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The Impact of Warming, CO2 and Iron Availability on a Southern Ocean Phytoplankton Assemblage

Paper Reviewed
Spackeen, J.L., Sipler, R.E., Bertrand, E.M., Xu, K., McQuaid, J.B., Walworth, N.G., Hutchins, D.A., Allen, A.E. and Bronk, D.A. 2018. Impact of temperature, CO2, and iron on nutrient uptake by a late-season microbial community from the Ross Sea, Antarctica. Aquatic Microbial Ecology 82: 145-159.

The Southern Ocean is one of the most biologically productive regions on Earth. Within its boundaries, the Ross Sea accounts for more than one-third of the total production that occurs in the Southern Ocean's shelf waters. Hence, the Ross Sea plays a significant role as a CO2 sink in the global carbon cycle. Yet much remains to be learned as to how the strength of that sink may or may not be altered in response to projected changes in future climate.

Hoping to provide some information in this regard, Spackeen et al. (2018) conducted two laboratory-based experiments on a phytoplankton community sampled at approximately 3 m depth near the ice edge at McMurdo Sound (77.62°S, 165.47°E). In the first experiment, they used a semi-continuous method of culturing to study the impact of elevated temperature (+4°C) and iron (Fe) availability (normal and added Fe) on the phytoplankton assemblage, while in the second they used a continuous method of culturing to examine the combined impact of temperature, Fe availability and elevated CO2 (400 or 800 µatm) conditions.

In discussing their findings, Spackeen et al. report that elevated CO2 "did not have a significant impact on the [phytoplankton] community, either individually or in combination with other factors," which lack of response they attributed to adaptation by the microorganisms to the naturally-fluctuating high and low CO2 environment that is characteristic of the Ross Sea.

With respect to the influence of temperature and Fe availability on phytoplankton production, the authors found that "temperature elevation had the largest impact." In this regard, they report that phytoplankton growth was "stimulated by temperature, as evidenced by relatively higher chl a concentrations and by relatively lower concentrations of most dissolved nutrients [in the +4 °C treatment], indicating nutrient drawdown by the microbial community." The effects of iron were less important, but the magnitude of its impact was greater when combined with elevated temperature.

Commenting on the significance of their findings, Spackeen et al. say that "it is likely that nitrogen and carbon uptake rates will increase in a warmer Ross Sea," adding that such increases may become "a positive feedback for the biological pump, resulting in increased export to the deep sea." Thus, it would appear that the CO2 sink operating in Ross Sea will likely be strengthened under predictions of future ocean warming and/or ocean acidification.

Posted 4 April 2019