The distribution and availability of iron in the Antarctic coastal ocean

Despite widespread iron (Fe) limitation in the Southern Ocean, Antarctic coastlines are highly productive, supporting biodiverse ecosystems and atmospheric carbon sequestration. Iron is an essential micronutrient for cellular processes such as photosynthesis; therefore, its scarcity in surface waters is a key limitation on primary productivity. Seasonally variable sources of Fe to coastal waters include sediment resuspension, basal ice melt, dust deposition and sea-ice melt, as well as upwelling and vertical mixing of offshore deep waters. Early spring phytoplankton blooms rapidly deplete Fe, after which internal Fe recycling processes support productivity into late summer. Bacterial Fe remineralisation and pelagic recycling by marine animals are thought to influence the distribution of recycled Fe during this period. The chemical nature of Fe influences its bioavailability to microbial communities. For example, complexation by ligands increases Fe solubility in surface waters by reducing scavenging onto particles. However, little is known about the late summer distribution and bioavailability of Fe from different Antarctic coastal sources, which are expected to be impacted by climate change. This dissertation explores the bioavailability of coastal Fe sources and directly assesses distribution and complexation of Fe in a coastal Antarctic environment during late summer when recycling processes are thought to dominate.

Chapter 2 assesses the bioavailability of Fe from seven different coastal Antarctic sources. These include two geological sources: Antarctic continental dust and shelf sediments, and five biological sources: sea ice (pack ice), and faecal material from four marine animals (Antarctic krill, baleen whales, penguins, and seals). The concentration of labile Fe, solubility of dissolved Fe and complexation by Fe-binding ligands were measured as an indicator of Fe bioavailability. Poorly soluble Fe (<3%) from physical sources was subject to scavenging by particles in seawater. Whereas up to 30% of total Fe was leached from biological sources which was highly complexed by ligands, thus increasing Fe bioavailability to phytoplankton. Iron and dissolved organic carbon (DOC) co-limitation may be alleviated when heterotrophic bacteria are exposed to high Fe and DOC concentrations present in sediments and faecal material of whales, penguins, and seals. The spatial variability and patchiness of marine animals however suggest their contributions to Fe and DOC recycling are localised compared to Fe and DOC delivered from widespread sources such as shelf sediments and sea ice.

Chapter 3 explores the distribution and potential sources of dissolved Fe in coastal Antarctica during the late summer when primary Fe sources are thought to be exhausted. Seawater from the productive and globally important Mertz Glacier Region was sampled during the Australian Antarctic Program's 2019 'Euphausiids and Nutrient Recycling in Cetacean Hotspots' (ENRICH) Voyage, following GEOTRACES protocols for trace element sampling and analysis. Upwelling of Circumpolar Deep Water and shelf sediment resuspension were found to provide high concentrations of Fe to the region, with evidence for microbial Fe remineralisation in surface and shelf waters. Dissolved Fe released by late summer sea-ice melt and pelagic recycling by marine animals (including Antarctic krill and baleen whales) was either localised or rapidly consumed by biota as no relationship could be determined from these potential Fe sources.

Chapter 4 investigates the complexation of Fe by ligands in the Mertz Glacier Region, thereby providing an estimate of Fe bioavailability to marine microorganisms in late summer. Across the region, dissolved Fe was complexed by strong ligands supplied from depth (>200 m). Strongest ligands were observed where microbial remineralisation was identified as a possible Fe source, suggesting that bacterial release of siderophores may complex Fe over the shelf break (200 - 400 m). In surface waters (<100 m), significant Fe uptake by phytoplankton communities resulted in excess free ligands with the capacity to complex new Fe in surface waters. This study is among 25 complexation surveys included in the new Southern Ocean Ligand (SOLt) Collection which was compiled for this dissertation. The SOLt Collection is publicly available, highlights priority areas for future complexation studies and notes underlying issues with intercalibration of ligand datasets.

Overall, this study demonstrates that the theoretical bioavailability of Fe differs between physical and biological coastal sources. It shows that multiple sources supply Fe to coastal Antarctic regions on varying spatial scales and complexation by strong ligands helps to prolong productivity into late summer. Considering the vulnerability of Antarctic coastlines to rapid environmental change, future studies should aim to further constrain seasonal Fe supply mechanisms and determine the direct response of Southern Ocean microbial communities to fertilisation by coastal Fe sources. A thorough conception of coastal Antarctic Fe supply will inform future primary productivity estimates, carbon sequestration measures and ecosystem functioning.