PHOSPHORUS AND RED TIDE 



The high values of total phosphorus found in the bloom of Gynnodinium 

 b re vis in 19h7 have yet to be satisfactorily explained. Nowhere in the 

 Gulf or in Charlotte Harbor at any time during this investigation were 

 phosphorus concentrations found corresponding to the high values which 

 occurred in the red tide of 19^7 except in a scum of Trichodesmium which 

 will be discussed below. 



Neither the rivers , nor the bottom of the Gulf, nor the deep water 

 of the Gulf can be held as a source of phosphorus sufficient to increase 

 the concentration in the body of coastal water to the values which wert 

 found in the red tide samples. 



Ketchum and Keen's (19U&) suggestion that the high values may have 

 been caused by a swarming of organisms to the 'surface after absorbing 

 all the phosphorus in the water column is as plausible an explanation 

 as any to date. Unfortunately no blooms of Gymnodinium occurred during 

 this study so that this hypothesis could not be tested. 



However, blooms of Trichodesmium are common along the Florida west 

 coast and an opportunity was afforded to make an analysis of this material. 

 This filamentous blue-green alga is always present in the plankton there 

 and frequently occurs in bloom proportions. It grows in the water and on 

 the surface. It was found in practically every plankton sample examined, 

 whether surface or subsurface. (See also King, 195>0.) Its propensity for 

 floating on the surface is the feature which is of importance to the present 

 problem. At times clumps of alga appearing as green flakes are spread over 

 a wide area. At other times the growth is so abundant as to cover the sur- 

 face of the sea with a scum so thick that it can be scooped up with the hand. 



Since it accumulates at the surface it is driven by the wind. Some- 

 times it piles ur? in such abundance as to create a nuisance at bathing 

 beaches. On April 12, 19U9 a bloom was blown into Sarasota Bay and up 

 onto the beach. The accumulated decomposing organic matter created such a 

 nuisance that it was necessary for the city of Sarasota to haul it away in 

 trucks . 



The color of the blooms varies from light green through straw-colored 

 to brownish-red. There is evidence to indicate that when brown the alga 

 is moribund. On one occasion when a mass of straw-colored Trichodesmium 

 was filtered, the filtrate contained a high concentration of red-blue pig- 

 ments indicating that the cells were breaking down. 



Phosphorus analyses of some of these blooms are presented in table 6. 

 It is evident that heavy blooms occurred in water containing very little in- 

 organic phosphorus. The analysis of the bloom of October 5, 195>0 is par- 

 ticularly significant in this respect. This bloom was found as a thick 

 straw-colored surface accumulation about 12 miles off Venice, Florida, in 



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