Diatoms and their influence on the biologically mediated uptake of atmospheric CO₂ in the Arabian Sea upwelling system

International audienceModel experiments have shown that diatoms can lower the atmospheric CO2-concentration when they grow at the expense of coccolithophorids, since this reduces the precipitation of calcium carbonate, which acts as an oceanic CO2 source. In the Arabian Sea we conducted long-term sediment trap experiments (water depth >1000 m) in order to study processes controlling shifts from diatom to non-diatom dominated systems. One of our major problems was to link sediment trap records to surface ocean processes. Satellite-derived observations on upper ocean parameters were helpful to reduce this problem in the past and gain a new quality by combining it with results obtained during the Joint Global Ocean Flux Study in the Arabian Sea. The new results imply that a deficiency of silicon (Si) in the euphotic zone terminates diatom blooms. Enhanced eolian iron inputs raise the availability of silicon in the surface water by decreasing the Si/N uptake ratios of diatoms. An enhanced abundance of diatoms within the plankton community seems to increase the biologically mediated uptake of atmospheric CO2 by suppressing blooms of calcium carbonate producing organisms and by elevating the carbon to nutrient uptake (Redfield) ratio. These results agree in principle with assumptions made in models but indicate also that enhanced iron concentrations hinder the development of diatom blooms. The latter could be responsible for the amplitude of derived changes in the Redfield ratio and in the ratio between organic carbon and calcium carbonate carbon production which fall below assumptions made in some model experiments