of 

 nutrients. In fact, dinoflagellates usxially bloom 

 after the diatoms have imipoverished the water. 

 Thus, Brongersma-Sanders (1957) said, "The 

 greatest outbreaks of red water probably 

 occur toward the end of a phytoplankton 

 season. . . ." Hornel and Nayudu (1923), ac- 

 cording to Ryther (1955), described red water 

 caused by peridineans occurring annually along 

 the Malabar Coast, following the cessation of 

 the southwest monsoon, diatom blooms, and 

 heavy rainfall. Blooms of Gymnodinium sp. 

 occur annually along the TrivandrumCoast of 

 India, following rains and diatoin maxima, 

 according to Menon (1945). 



In summing up this reasoning, Ryther (1955) 

 wrote that dinoflagellates have often been 

 credited with the ability to utilize and flourish 

 in extremely low concentrations of nitrogen 



and phosphorus. He stated that this concept 

 stems largely from observations that dino- 

 flagellate maxima, in temperate waters, fol- 

 low the decline of the spring diatom flowering, 

 and that relatively large populations often 

 persist throughout the summer, when the 

 supplies of these nutrients are alnnost unde- 

 tectable. 



Niimann (1957) appeared to hold similar 

 views. He stated, "It also appears--and this 

 is our final conclusion--that a mass outburst 

 of phytoplankton occurs when fresh-water 

 growth-promoting substances (trace elements, 

 enzymes, or other biologically active sub- 

 stances) reach the sea. Due to the presence 

 of a necessary quantity of nutrients in the 

 sea, a pre-condition for the outburst of 

 plankton in the sea exists already. Accordingly, 

 these outbursts occur only near the coast." 



In studying the phosphorus values within and 

 without red-tide water from the work of 

 several authors, Bein (1957) concluded con- 

 cerning G. breve that, "It is possible that the 

 threshold level of total phosphorus necessary 

 to support dense populations of this organism 

 is lower than originally assumed. ... It seems 

 very probable that, insofar as phosphorus is 

 concerned, the areas of the west coast of 

 Florida which have recorded Red Tides are, 

 at all times, capable of supporting an out- 

 break. . . ." 



It also appears that G. breve does not re- 

 quire much nitrogen. Lackey and Hynes (1955) 

 stated that Howard Odum failed to find any 

 nitrate nitrogen in a series of about 15 field 

 samples from red-tide water, taken October 6, 

 1953. Dragovich (1960b) stated, "No relation- 

 ship was observed between the incidence of 

 G. breve and nitrate-nitrites." 



In considering the nutrients available, one 

 must note that the rivers of the Florida west 

 coast- -especially those draining phosphatic 

 formations --contain heavy concentrations of 

 phosphorus (Odum, 1953; Specht, 1950; Graham 

 et al., 1954; Dragovich and May, 1962). After 

 weighting the discharges of seven rivers 

 according to relative concentration of phos- 

 phorus, which differ considerably among the 

 rivers, we estimated the metric tons of phos- 

 phorus discharged by them. The results are 

 shown in figure 4. The Tampa Basin rivers 

 are the Hillsborough, Alafia, Manatee, and 

 Little Manatee; the south Florida rivers are 

 the Peace, Myakka, and Caloosahatchee. The 

 picture does not vary too markedly from that 

 of rainfall. From these data one cannot assume 

 that phosphorus necessarily had any more 

 effect than another ingredient of river water. 



Are Preceding Diatom Blooms Conducive to 

 Red Tide? 



It seems to be fairly well established that 

 red-tide organisms can and do bloom when 

 nutrient levels are low. There has been 



