Effect of Ammonium and Nitrate Concentration on Chlorophyll Increases in Natural Tropical Pacific Phytoplankton Populations1

Samples of nutrient‐poor water from the eastern tropical Pacific were enriched with a complete mixture of nonnitrogenous nutrients and with varying concentrations of nitrate and ammonium. They were then incubated in natural light on the deck of the ship and phytoplankton growth was measured daily by means of in vivo chlorophyll fluorescence. Growth rates were calculated from increases in fluorescence, and final yield of fluorescence as a function of nitrogen added was also calculated. In two out of four experiments plots of rates against concentration of nitrate or ammonium were hyperbolic and the concentrations giving a half‐maximal rate were about 1.5 µ g‐atom/liter for ammonium and about 0.75 µ g‐atom/liter for nitrate. In two other experiments a maximal growth rate was not achieved even at 10 µ g‐atom/liter of added nitrogenous nutrient. The mean yield of chlorophyll for all experiments was 0.22 µ g per µ m of nitrogen added. Growth rates calculated from ammonium concentration at two of these experimental stations agreed well with growth rates calculated from 14 C productivity and chlorophyll concentrations.