Temporal variation of phosphate concentration in the mixed layer of western subarctic North Pacific from 1968 to 1998 was investigated with quasi-monthly resolution. February and July phosphate time series showed a linear decreasing trend of -0.008 ¿ 0.003 and -0.019 ¿ 0.005 ¿mol/l/y, respectively, while no multi-decadal trend was observed in April/May phosphate time series. With this result, net biological phosphate consumption between February and April linearly decreased at the rate of -0.43 ¿ 0.11 mmolP/m2/y, while that between May and July increased at the rate of +0.16 ¿ 0.05 mmolP/m2/y. Net biological phosphate consumption from February through August was linearly diminished at the rate of -0.40 ¿ 0.07 mmolP/m2/y, corresponding to a -0.51 ¿ 0.09 gC/m2/y decrease of net community production under assumption of the constant C/P ratio of 106. Chl.a inventory in the April mixed layer also showed a decreasing trend of -2.35 ¿ 1.22 mg/m2/y, which correlated with the observed net production decrease between February and April.