The Uptake of Urea by Natural Populations of Marine Phytoplankton1

Nitrogen‐15 isotopes were used to study the uptake of nitrate, ammonium, and urea by natural phytoplankton populations in 36 samples collected at nine stations off the coast of southern California. The percentage of the total phytoplankton nitrogen productivity accounted for by urea varied from 60% and for the entire study averaged 28%. The percentage of total available nitrogenous nutrient (ambient nitrate, ammonium, and urea plus the 15 N additions) utilized per day varied among the stations from a minimum of 5% at station 4 (12 km off San Diego) to a maximum of 46% at station 19 (off White Point). The average C:N uptake ratio was 12.4.