DINOFLAGELLATES OF THE GULF OF MEXICO 



By Herbert W. Graham, United States Department of the Interior, Fish and Wildlife Service 



Dinoflagellates are important in the natural 

 economy of the Gulf of Mexico as they are in all 

 waters of the world. In marine phytoplankton 

 thej' are usually outnumbered by diatoms, but 

 they are second in importance to the diatoms as 

 fundamental synthesizers of organic material in 

 the sea. On the other hand, to the dinoflagellates 

 belong most of the organisms which cause "red , 

 water," mass mortalitj' of marine organisms, and 

 paralytic shellfish poisoning. A thorough knowl- 

 edge of the dinoflagellates is necessary to a clear 

 understanding of the basic biology of the Gulf of 

 Mexico. 



Despite the importance of these organisms, the 

 Gulf of Mexico is almost a terra incognita in 

 respect to our knowledge of the dinoflagellate 

 plankton. Very few oceanographic expeditions 

 have included the Gulf in their itinerary, and 

 those that visited the Gulf have not reported on 

 any dinoflagellate collections. 



Many species of dinoflagellates have a world- 

 wide distribution, especially the offshore forms. 

 Many of these can be expected in the Gulf. It is 

 very likely that the pelagic species of the Gulf 

 will be found to be similar to those of the tropical 

 Atlantic, although the general composition of the 

 flora may be different. The inshore, or neritic, 

 plankton may well contain species peculiar to or 

 at least characteristic of the Gulf of Mexico or of 

 certain areas of the coast line. The dinoflagellate 

 fauna of the open Gulf is very likely quite similar 

 to that of the Caribbean and the tropical Atlantic. 



As far as the dinoflagellates are concerned, there 

 are three general habitats in the Gulf of Mexico: 

 the offshore waters, the neritic waters, and the 

 sandy beaches. 



The offshore waters of the Gulf are clear and 

 blue, characteristic of tropical waters the world 

 over. Surface temperatures are high, the con- 

 centration of nutrients is low, and the salinity 

 high throughout the year. The quantity of 

 plankton in these waters (the standing crop) is 

 low (Riley 1938), but the number of species is 



probably relatively high. The temperature of 

 this water drops markedly in the northern part 

 of the Gulf for a few weeks during the winter, 

 but as far as we know there is no seasonal change 

 in the dinoflagellate fauna during this period. 



The neritic waters may be considered to include 

 the shallow periphery of the open Gulf in which 

 the water is often of very high temperature, with 

 variable salinity and nutrient content and in 

 which wind mixing creates high turbidity, par- 

 ticularly in the winter when the density of the 

 water is uniform from surface to bottom. The 

 bays, bayous, and lagoons are also within this 

 zone. These include mangrove swamps and other 

 brackish \rater areas. Tidal effects are strong in 

 the neritic zone, and the physical and chemical 

 conditions of the water vary greatly tliroughout 

 the year and, in some cases, within a daily tidal 

 cycle. As a general rule, the species of dinoflagel- 

 lates found in the neritic zone are distinct from 

 those in open waters. However, the invasion of 

 the coastal area with open Gulf water frequently 

 obscures the zonation. 



Sandy beaches in the intertidal zone constitute 

 the third type of environment for the Gulf dino- 

 flagellates. Certain specialized species belonging 

 chiefly to the genus Amphidinium thrive in this 

 situation in some parts of the world (Herdman 

 1924) causing discoloration of the sand and lu- 

 minescence. When they are abundant, each kick of 

 the heel at night on a wet, sandy beach will cause 

 a flash of light. There are apparently no reports 

 of such "dinoflagellate sand" for the Gulf coast, 

 but a careful investigation of this zone might 

 reveal a rich fauna. 



Interest in the dinoflagellates of the Gulf, par- 

 ticularly along the west coast of Florida, was 

 stimulated by the disastrous outbreaks of red 

 tide which occurred in that area in 1946 and 1947 

 (Galtsoff, 1948; Gunter et al., 1948; Gunter, 

 Smith, and Williams, 1947; Smith 1949). This 

 red water was caused by a previously undescribed 

 species, Gymnodinium hrevis Davis (1948). The 



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