Ironing Out Algal Issues in the Southern Ocean Science


Periods of iron supply to the ocean over geological time scales may have altered climate by increasing the amount of CO2 fixed by anaemic
(iron-limited) phytoplankton. The papers (Coale et al., Bishop et al., and Buesseler et al.) report results from recent large-scale experiments in which ocean waters have been enriched with iron. By measuring how much carbon fixed by phytoplankton is sequestered into the abyss, the experiments test the hypothesis that enhanced iron supply leads to enhanced export of particulate organic carbon.

Southern Ocean Iron Enrichment Experiment: Carbon Cycling in High- and Low-Si Waters

Kenneth H. Coale, et al. Science, Vol 304, Issue 5669, 408-414 , 16 April 2004

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron’s pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

The Effects of Iron Fertilization on Carbon Sequestration in the Southern Ocean

Ken O. Buesseler, et al. Science, Vol 304, Issue 5669, 414-417 , 16 April 2004

An unresolved issue in ocean and climate sciences is whether changes to the surface ocean input of the micronutrient iron can alter the flux of carbon to the deep ocean. During the Southern Ocean Iron Experiment, we measured an increase in the flux of particulate carbon from the surface mixed layer, as well as changes in particle cycling below the iron-fertilized patch. The flux of carbon was similar in magnitude to that of natural blooms in the Southern Ocean and thus small relative to global carbon budgets and proposed geoengineering plans to sequester atmospheric carbon dioxide in the deep sea.

Robotic Observations of Enhanced Carbon Biomass and Export at 55°S During SOFeX

James K. B. Bishop, Science, Vol 304, Issue 5669, 417-420 , 16 April 2004

Autonomous floats profiling in high-nitrate low-silicate waters of the Southern Ocean observed carbon biomass variability and carbon exported to depths of 100 m during the 2002 Southern Ocean Iron Experiment (SOFeX) to detect the effects of iron fertilization of surface water there. Control and “in-patch” measurements documented a greater than fourfold enhancement of carbon biomass in the iron-amended waters. Carbon export through 100 m increased two- to sixfold as the patch

subducted below a front. The molar ratio of iron added to carbon exported ranged between 104 and 105. The biomass buildup and export were much higher than expected for iron-amended low-silicate waters.