High silicate:nitrate ratios in eastern boundary upwelling waters may produce greater carbon drawdown than predicted from Redfield C:N ratios

The Redfield ratio defines the average ratio of changes in major nutrient concentrations during primary production as 106:16:1, C:N:P. This ratio and the phytoplankton uptake or drawdown of nitrate (new production in the ocean) are often used to estimate carbon production and export of carbon to the deep ocean. Elevated nitrate in upwelled water is rapidly drawn down by diatoms, usually within 3-5 days and the assumption is that carbon drawdown ceases at that end of that time. However, in large-volume enclosure experiments using silicate-rich San Francisco Bay water, silicate drawdown continued well after nitrate was exhausted by phytoplankton growth. Enclosure experiments made with water upwelled at Point. Reyes, northern California followed the same pattern of silicate drawdown continuing past nitrate exhaustion. Dissolved inorganic carbon (DIC) drawdown tracked silicate drawdown after nitrate exhaustion; i.e. the DIC drawdown followed more closely the drawdown of silicate than nitrate. The drawdown of DIC calculated from nitrate drawdown using Redfield resulted in an underestimate of the measured DIC disappearance. In upwelling waters with Si:N ratios of greater than l, the uptake of DIC may be substantially underestimated. The implication of these preliminary results is that coastal upwelling in basins rich in silicate, e.g. in the North Pacific, may account for substantially more drawdown of CO2 than would be calculated from upwelled nitrate concentrations. In eastern boundary upwelling areas, a modification of the Redfield ratio to incorporate C:Si is necessary since these areas are dominated by diatoms. Victor Smetacek’s designation of diatoms as the "workhorses of the sea" becomes more appropriate than ever. Their obligate requirement for Si to construct their frustules makes them responsible for this re-interpretation of estimating carbon drawdown using the Redfield ratio. In these circumstances we may better define new production in terms of silicate rather than nitrogen processes.