164 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



FiGUBE 44. — Stations established by Riley (1937) in the northern portion of the Gulf of Mexico. 



SURFACE PHOSPHATE CONTENTS 



MILLIGRAMS PER CUBIC METERS /^y 



J 



Figure 45. — Distribution of phosphates in the waters of the northern portion of the Gulf of Mexico. 



The later work by Riley (1938) was done in the 

 Dry Tortugas at the end of the chain of the 

 Florida Keys in the eastern part of the Gulf. 

 Here the water was shallow and with no influence 

 of land drainage of any consequence. Some 

 samples were taken at the edge of the Gulf Stream, 

 but most of them were taken at two regular 

 stations between Loggerhead Key and Garden 

 Key. Plankton samples were obtained by sieving 

 400 liters of water through a No. 20 silk net, and 

 a second set of samples was obtained by filtering 

 from 3 to 10 liters through a Whatman No. 2 



filter paper. Part of each net sample was studied 

 for number of animals present, for plankton 

 weight, and for organic material weight, while 

 the remainder was studied for the quantity of 

 plant pigments. It was found that the plant 

 pigments of the net plankton constituted less 

 than 2 percent of that occurring in the filtered 

 samples. Thus, the mean value for the net plank- 

 ton was 17 Harvey units per m.^, while the average 

 for the total plankton was 924 Harvey units per 

 m.', indicating a very high proportion of nan- 

 noplankton. The total quantitj' of plankton 



