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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



heavily from this loss. The area affected by this 

 red tide extended from Sarasota southward to the 

 Dry Tortugas. This was approximately 150 miles 

 long afid encompassed an area of several thousand 

 square miles. 



Davis (1948) identified the causative agent of 

 the noxious water and the consequent fish deaths 

 as Gymnodinium brevis, a new species of dino- 

 flagellate. In some places the concentration of 

 the organism was nearly 15 million cells per liter. 

 Also, a definite sequence of discoloration took 

 place. The initial discolored water was an 

 ■'opaque" green as distinguished from the normal 

 clear green. Following this came a yellowish 

 green and later a greenish yellow. The water 

 next took on a bright saffron yellow and became 

 viscid and oily to the touch. This yellow water 

 was associated with dying fish and was apparently 

 caused by G. brevis; this organism contained 

 individually several yellow-green chloroplasts. 

 Other organisms were dominant at the same time 

 and wUl be given consideration later. After the 

 yellow water stage the water turned brown slowly 

 and then "red." The "red" was actually an 

 opaque, dull, dark amber with a greenish yellow 

 cast. At the end of the cycle the water reverted 

 to an opaque green and finally to its normal 

 appearance. 



Red water similar to that which appeared at 

 the end of the discoloration cycle is believed to 

 be caused by purple sulfur bacteria (Hayes and 

 Austin, 1950). ZoBell (1946, p. 165) states, "E.x- 

 tensive populations of purple sulfur bacteria . . . 

 growing associated with decomposing plankton 

 including algae, jellyfish, etc., imparted a dis- 

 tinctly red coloration to the sea. ... It is sig- 

 nificant that the hydrographic conditions in 

 'Bloody seas' are generally precisely those which 

 would promote the growth of purple sulfur bac- 

 teria; namely, the presence of an abundance of 

 decomposing plankton material which provides 

 for H2S production and reduced oxygen tension." 

 In the red water stage in the Gulf few Gymno- 

 dinium were found, since most had previously 

 perished. Animal plankton was found to dom- 

 inate at that time but did not cause the red 

 water (Gunter et al., 1948). 



The poison produced by huge numbers of G. 

 brevis is a powerful toxic agent to fish. This 

 was proved experimentally in the laboratory by 

 placing live and healthy fishes into Florida Bay 



water containing dense masses of Gymnodinium; 

 similar fishes were placed into a control tank 

 containing Biscayne Bay water. Both tanks were 

 aerated strongly by means of electric pumps. 

 All the fishes in the test tank died, while those in 

 the control were unaffected (Gunter et al., 1948). 



Pharmacological studies have been made of a 

 dinoflagellate-produced poison which infected 

 shellfish off the coast of California (Sommer et 

 al., 1948; Riegel et al., 1949), but as yet its exact 

 chemical structure has not been described. 



Connell and Cross (1950) state that a species 

 of the dinoflagellate Gonyaulax produces a lethal 

 anaerobic condition in Ofi'atts Bayou (Galveston 

 Bay) by its own high biochemical oxygen demand. 

 This condition was suggested as the cause of the 

 fish deaths that took place therein. In the red 

 tide observations of 1946-1947 the oxygen content 

 in the affected waters, as a rule, was normal. 



It is evident that one or several genera of dino- 

 flagellates are the causative agents of death- 

 producing blooms of phytoplankton. These or- 

 ganisms, studied by both botanists and zoologists, 

 exhibit a wide range of morphological and physio- 

 logical differentiation. Thus, any satisfactory ex- 

 planation of the red tide must take into account 

 their diverse phj'siological characteristics. 



The conditions necessary for the growth of 

 phytoplankton and for its maintenance include 

 proper temperature, salinity, dissolved oxygen, 

 illumination, hydrogen-ion concentration, the pres- 

 ence of mineral nutrient salts, and possibly of 

 organic substances as well as other less known 

 factors. In the ocean, under normal conditions, 

 the quantity of nutrient salts, particularly the 

 phosphate-phosphorus content, is believed to be 

 the factor limiting the growth of phytoplankton, 

 although the role of organic nutrients cannot be 

 dismissed as unimportant. 



The areas in which the outbreaks of red tide 

 occur in the Gulf of Mexico are those where 

 normally a low plankton content is coupled with o 

 low phosphorus content (Smith 1949). In the red 

 tide waters of 1946-47, Ketchum and Keen (1948) 

 discovered that waters containing a dense G. 

 brevis population have 2}i to 10 times the maximum 

 total phosphorus concentration, both organic and 

 inorganic, to be found normally m the Gulf (table 

 1). Dissolved oxygen, salinity, and other condi- 

 tions deviated little from the average for the areas 

 involved. 



