The Southern ocean’s biological response to atmospheric iron fertilisation

The role of atmospheric iron deposition in influencing primary productivity in the iron-limited Southern Ocean has been a subject of longstanding interest. This thesis provides a comprehensive analysis of iron fertilisation by mineral dust and wildfire smoke in the Southern Ocean, using an integrated approach of ocean colour satellite imagery, biogeochemical ocean profiling float observations, and atmospheric and biogeochemical simulations.
The primary findings of this research include a significant basin-scale correlation between dust-derived iron deposition and annual net community production (ANCP) in the present-day Southern Ocean’s epipelagic zone. The ANCP-dust relationship enables the calculation of ANCP and dust-supported ANCP maps from dust fluxes. From these we infer that, at present, one third of productivity in the iron-limited ice-free Southern Ocean is supported by dust. This correlation further offers valuable insights into the potential impacts of dust-iron fertilisation on glacial-interglacial timescales, underscoring the pivotal role of dust in past climate dynamics. Dust-impacted regions further reveal distinct phytoplankton physiological characteristics, suggesting a of phytoplankton community structure and a potential increase in carbon export efficiency due to dust supply.
The 2019–2020 Australian wildfires provided the opportunity for an insightful case study on the impacts of wildfire smoke plumes on the biogeochemistry of the Pacific sector of the Southern Ocean. Satellite observations and a regional iron budget showed a prolonged phytoplankton bloom in response to iron deposited by these smoke plumes. The bloom exhibited unique physiological responses associated with iron stress relief. This study further unveils prolonged iron recycling in the bloom, which persisted for up to 9 months after the initial fertilisation event, suggesting a sustained influence of such extreme wildfire events on Southern Ocean biogeochemical cycles.
In conclusion, this research underscores the varied and profound influence of atmospheric iron sources on the Southern Ocean’s primary productivity and carbon cycling. The findings present critical insights for future biogeochemical projections and emphasise the importance of including dust and wildfire aerosols in ocean biogeochemical and global climate models.