Half‐saturation Constants for Uptake of Nitrate and Ammonium by Marine Phytoplankton1

Uptake rate of nitrate and ammonium was studied as a function of nitrate or ammonium concentration with cultures of 16 species of marine phytoplankton. Half‐saturation constants (the concentration supporting an uptake rate one‐half the maximum rate) were computed as a measure of the ability of a species to use low levels of nitrate and ammonium. The half‐saturation constant (K s ) varied approximately in proportion to cell size and inversely with specific growth rate. Small‐celled oceanic species, such as Coccolithus huxleyi, showed the lowest K 8 values (⩽0.5 μ m ) and Gonyaulax polyedra the highest (K s > 5 µ m ). The K s values are considered to be important characteristics of organisms living in nitrogen‐limited environments. Some apparent success was achieved in predicting the competitive advantage of one species over another by calculating specific growth rates as functions of nitrate and ammonium concentration for species with known growth response to irradiance, temperature, and daylength. For example, at about 20C and 16 hr daylength C. huxleyi should be able to compete successfully with Skeletonema costatum when nitrate, ammonium, and irradiance are low. Skeletonema costatum should grow faster than C . huxleyi only when irradiance and nitrate or ammonium are fairly high. Rock pool forms appear to be relatively unsuccessful in the ocean because of a sluggish response to irradiance and not because of an inability to use low levels of nitrate and ammonium. The success of G. polyedra, often the dominant dinoflagellate in local red water blooms, appears to be due to its vertical migration from the sea surface in daylight to nitrate‐rich water at 10‐15‐m depth at night, rather than to any special ability to grow rapidly or use low levels of nitrate and ammonium.