Iron(II) concentrations and availability for phytoplankton – Multiple stressor studies of a future Southern Ocean
Helene Aflenzer, Antarctic Climate & Ecosystems Cooperative Research Centre (ACE CRC), University of Tasmania, Hobart Tasmania 7001, TAS, Australia; Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Hobart, TAS, Australia, Philip W Boyd, Antarctic Climate & Ecosystems Collaborative Research Centre (ACE CRC), Univesity of Tasmania, Hobart, TAS, Australia, Pier van der Merwe, Antarctic Gateway Partnership, University of Tasmania, Hobart, Australia, Kathrin Wuttig, Antarctic Climate & Ecosystems Cooperative Research Centre (ACE CRC), University of Tasmania, Hobart, TAS, Australia and Andrew R Bowie, Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC), University of Tasmania, Hobart, TAS, Australia; Institute for Marine and Antarctic studies (IMAS), University of Tasmania, Hobart, TAS, Australia
Abstract:
Iron (Fe) is a vital yet limiting element for phytoplankton growth in the Southern Ocean (SO). Of the two redox species of dissolved Fe available for phytoplankton (dFe(II) and dFe(III)), dFe(III) is thermodynamically favored in oxygenated oceans, while dFe(II) is an oxidizing species and thought to be more accessible to phytoplankton.
Overall, dFe(II) occurs in low concentrations compared to dFe(III) (pico- vs. nanomolar) and its half-life is dependent on pH (Ocean acidification, OA) and temperatures (global warming). These two parameters will change the oxidation rate of dFe(II) in our future ocean. A projected increase in sea surface temperature should promote the oxidation of dFe(II), shortening its half-life, whereas an OA shift extends it.
Here we present data obtained during oxidation and incubation experiments in coastal and open ocean seawater. Based on our oxidation experiments we found that an increase by 0.85°C (period from 1880 to 2012, IPCC report 2014) shortened the half-life by 1.3 minutes in open ocean water and by 2.5 minutes in coastal water. In regard to OA at 10°C, we found that a drop in pH by 0.05 units led to a dFe(II) half-life extension of 3.1 minutes in open ocean water and 6 minutes in coastal water.
Our incubation experiment with additions of dFe(II) and elevated CO2 revealed enhanced growth rates of the diatom F. cylindrus in open ocean water. The haptophyte P. antarctica had increased growth rates in coastal water under the same conditions. Together with the outcome of a modelling approach (MINEQL+), these results imply that acidification and increased temperatures will have noticeable impacts on the oxidation and availability of dFe(II) to SO phytoplankton. Changes in dFe(II) concentrations could lead to phytoplankton abundance shifts due to altered availability of Fe for growth. This will likely result in a change in the productivity of coastal and open ocean phytoplankton species, with further implications for ocean carbon storage.
