Standard Presentation Australian Marine Sciences Association 2026 Conference

Iron biogeochemical cycling in the northern subantarctic at the Southern Ocean Time Series Site (139603)

Michael Ellwood 1 2 , Pier van der Merwe 2 , Robert Strzepek 2 , Sam Eggins 1 , Robin Grun 1 , Pamela Barrett 1 , Naomi Gilbert 3 , Gary LeCleir 3 , Steve Wilhelm 3 , Matthieu Bressac 4 , Elizabeth Shadwick 5 , Cathryn Wynn-Edwards 5 , Tom Trull 5 , Philip Boyd 2
  1. Australian National University, Acton, ACT, Australia
  2. Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
  3. Department of Microbiology, The University of Tennessee, Knoxville, Tennessee, USA
  4. CNRS, Laboratoire d’Océanographie de Villefranche, Sorbonne Université, Villefranche, Villefranche-sur-Mer, France
  5. Commonwealth Scientific and Industrial Research Organisation, Hobart, Tasmania, Australia

 

The Southern Ocean Time Series (SOTS) site (47°S, 142°E) is located on the northern margin of the subantarctic zone, near the interface between subtropical and subantarctic waters. Its location and sustained observations provide an opportunity to examine how physical circulation influences trace metal distributions and biological processes in the Southern Ocean. This study synthesises measurements of iron and other interacting trace metals collected during five voyages between 2016 and 2020, together with monthly surface-water sampling from 2021 to 2022 and complementary biological observations.

Seasonal forcing and episodic intrusions of subtropical water drive pronounced variability in upper-ocean temperature and salinity, which is reflected in dissolved iron (dFe) concentrations in surface and upper mesopelagic waters (<80 m). Upper-ocean dFe concentrations ranged from 0.06 nmol kg⁻¹ to 0.33 nmol kg⁻¹ and are associated with subtropical water inputs, atmospheric deposition, and biological processing. Deep-water dFe supply is also influenced by sedimentary inputs from the nearby South Tasman Rise.

Surface chlorophyll showed a weak but positive relationship with dFe, consistent with strong biological recycling and multiple interacting controls on biomass. Size‑fractionated dFe and carbon uptake measurements indicate that picoeukaryotes dominate mixed‑layer Fe cycling, with short turnover times (3–42 days) and substantial recycling by grazers and heterotrophic bacteria. Agreement between dissolved and particulate Fe:C ratios suggests that newly produced material largely retains its stoichiometry during export. Extended Redfield stoichiometries for newly formed organic matter [(C107N17P1)1000Fe5.0Ni1.7Zn0.55Mn0.55Cu0.45Cd0.23Co0.09] yield Fe:C and Fe:P ratios consistent with literature values, but elevated Ni and low Mn, suggesting potential secondary Mn limitation. Taken together, our data suggest that SOTS is less iron-stressed than other subantarctic locales due to its proximity to subtropical and sedimentary iron sources.