- 5- 



of the Gulf were between 30° and 31 °C, were below 

 28 °C at the top of the thermal ridge, where surface 

 phosphorus concentrations were also higher than to 

 the east or west. Surface circulation measured by 

 GEK was in good agreement with that indicated by the 

 dynamic topography, especially between the above- 

 mentioned ridge and hollow where it was of the order 

 of 1 to 2 kt in a southwesterly direction. The distri- 

 bution of zooplankton standing crop was what might 

 be expected if a crop recently produced on the ridge 

 were in process of transportation downcurrent. 



The area is to be resurveyed further. 



Experimental Assessment of Phytoplankton - limlting 

 Nutrients and the Culturing of Pelagic Phytoplankton 

 (W. H. Thomas) 



At SCOT Stations 4 and 49 (figure 1) 5-gallon water 

 samples were taken at a depth of 10 m. Station 4 was 

 located in the oligotrophic area west of Baja Califor- 

 nia and Station 49 was located in the eutrophic "Dome" 

 area off Central America. Fifty-ml aliquots of these 

 samples were dispensed into forty-two 125-ml flasks 

 to which suspected nutrients were added in various 

 combinations. Fourteen such treatments were set up 

 in triplicate. The nutrients used were N, P, Si, trace 

 elements, soU extract, purines and pyrimidines, and 

 amino acids. After addition of nutrients the flasks 

 were incubated at 25 °C and were illuminated contin- 

 uously at 600-900 foot -candles. The response of the 

 phytoplankton in each flask was estimated at dally in- 

 tervals by examination with the naked eye and by mi- 

 croscopic examination. The latter task proved quite 

 feasible in the calm weather experienced. A milli- 

 pore filter slide was made initially to determine 

 which phytoplankton genera were the dominants in 

 the original sample. 



One of the purposes of these experiments was to 

 determine which nutrients might be limiting in these 

 waters. If the response to a given single nutrient was 

 greater than the response to any other, then presum- 

 ably the former would be the more likely limiting nu- 

 trient. If, when a given nutrient was omitted from a 

 complete mixture and the response was less than in 

 the complete mixture, then the omitted nutrient might 

 be limiting. Another purpose of these experiments 

 was the establishment of cultures of truly pelagic 

 phytoplankton which could be used for photosynthesis 

 and nutritional studies. 



The first experiment (Station 4) was started on 



April 26, 1958. Approximately 500 cells per liter 

 were found in the original water. The following gen- 

 era were observed on the Millipore slide: Navicula . 

 Ditylum , Coscinodusous , Ceratium , Dinophysis . 

 Gonyaulax , Peridinium . A small unidentified pennate 

 diatom was the most prevalent organism seen, how- 

 ever. None of these organisms grew in any treat- 

 ment. After 3 days cultures containing the complete 

 supplement (N, P, Si, trace elements, soil extract) 

 plus purines, pyrimidines, and amino acids, were 

 visibly turbid owing to intense bacterial growth. 

 After 5 days very small green flagellates were seen 

 in these cultures. These organisms were also ob- 

 served after 6 days in cultures containing the com- 

 plete supplement without purines, pyrimidines, or 

 amino acids. They also developed in the treatment in 

 which phosphate was omitted from the complete sup- 

 plement. Nothing developed in the other 10 treat- 

 ments containing no additions or nutrients added 

 singly, or when N, Si, trace elements, or soil extract 

 were respectively omitted from the complete mixture. 

 Since none of the more prevalent organisms seen in 

 the original sample grew, the experiment did not 

 show what factors might be limiting the major portion 

 of the phytoplankton. The small flagellate was trans- 

 ferred to fresh complete medium and persisted 

 through several subcultures. 



The second experiment was started on May 13, 1958. 

 Five hundred thousand cells per liter were found in 

 the original water sample, which was visibly green. 

 The dominant genera were Nitzschia , Rhizosolenla , 

 and Chaetoceros . Other diatoms seen on the Milli- 

 pore slide were Navicula and Planktoniella . About 

 10-20 percent of the population consisted of dinoflag- 

 ellates of the genera Gymnodinium and Peridinium . 

 Ceratium and Gonyaulax were also seen in net hauls. 



The results of this ejqjeriment are shown in table 1. 



No growth occurred In the second experiment when 

 nothing was added or when nutritional factors were 

 added singly, with the exceptions of the treatment in 

 which soil extract was added alone and possibly in 

 the trace element treatment. Growth occurred in all 

 the combination type treatments, but no diatoms de- 

 veloped in cultures containing no added Si, trace ele- 

 ments, or soil extract. Since diatoms did grow in 

 cultures containing no added N or P, it is possible 

 that these elements were not limiting and that Si, 

 trace elements, and the components of soil extract 

 were limiting. However, one of the dominant diatoms, 

 Rhizosolenla , failed to grow at all. It is interesting 



