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Biogeochemistry

Icebergs Fertilize Phytoplankton Growth

Source: Hopwood, M. J., D. Carroll, J. Höfer, E. P. Achterberg, L. Meire, F. A. C. Le Moigne, et al. (2019), Highly variable iron content modulates iceberg-ocean fertilization and potential carbon export. Nature Communications, 10, 5261. doi:10.1038/s41467-019-13231-0

The ocean helps regulate the global climate system by taking up carbon dioxide (CO2) from the atmosphere. The polar regions account for a particularly large proportion of the total global oceanic carbon uptake. One of the ways that the ocean absorbs CO2 is with the help of tiny microalgae called phytoplankton. Phytoplankton take up carbon dioxide from the atmosphere through photosynthesis, just like trees and other land plants. Therefore, understanding the processes that determine the distribution of phytoplankton has implications for the global carbon cycle.

One of the factors limiting phytoplankton growth in the polar regions is the nutrient, iron. Therefore, adding iron often stimulates large growth events called phytoplankton blooms. There are several different sources of iron to the ocean including dust, sediments, and ice. Past studies have suggested that icebergs, large chunks of ice sheet that break off and drift around the ocean, support a significant portion of the total biological production and carbon export in the Antarctic. However, the ability of an iceberg to support a bloom will depend on the amount of iron contained within it, which is not well known. Therefore, a recent study published in Nature Communications sought to quantify the variability in iceberg iron content.

Icebergs, like this one, can fertilize blooms by supplying iron, a nutrient necessary for phytoplankton growth (Wikimedia Commons)

The study uses measurements of dissolved iron in melted ice samples collected from 206 icebergs off the coasts of Antarctica, Greenland, Norway, Iceland, and Chile. This unique dataset reveals that iceberg iron concentrations vary hugely (by over 6 orders of magnitude!). These large differences also suggest that there is significant variability in the potential for icebergs to stimulate phytoplankton growth.

Iceberg iron content for 206 samples taken in the Arctic and Antarctic (Hopwood et al., 2019)

The results from this study point to the importance of collecting more measurements of iceberg iron content in order to better constrain the range of values, as well as the magnitude of associated carbon uptake. This will be particularly important in the future as global temperatures rise and ice melt increases. However, these results also show that iceberg properties (in addition to increasing total iceberg flux) must be considered when predicting changes in future biological productivity associated with global climate change. Accurately accounting for iron inputs from icebergs is necessary to predict the strength of the Southern Ocean and Arctic carbon sinks – and how these might change in the future.

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