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

Abstract

We examined interannual variability in the 1965–1990 winter‐summer differences in surface nitrate (ΔNO3) in the northeast subarctic Pacific Ocean (Station P, 50°N, 145°W). Increases in ΔNO3, of 30–70% above the mean value occurred during four events (1969, 1971‐1972, 1979, 1983). The ΔNO3 time series was highly correlated (r = 0.86) with the spring‐summer nitrate utilization rate from 1970 to 1980. The ΔNO3 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 ΔNO3 and sedimenting particulate organic nitrogen (PON) showed that the high ΔNO3 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 ΔNO3. The increase in f‐ratio implies a change in the community structure of the phytoplankton during the elevated ΔNO3 events that appears connected to changes in the NPI.