The Influence of the Mineral Composition of the Medium on the Growth of Planktonic Algae: Part II. The Influence of the Concentration of Inorganic Nitrogen and Phosphate Phosphorus

Investigations of phytoplankton, in relation to the chemical composition of the sea and of various fresh waters, during the last twenty years show that phytoplankton is an important consumer of nitrogen and that its amount is often markedly affeqted by increase or decrease of the nitrogen content of the water. Exact relations between plankton growth and the amount of inorganic nitrogen have, however, not yet been precisely determined. Among higher plants the rate of development is proportional to the uptake of nitrogen, and the latter is in turn dependent on the nitrogen concentration,in the medium (Crowther, 1935; Miller, 1938, p. 643). A similar relation has been suggested in limnological studies by the fact that larger amounts of phytoplank,,on are often obtained in waters with a higher nitrogen content (Whipple, 1927, p. 213). Ketchum (1939), however, experimenting with the marine diatom Nitzschia closterium, has shown that the rate of division is independent of nitrate concentration. On the other hand, deficiency in nitrogen is usually considered to be one of the most important limiting factors in plankton production. It has also been claimed that nitrogen in solution may be concentrated to the point of injury (Welch, 1935, p. 186). There is, however, no general agreement on these matters. Wiebe (1930, pp. 141 and 169), investigating the plankton production in fish ponds, concludes that inorganic nitrogen is not a limiting factor, and a similar conclusion is reached in his investigation on the dissolved phosphorus and inorganic nitrogen in the water of the Mississippi (1931, p. 652). The following problems, therefore, await solution: (a) whether the nitrogen concentration in natural waters is commonly a limiting factor, (b) to what extent and at what concentration the growth of phytoplankion is limited by it, (c) whether higher concentrations are harmful and what are the injurious concentrations, and (d) within what limits an increase beneficially affects the development of phytoplankton, and within what limits the latter is indifferent to increase or decrease cXf nitrogen concentration. Certain algae are found in waters' with a low nitrogen concentration and others in waters with a higher concentration, while still others are met with both in low and high concentrations. Thus, Pearsall (1932) found that diatoms flourished when the water was richest in nitrates and that the appearance of Asterionella was correlated with higher concentrations than those occurring when Tabellaria fenestrata and green. algae were found; further, that desmids occurred in summer when nitrates were low, while colonial