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.