INTRODUCTION In addition to the well-known conservative elements which constitute most of the ions in sea water (Chapter 8) there are other ions, present by comparison in small quantities, yet of fundamental importance in the economy of the sea. In contrast to the conservative elements, their concentrations, which vary considerably in both space and time, are not a function of salinity. These substances, the nutrient elements, are required in the nutrition of the primary producers, the phytoplankton, and their study has therefore often been closely bound up with these organisms. The substances concerned are compounds of phosphorus, nitrogen, and silica (required for the skeletons of diatoms and radiolarians); the other elements required in phytoplankton nutrition are present either in relatively large quantities and are maintained as conservative elements (the inorganic cations, for example) or as trace elements (Chapter 12). It will be necessary to consider both the spatial and seasonal distribution of each of the above substances in the various forms in which they occur, as well as their relation to one another. In their simpler form as components of the inorganic ions, phosphate, nitrate, and silicate, they are used up in phytoplankton nutrition. Considerable attention will therefore be given to the cycle by which they are regenerated. The detailed study of these nutrient elements in the sea was begun much later than that of the conservative elements; at first their importance was not appreciated, and later, when it became recognized, the analytical methods for their quantitative estimation, where such...