FIG. 48.20

Does iron limit primary production in some parts of the oceans?

BACKGROUND Nitrogen and phosphorus are nutrients that are thought to limit primary production in the oceans, and, consistent with this hypothesis, the concentrations of nitrate and phosphate in surface oceans are generally very low. Regionally, however, nitrogen and phosphorus concentrations are high, indicating that primary producers are not making use of all available nutrients.

HYPOTHESIS In some parts of the oceans, iron—required for the functioning of enzymes critical to photosynthesis and nitrogen uptake—is the nutrient in lowest supply relative to the needs of primary producers and so limits rates of photosynthesis.

EXPERIMENT Biologists John Martin and Steve Fitzwater sailed to subarctic waters of the northeastern Pacific Ocean, where high nitrogen and phosphorus concentrations suggested limitation by another nutrient. They collected samples of seawater, along with its phytoplankton, and added small amounts of iron to different samples: 1, 5, or 10 nanomoles per kilogram of sample (in the control sample, no iron was added). As photosynthesis proceeded over the course of a week, the researchers made daily measurements of chlorophyll concentration (an indicator of photosynthetic rate), as well as of the concentrations of nitrate and phosphate in the sample treatments.

RESULTS As the figure shows, over the course of the experiments, samples to which iron was added showed increased chlorophyll concentration and decreased levels of nitrogen and phosphorus, with higher additions of iron generally showing more pronounced effects. Controls with no added iron showed little change over the course of the experiment.

CONCLUSION In samples from the northeastern Pacific Ocean, addition of iron increased rates of photosynthesis, with more uptake of nitrogen and phosphorus. This supports the hypothesis that, in these waters, iron and not nitrogen or phosphorus limits primary production.

FOLLOW-UP WORK Since this initial experiment, many researchers have done similar research in areas of the surface ocean with relatively high levels of nitrogen and phosphorus, with consistent results. Some researchers have suggested that seeding these ocean waters with iron could provide a sink for atmospheric carbon dioxide, helping to alleviate global change associated with anthropogenic CO₂ emissions. Results, however, have been mixed, with many experiments showing a spike of increased photosynthesis followed by an interval of enhanced respiration—with little carbon sinking into the deep sea.

SOURCE Martin, J. H., and S. E. Fitzwater. 1988. “Iron-Deficiency Limits Phytoplankton Growth in the Northeast Pacific Subarctic.” Nature 331: 341–343.