Whilst forests – aided and abetted by cryptogams (see my previous post) – have a major role as biotic carbon sinks on land, in the oceans that role is largely down to the activity of cryptogamous phytoplankton, which ‘draw-down’ vast amounts of CO2 during photosynthesis. However – and unlike trees – much of that aquatic primary productivity is consumed by herbivores, which in turn are preyed upon by various levels of carnivores. Ultimately, a lot of the CO2 that is fixed is released quite soon thereafter in respiration. Which is why attempts to consign such fixed carbon that is retained in the bodies of the algae (before it can be consumed and respired by hungry herbi-/carnivores) to the ocean depths – and thereby place it out of reach of the atmosphere where it could contribute to global warming – are quite attractive. Hence the notion of iron fertilisation, which aims to promote phytoplankton growth by addition of that essential plant nutrient, which is in short supply in large parts of the oceans. Whilst attempts at this manipulation to date have succeeded in promoting algal growth, none have unambiguously demonstrated the necessary mass deep-sinking events of the fixed carbon that would lead to carbon being appropriately sequestered at depths that preclude its rapid return to the atmosphere. However, Victor Smetacek et al., analysing the European Iron Fertilization Experiment (EIFEX) – carried out early in 2004 in ‘the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current’ – conclude that at least half the diatom bloom biomass sank far below a depth of 1000 m and that a substantial portion is likely to have reached the sea floor as a ‘fluff layer’. And – encouragingly – iron-fertilised diatom blooms ‘may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments’. But I’ve yet to find out why these promising results were seemingly sequestered from public gaze for 8 years… And, if this biological sequestration isn’t up to the job, we’ll just have to hope that the oceans themselves continue to soak up the excess CO2.
About the author
Nigel is a botanist and full-time academic at Bath Spa University (Bath, near Bristol, UK). As News Editor for the Annals of Botany he contributes the monthly Plant Cuttings column to that august international botanical organ. His main goal is to inform (hopefully, in an educational, and entertaining way...) about plants and plant-people interactions.