The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August-September 1985. The measurements were performed by using 15N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 .mu.mol l-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m-2 d-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands[USA] were very close to the rates found at the oceanic station. Organisms > 3 .mu.m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.
