Several studies have been made of the seasonal changes in th, chemical composition of sea water, especially of the so-called "nutrient substances." But most of these have been in localities relatively near shore and in the temperate zones where climatic fluctuations are relatively great. During the years 1937 and 1938 an opportunity arose to study such changes in the open ocean in connection with an investigation of the variations of the Gulf Stream (Iselin, 1940).