Abstract
The maximum uptake rate (ρmax) and affinity constant (Ks) for nitrate and ammonium were estimated in the surface water of offshore Oyashio in May (spring) and September (summer), 1990. The average ρmax/Chl.a for ammonium was 2.1 times larger than that of nitrate in both seasons. The average ρmax/Chl.a for both nitrogens were 3.5 times larger in summer than in spring. Water temperature and size composition of phytoplankton population were related to the seasonal difference in the ρmax/Chl.a. Phytoplankton population showed high affinity for both nitrogens in the spring and summer. In addition, the contribution of “new” production to total production was estimated by ρmax[ρmax−No3/(ρmax−NO3+ρmax−NH4)]. The spring value was in the range of 0.26 to 0.45 (mean±SD=0.35±0.092), and the values in spring bloom were especially a little over 0.4. The summer value was in the range of 0.30 to 0.37 (0.34±0.04).
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