Enhancement of new production in the northeast subarctic Pacific Ocean during negative North Pacific index events

We examined interannual variability in the 1965-1990 winter-summer differences in surface nitrate (Delta NO3) in the northeast subarctic Pacific Ocean (Station P, 50 degrees N, 145 degrees W). Increases in Delta NO3 of 30-70% above the mean value occurred during four events (1969, 1971-1972, 1979, 1983). The Delta NO3 time series was highly correlated (r = 0.86) with the spring-summer nitrate utilization rate from 1970 to 1980. The Delta NO3 time series was negatively correlated with the North Pacific index (NPI) (r = -0.72) and positively correlated with solar radiation in the spring (r = 0.48) and summer (r = 0.46). A simple ecosystem model forced with observed incident solar radiation and mixed-layer depth during the spring-summer period predicted increases in export production that were 50% of the observed increase for 1970-1972 and 1977-1979. Comparison between the Delta NO3 and sedimenting particulate organic nitrogen (PON) showed that the high Delta NO3 event of 1983 was associated with a dramatic increase in PON fluxes. A simple ecosystem model underpredicts observed interannual variations in export production, and this result combined with the observed PON fluxes suggests that a combination of increased phytoplankton production and increased f-ratio produced the observed increase in Delta NO3. The increase inf-ratio implies a change in the community structure of the phytoplankton during the elevated Delta NO3 events that appears connected to changes in the NPI.