Silicate Pumping of Excess Carbon in Coastal Upwelling Ecosystems

Diatoms dominate new production in upwelling ecosystems and control the rate at which nitrate and associated carbon are assimilated into the euphotic zone. The Redfield ratio defines the average ratio of major nutrients C:N:P in phytoplankton as 106:16:1 and the phytoplankton uptake of nitrate (new production in the ocean) is often used as a proxy for carbon production and export of carbon to the deep ocean. Elevated nitrate and CO₂ in upwelled water is rapidly drawn down by diatoms, usually within 3-5 days. From the Redfield ratio the ensuing drawdown of CO₂ would be expected to be a function of the initial concentration of nitrate. If the ratio of Si:N in upwelled water is >1:1, and the canonical ratio of Si:N in coastal diatoms is taken as 1:1, silicate in excess of nitrate would be expected to have no effect on carbon drawdown and export. However, in enclosure experiments using silicate-rich upwelled water, silicate uptake continued well after nitrate was exhausted by phytoplankton growth. Dissolved inorganic carbon drawdown followed silicate drawdown stoichiometrically and also continued after nitrate exhaustion. The implication is that coastal upwelling basins rich in silicate, e.g. North Pacific, may account for substantially more drawdown/absorption of CO₂ than would be calculated from upwelled nitrate concentrations. The excess carbon exported to the underlying waters by silicate pumping also may be a factor in the development of hypoxia along the US West Coast upwelling areas. In these circumstances where there is a direct role of silicate in CO₂ drawdown new production may be better defined in terms of silicate rather than nitrogen and be a better indicator of biological carbon processes.