The expected impact of ocean nourishment on ocean acidity
Ian S.F. Jones
University of Sydney, Australia. E-mail: ian.jones@sydney.edu.au
If the partial pressure of carbon dioxide in the atmosphere were to increase by 200 ppmv, the
pH of the surface ocean would decrease by about 0.2. Business as usual would see this
happen in 40 years. One counter to this is to fertilise a patch of ocean with nitrate in those
areas where photosynthesis is limited by nitrogen. Photosynthesis in the photic zone takes
inorganic carbon and exports it out of the surface ocean. The organic carbon produced may
be cycled back and forth between organic and inorganic carbon but eventually almost all the
carbon involved is exported to the deep ocean and its place taken by carbon fluxed from the
atmosphere. The lower atmospheric concentration of carbon dioxide as a result of fertilisation
reduces heat trapping (global warming). Redfield showed that ocean photosynthesis can also
increase the total alkalinity. Carbon flux from the atmosphere replaces the exported carbon
but it does not lower the total alkalinity. Over much of the ocean, the addition of (say) 16
micromoles of nitrate in an ocean of pH 8.1, increases the pH by .075 to a pH value of pH of
8.175. The hydrogen ions exported from the fertilised patch appear again in the deep ocean.
Iron fertilisation of high nutrient low chlorophyll regions of the ocean have the same
reduction of acidity but other reactive forms of nitrogen produce smaller pH changes per
mole.