THE RED TIDE — SMITH 379 



a water mass were to remain red-tide active while moving along the 

 coast, it must not mix too quickly with surrounding harmless water 

 and so be dissipated. Therefore, said the oceanographers as they 

 reviewed the results of the Gerda cruises, we must next find out just 

 what the conditions are that prevent mixing. These will be the con- 

 ditions which allow a series of red-tide outbreaks to occur and they 

 may well lead us to a method of prediction. 



Chew and his group from Miami worked out a mathematical for- 

 mula. In simple language it said that "sea water becomes heavier 

 or denser as it becomes cooler or more salt, but less dense as it warms 

 up or becomes fresher. The mixed bay and Gulf water which supports 

 red tides is lighter than Gulf of Mexico sea water. The red-tide water 

 therefore tends to float above the rest. If it is very much lighter, 

 though, it spreads out like an oil film and so begins to disappear. 

 If it is only slightly lighter than the Gulf water it will mix more 

 easily." So, for red tide to progress into a major series of outbreaks 

 the difference in density must be neither too much nor too little. 



But how could this density be predicted ? Clearly it was related 

 to the amount of brackish water entering the ocean and so to the 

 fresh water entering the bays and this in turn to river drainage and 

 rainfall during the previous months. It was also related to the dif- 

 ference in temperature between Gulf water and bay water and con- 

 sequently to the air temperature of winds which influence them. 



SUCCESS 



It seemed a long shot, but after taking meteorological figures for 

 26 past years and performing numerous calculations with different 

 combinations of the data, a formula emerged which worked. The 

 weather information for any year was placed into the formula. When 

 the numerical result fell within a certain narrow range, then a red- 

 tide outbreak happened during the next 12 months. If outside the 

 range, there was no red tide. 



But this was only a start. The test would come when predictions 

 for future years could be checked. Time was of the essence, since 

 a red-tide outbreak is a serious matter to the west-coast residents, 

 and might well cause millions of dollars of lost business if not con- 

 trolled. So, though a scientist does not like to take chances, it was 

 decided, even before the theory had been fully worked out, to risk a 

 forecast. In November 1955 the State Board of Conservation in 

 Florida was notified that there was little likelihood of major red-tide 

 outbreaks in the year 1956. It turned out that there was none. A simi- 

 lar prediction was made for 1957. The west-coast waters of Florida 

 will be watched with interest to see if it holds good. 



