Multi‐decadal decrease of net community production in western subarctic North Pacific

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 μmol/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.