Differential effects of iron additions on organic and inorganic carbon production by phytoplankton

Bottle and mesoscale experiments have demonstrated that iron additions enhance phytoplankton growth and reduce surface pCO₂ in high-nutrient, low-chlorophyll (HNLC) regions of the world oceans. Here we show that iron additions specifically stimulate organic but not inorganic carbon production in the HNLC Subarctic Pacific. Five-hour ¹⁴C labeling experiments performed during incubation of surface water samples demonstrated a large increase in the rate of organic carbon produced but no change in the rate of inorganic carbon production. The same result was obtained on two different dates: one when coccolithophores formed a relatively large proportion of total autotrophic biomass; the other when coccolithophores were less abundant. Together with previous taxonomic observations, our results imply that iron fertilization may be particularly effective in drawing down CO₂ in surface waters by stimulating primary production but not calcium carbonate precipitation, which augments CO₂.