06 Jan 2023
06 Jan 2023
Status: this preprint is currently under review for the journal BG.

Photophysiological response of autumn phytoplankton in the Antarctic Sea-Ice Zone

Asmita Singh1,2, Susanne Fietz1, Sandy J. Thomalla2, Nicolas Sanchez3, Murat V. Ardelan3, Sébastien Moreau4, Hanna M. Kauko4, Agneta Fransson4, Melissa Chierici5, Saumik Samanta1, Thato N. Mtshali6, Alakendra N. Roychoudhury1, and Thomas J. Ryan-Keogh2 Asmita Singh et al.
  • 1Department of Earth Sciences, University of Stellenbosch, Stellenbosch, South Africa
  • 2Southern Ocean Carbon-Climate Observatory, CSIR, Cape Town, South Africa
  • 3Department of Chemistry, Norwegian University of Science and Technology (NTNU), Trondheim, Norway
  • 4Norwegian Polar Institute (NPI), Tromsø, Norway
  • 5Institute of Marine Research, Fram Centre, Tromsø, Norway
  • 6Oceans and Coast, Department of Environment, Forestry and Fisheries, Cape Town, South Africa

Abstract. The High Nutrient-Low Chlorophyll condition of the Southern Ocean is generally thought to be caused by the low bioavailability of micronutrients, particularly iron, which plays an integral role in phytoplankton photosynthesis. Nevertheless, the Southern Ocean experiences seasonal blooms that generally initiate in austral spring, peak in summer and extend into autumn. This seasonal increase in primary productivity is typically linked to the seasonal characteristics of nutrient and light supply. To better understand the constraints on productivity in the Antarctic Sea-Ice Zone (SIZ), the photophysiological response of phytoplankton to iron addition was investigated during autumn along the Antarctic coast off Dronning Maud Land. Five short-term (24 hr) incubation experiments were conducted around Astrid Ridge (68° S) and along a 6° E transect, where an autumn bloom was identified in the region of the western SIZ. Surface iron concentrations ranged from 0.27 to 1.39 nM around Astrid Ridge, and 0.56 to 0.63 nM along the 6° E transect. The photophysiological response of phytoplankton to iron addition, measured through the photosynthetic efficiency and the absorption cross-section for photosystem II, showed no significant responses. This confirms that phytoplankton were not iron-limited at the time and that ambient iron concentrations were sufficient to fulfil the cellular requirements. This provides new insights into extended iron replete post-bloom conditions in the typically assumed iron deficient High Nutrient-Low Chlorophyll Southern Ocean.

Asmita Singh et al.

Status: open (until 17 Feb 2023)

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Asmita Singh et al.

Asmita Singh et al.


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Short summary
Despite the low bioavailability of the micronutrient iron (Fe), the highly productive Southern Ocean experiences seasonal blooms due to changes in nutrient and light supply. The photophysiological response of phytoplankton to Fe addition was studied using five short-term incubations during an austral autumn bloom in the Antarctic Sea-Ice Zone. Phytoplankton exhibited no photophysiological response to Fe implying no Fe limitation as ambient Fe concentrations fulfilled their cellular requirements.