samples for nitrate, nitrite, and silicate meas- 

 urements in the laboratory; phosphate concen- 

 trations were measured at sea. The summary 

 of surface water data given in table 3 shows 

 higher values at the Dome than off the coast 

 of southern Mexico, off Cocos Island, or off 

 San Diego (fig. 1). The high values at the 

 Dome are consistent with previous information 

 about the distribution of physical, chemical, 

 and biological properties in this characteris- 

 tically eutrophic area (e.g., Holnies et al., 

 1957; Wooster and Cromwell, 1958). 



Culture and Physiology of Tropical Oceanic 

 Phytoplankton (W. H. Thomas) 



Oceanic tropical phytoplankton from the 

 Costa Rica Dome and from the Gulf of Tehuan- 

 tepec have been isolated into unialgal cultures. 

 These cultures were used in laboratory ex- 

 periments to establish the optimum chemical 

 and physical conditions for growth. Optimum 

 conditions were taken as those which sup- 

 ported a maximum rate of growth in the 

 laboratory. 



The optima were then compared with oce- 

 anographic conditions m the eastern tropical 

 Pacific Ocean, as measured during STOR 

 cruises. This comparison is intended to in- 

 crease our understanding of the temporal and 

 spatial distribution of phytoplankton in this 

 region. 



Most previous studies of this type have been 

 done with more easily grown coastal species, 

 and the results may not be completely appli- 

 cable to the open sea. The present work is 

 wholly concerned with species taken from 

 open sea areas of particular interest to the 

 STOR program. 



The following algae have been cultured in 

 this investigation: Gymnodinium simplex ? (two 

 strains), /VaanocAioris (possibly A/, atomus ) , Chae'- 

 toceros spp. (four strains), Nitzschia spp., and 

 several unidentified forms. 



Results with Gymnodinium .--Two isolates of 

 Gymnodinium were obtained. One was from sta- 

 tion 49 of Expedition SCOT (TO-58-1) 

 (9°48.5' N., 89°14.5' W.) in the area of the 

 Central American Thermal Dome; the other 

 was from station 19 of cruise TO-58-2 

 (14°40.3' N., 96°58.6' W.) m the Gulf of 

 Tehuantepec. 



Morphologically, these isolates are iden- 

 tical. The cells are about 17 (j. in length and 

 7 fi wide. The sulcus is not well developed. 

 The cells are motile with the typical whirling 

 motion of dmoflagellates. Dr. J. B. Lackey 

 has tentatively identified this organism as 

 G. simplex. 



Logarithmic growth rates have been meas- 

 ured at different temperatures in various 

 experiments. For both isolates the optimum 

 range is 21-29° C. Maximum growth of the 

 Dome isolate occurred at 29 C, and at 26 C. 

 for the Tehuantepec isolate. A comparison of 

 the optimum range with sea surface tempera- 

 ture charts of the eastern Pacific Ocean shows 

 that this organism would grow most success- 

 fully, if supplied with other requirements, as 

 far north as 27° N. in the summer months and 

 south of 23° N. in the winter. This conclusion 

 is based on the position of the 21° C. surface 

 isotherm. Gymnodinium might occur in waters 

 having a lower temperature, but would not 

 grow at an optimum rate. Thus, it appears 

 to be a tropical organism. The growth rates 

 of the Tehuantepec isolate are slightly less 

 than those of the Dome isolate at most tem- 

 peratures. This difference has not yet been 

 explained. 



The growth rate of the Tehuantepec isolate 

 was measured at several light intensities. 

 Saturation occurred at about 700 foot-candles; 

 the half- saturation value is 200 ft.-c, and 

 compensation is about 30 ft.-c. For the Dome 

 isolate similar figures are 800, 250, and 

 30 ft.-c, respectively. No inhibition of either 

 isolate was observed at illuminances from 

 800 to 2,000 ft.-c. Growth at higher intensities 

 was not measured; fluorescent lights were 



Table 3. --Mean surface nutrient concentrations, Tuna Commission cruise to Costa Rica Dome 



and Cocos Island, November 1959 



[Concentrations in ^g.-atyi., numbers of observations in parentheses] 



26 



