SECTION VI 

 PHYTOPLANKTON DISTRIBUTION 



Patricia A. Tester 



Southeast Fisheries Science Center 

 Beaufort Laboratory 

 Beaufort, NC 28516 



Gymnodinium breve (Steidinger 1990; fonnerly Ptychodiscus brevis) is a toxic 

 dinoflagellate species generally restricted to the Gulf of Mexico where it is responsible for 

 red tides, particularly off the west coast of Florida (Baden et al. 1984). This species blooms 

 sporadically (bloom = 5 x ICP cells 1"^) and can reach concentrations of 2-5 x 10'' cells 1"' 

 (Woodcock 1948; Steidinger and Ingle 1972). G. breve blooms are thought to be initiated 

 in offshore coastal waters, primarily in late summer-fall months and transported inshore 

 (Steidinger, 1975). Dinoflagellates are phototactically positive organisms and are frequently 

 more abundant in surface waters than at depth. Because of this, winds, currents, and tides 

 are important factors in the transport (concentration or dispersal) of red tide cells. 

 Consequently, areas of high cell counts may be patchy even during blooms. Bloom 

 conditions though, are associated with a stratified water colimin or a defined water mass and 

 the integrity of this water mass is believed to affect the durations and extent of a bloom 

 (Steidinger and Haddad 1981). 



The more or less armual red tide blooms along the west Florida coast and sporadic 

 blooms elsewhere in the Gulf cause fish kills, neurotoxic shellfish poisoning, and respiratory 

 irritation in humans (Gunter et al. 1947; Woodcock 1948; Pierce 1986). Prior to 1987, the 

 knowledge of the effect of these toxins (collectively known as brevetoxins) on marine 

 mammals was little more than anecdotal (Gunter et al. 1948). Since the implication of 

 dinoflagellate brevetoxins in the dolphin deaths of 1987 (Geraci 1989), examination of 

 phytoplankton samples from areas of high dolphin mortality (especially in the Gulf) is 

 indicated. 



Methods 



Water samples for phytoplankton analysis were collected 22-25 March 1990 in the 

 primary study area between Galveston Bay and the Mississippi delta region (Figure 1). Four 

 samples were collected at each station: one at the surface, one near the bottom, and the 

 other two equidistant from each other and the surface and bottom samples. Since no basin- 

 wide (Gulf) data exist on background G. breve cell concentrations, and little is kiown of its 

 seasonal occurrences or natural variation except in Florida waters, we obtained a number 

 of water samples from the northern Gulf for comparative purposes. 



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