In the Southern Ocean, primary productivity—the rate at which living organisms such as phytoplankton produce organic compounds—is limited by low concentrations of iron. Although earlier studies in the Ross Sea have shown the most important sources of this nutrient include icebergs, windblown dust, and melting sea ice, seasonal streamflow from ice-free areas is another potential contributor. To date, however, the amount of iron these streams supply to coastal Antarctic waters has been poorly constrained.
Now Olund et al. have measured iron concentrations in four streams that flow from the McMurdo Dry Valleys, the southern continent’s largest ice-free area, into the Ross Sea to determine their potential impact on coastal water biogeochemistry. These streams, which flow only from 4 to 10 weeks per year, were sampled along their lengths from late December 2015 through late January 2016.
The results indicate that two of the streams, Commonwealth and Wales, contribute an average of 240 moles of filterable iron to the Ross Sea each year, an amount that is several orders of magnitude less than the contributions from other sources. The team also discovered that the ratio of iron to other vital nutrients, including nitrogen, phosphorous, and silicon, differs substantially from the ratios found in coastal phytoplankton communities. This finding indicates that seasonal streams are important sources of both phosphorous and iron for the Ross Sea’s plankton communities.
By increasing our understanding of iron fluxes into the seas surrounding Antarctica, this study highlights the importance of local nutrient inputs to the Southern Ocean. In addition, because primary production can boost the uptake of carbon dioxide and the consequent sequestration of carbon in marine sediment, this study has implications for understanding future changes in productivity and the cycling of carbon in the region as increased melting augments the flux of iron to the sea via these coastal streams. (Journal Geophysical Research: Biogeosciences, https://doi.org/10.1029/2017JG004352, 2018)
—Terri Cook, Freelance Writer