Sporadic silicate limitation of phytoplankton productivity in the subarctic NE Pacific

A time series (1970–1980) of silicate concentrations in the surface mixed layer at Ocean Station P (OSP, 50°N, 145°W) in the subarctic NE Pacific Ocean in high-nutrient and low-chlorophyll (HNLC) waters shows nearly total depletion of silicate (<1 μmol kg−1) in the summers of 1972, 1976 and 1979. From a mixed-layer model for the spring–summer period, we calculated silicate and nitrate utilization. The silicate utilization (ΔSiO4) during the growing season displays large interannual variations, suggesting that diatom production would experience similar fluctuations. The years 1972 and 1979 had both high-silicate utilization (ΔSiO4) and high-nitrate utilization (ΔNO3). During these two high-production years, the lack of available silicate appeared to limit diatom production. For 1972 and 1979, the ratio of ΔSiO4 to ΔNO3 was 1.4 and 2.5, respectively. The 1979 ratio supports the conclusion that high-nutrient utilization in the mixed layer is dominated by diatoms. The 1972 ratio is consistent with the average value calculated from the time-series data and suggests that the high-nutrient utilization resulted from a combination of diatom and non-siliceous production. A time series of particle fluxes (1980–1994) collected in deep-moored sediment traps at OSP showed that the averaged monthly flux ratio of opal to particulate organic nitrogen (PON) remained constant except during two high-PON flux periods. These two periods occurred during the late summers of 1983 and 1993 when the PON flux was more than double the average. During these two high-PON flux periods a significant decrease in the opal to PON flux ratio occurred. If these high-PON fluxes were caused by increased diatom production, the composition and preservation of nitrogen in the sinking organic matter also must increase during these diatom blooms. In 1983, the high-PON flux period was associated with a high-opal flux but not in 1993. It appears that the two high-PON periods were driven by two different processes. In 1983, the high-PON and opal fluxes are consistent with increased diatom production, while in 1993, the high-PON flux and average opal flux suggest increased non-diatom production. The switch between high production of diatoms and high production by non-diatoms is consistent with the silicate and nitrate utilization, which also had years with both high silicate and nitrate utilization and years with only high-nitrate utilization.