The dependence of the production of plankton upon an adequate supply of such inorganic salts as phosphates and nitrates in the photic zone is now an established planktological concept (Harvey, 1928; Ekman, 1935; Russell and Yonge, 1936). It is also well known that the photic zone often becomes depleted of such essential mineral substances and that when this occurs any increase in the total production of plankton will depend upon a renewal of these substances from some source....