Abstract

The rise in anthropogenic CO2 and the associated ocean acidification (OA) will change trace metal solubility and speciation, potentially altering Southern Ocean (SO) phytoplankton productivity and species composition. As iron (Fe) sources are important determinants of Fe bioavailability, we assessed the effect of Fe-laden dust versus inorganic Fe (FeCl3) enrichment under ambient and high pCO2 levels (390 and 900 μatm) in a naturally Fe-limited SO phytoplankton community. Despite similar Fe chemical speciation and net particulate organic carbon (POC) production rates, CO2-dependent species shifts were controlled by Fe sources. Final phytoplankton communities of both control and dust treatments were dominated by the same species, with an OA-dependent shift from the diatom Pseudo nitzschia prolongatoides towards the prymnesiophyte Phaeocystis antarctica. Addition of FeCl3 resulted in high abundances of Nitzschia lecointei and Chaetoceros neogracilis under ambient and high pCO2, respectively. These findings reveal that both the characterization of the phytoplankton community at the species level and the use of natural Fe sources are essential for a realistic projection of the biological carbon pump in the Fe-limited pelagic SO under OA. As dust deposition represents a more realistic scenario for the Fe-limited pelagic SO under OA, unaffected net POC production and dominance of P. antarctica can potentially weaken the export of carbon and silica in the future.

Highlights

  • At present, due to anthropogenic emissions, atmospheric carbon dioxide (CO2) levels are increasing at an unprecedented rate (Hoegh-Guldberg & Bruno 2010) and are projected to reach between 720 and 1000 μatm by the end of this century (RCP6.0 scenario; IPCC 2014)

  • Due to the importance of the Southern Ocean (SO) in sequestering anthropogenic CO2 (Sabine et al 2004, Landschützer et al 2015), understanding the effects of natural Fe sources under different CO2 scenarios on SO primary productivity and phytoplankton species composition can help to elucidate their combined effects on the biological carbon pump, under both present-day and future conditions. To assess their potential impacts, CO2−Fe perturbation bottle experiments with natural phytoplankton assemblages can serve as valuable tools, as they account for species interactions and reduce complexity by targeting the investigated environmental factors

  • To date multiple stressor experiments represent the only tool to simulate future potential climate change scenarios and their impact on future SO phytoplankton, these experiments can facilitate the interpretation of cause−

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Summary

Introduction

Due to anthropogenic emissions, atmospheric carbon dioxide (CO2) levels are increasing at an unprecedented rate (Hoegh-Guldberg & Bruno 2010) and are projected to reach between 720 and 1000 μatm by the end of this century (RCP6.0 scenario; IPCC 2014). In situ Fe fertilization of SO surface waters relieved Fe limitation and triggered growth of predominantly diatoms (Boyd et al 2007, Smetacek & Naqvi 2008), accompanied by significant CO2 drawdown, and in some cases, sinking of organic matter (Blain et al 2007, Smetacek et al 2012). These studies, lack an assessment of OA impacts. Some important dust-producing areas such as central Australia are predicted to become dryer (Durack et al 2012), resulting in a change in atmospheric Fe-laden dust inputs to the SO, in the context of OA

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