GULF OF MEXICO 



175 



Table 1. — Phosphate-phosphorus determinations (Smith 

 1949) 



The source of this increased phosphorus is the 

 main problem faced by the red tide scientist. 

 Smith (1949), Ketchum and Keen (1948) suspect 

 that the presence of this element can be explained 

 bj- a simple process of accumulation. If the initial 

 absorption of phosphorus by G. hrevis takes place 

 over an entii'e water column (approximately 10 

 meters in depth) , it is possible for the organism to 

 concentrate this element by migrating to, and 

 aggregating at, the surface. Unfortunately, chem- 

 ical analyses of the water at different depths were 

 not made, hence, no direct evidence is available 

 to test this hypothesis. It appears that even with 

 the removal of all the phosphorus from the water 

 column, the phosphorus content was excessive. 

 George L. Clarke of Harvard (personal communi- 

 cation) theorizes that vertical migration linked 

 with a subsequent horizontal concentration due 

 to convergences of water masses may be the 

 complete solution. 



Coincident with the large amount of total 

 phosphorus the total nitrogen in red tide waters 

 bears consideration. Sverdrup et al. (1942), in- 

 dicate that the ratio of nitrogen to phosphorus 

 in phytoplankton is about 15 atoms to 1. Al- 

 though no analysis for this element was made, its 

 presence or absence has a bearing on the problem. 

 In order to provide for the large quantity of nitro- 

 gen that ordinarily accompanies the metabolic 

 absorption of phosphorus, it is possible that red 

 tide organisms might be able to utilize atmospheric 

 nitrogen as some of the blue-green algae are capable 

 of doing. On the other hand, if this is not so, the 

 organism must be able to survive and develop 

 under far lower nitrogen concentrations than ever 

 before were recorded. 



King (1950), while attempting to produce red 

 tide in the laboratory, states that Gymnodinium 

 simplex, a dinoflagellate closely related to G. 

 hrevis, was able to utilize dissolved organic 

 nitrogenous matter. Though the concept of 



organic utilization by obligate phototrophs is 

 not a new one to the field of protozoology (LwofI 

 1947), it does suggest that in red tide dissolved 

 organic material could be a source of nitrogen. 



The concentration of nutrients may have been 

 the result of the lateral or vertical migration of 

 some organism or organisms other than Gymno- 

 dinium. Comcident blooming of other phytoplank- 

 ton was pointed out by Gunter et al. (1948). 

 These were primarOy naviculoids and other dia- 

 toms. Utilization of phosphorus accumulated 

 this way and later released could account for the 

 bloom of the death-causing Gymnodinium. 



The theory of accumulation as it stands does 

 not explain the fact that swarming occurs at 

 infrequent intervals as much as 30 years apart, 

 nor does it explain any possible causative mech- 

 anism. 



The upwelling of nutrient-rich water has been 

 proposed but is not known to occur normally 

 in the Gulf of Mexico. For red tide in other 

 areas of the world, such as along the coast of Peru, 

 this is an adequate explanation. However, the 

 phosphorus present in the Gulf as reported by 

 Ketchum and Keen was far in excess of that 

 found normally in deep water. Nor are the 

 deeper water layers of the Gulf rich enough in 

 nutrient content to provide the amount found. 



The presence of nutrient salts in bottom 

 sediments is a likely source of nitrate and phos- 

 phate concentrations. Recently, Robert H. Stew- 

 art, a government geologist, in 1950 discovered 

 phosphate deposits covering a 25-mile area off 

 Tampa, Florida. Unfortunately, he has not 

 suppHed the authors with any further information. 

 The coincidence of these deposits with the area 

 in which red tide outbreaks have occurred sug- 

 gests that this may be a partial explanation of 

 phosphorus availability. 



The problem presented is to account for periodic 

 releases of nutrient salts from these bottom de- 

 posits. Possible explanations include shifts due 

 to cataclysmic upsets in the ocean bottom or 

 simple mechanical shifting of bottom muds due 

 to strong bottom currents. These, of course, are 

 still speculative. 



River drainage as a soiu-ce of mineral deposits 

 in the Gulf has been suggested (Smith 1949). 

 Since Florida is a major source of rock phos- 

 phates, this possible origin cannot be discarded. 

 The remoteness of places like Key West and Cape 



