The goals of this thesis were to investigate the uptake rates of inorganic and organic forms of nitrogen, the relative proportions of new and regenerated primary production, and the interactions between uptake of ammonium and nitrate by phytoplankton from the NE subarctic Pacific Ocean.

Nitrogen uptake rates, and physical, chemical and biological characteristics of the euphotic zone were studied during winter, spring and late summer for two years along a transect extending from 49N, 127W to 50N, 145W. New and regenerated primary production were estimated by means of N-15 isotopes. Ammonium (NH4) was usually taken up at the highest rates throughout the euphotic zone during all seasons. Urea uptake rates were lower than those of NH4, but higher values were occasionally observed, particularly during the spring of 1993. Nitrate (NO3) uptake rates represented on average 21% of the depth integrated total nitrogen uptake, both longitudinally and seasonally. The f-ratio was overestimated on average by 36% when urea was excluded from the calculation. The order of preference for nitrogen by the entire phytoplankton assemblage was NH4 > urea > NO3. This system functioned mainly on regenerated nitrogen forms year round, despite the availability of NO3 in most of the region. Small (< 2 micrometers) and large (> 2 micrometers) phytoplankton took up NH4 at higher rates than urea and NO3. Depth integrated f-ratios were 0.16 for small cells and 0.25 for large cells. The order of preference for the nitrogen sources by small cells was NH4 > urea > NO3, and by larger cells it was NH4 = urea > NO3. Picoplankton were responsible for the greatest proportion of new and regenerated primary production.

The presence of NH4 inhibited NO3 uptake in the coccolithophore, EMILIANIA HUXLEYI. Nitrate uptake rates were reduced to half the maximum value at 0.24 micromolar NH4, and maximum inhibition was ca. 100% at 2.2 micromolar NH4. Extrapolating this laboratory result to field conditions, the inhibition of NO3 uptake rates for the small size class of phytoplankton is predicted to be 37 to 69% for the range of ambient NH4 concentrations found in the NE subarctic Pacific.