Abstract
Iron (Fe) is an essential micronutrient that controls phytoplankton growth in the Southern Ocean. Dissolved Fe ( 0.4 μm) is far more abundant and may also become bioavailable through biogeochemical processing. To assess natural Fe fertilisation from the particulate fraction, we surveyed suspended particles in the water column at 11 stations in the vicinity of Heard and McDonald Islands (HIMI), in the Indian sector of the Southern Ocean and compared these to downstream plateau and reference stations. We quantified the labile (potentially bioavailable) fraction using a chemical leach. Suspended particles sourced from glacial erosion and fluvial outflow, including nanoparticulate Fe oxides near Heard Island, contained a significantly higher fraction of labile Fe (18 ± 2.8% of total Fe, or 115 ± 34 nM, n = 9) than all other coastal areas surveyed. In contrast, waters around McDonald Island, proximal to diffuse gasohydrothermal sites, contained poorly labile, highly refractory titanium and Fe bearing minerals such as ilmenite. We conclude that glacial erosion of Heard Island in combination with a unique elemental signature of the source rock, is an important mechanism of Fe supply to downstream waters. Our calculations show that the labile Fe supplied from primarily glacial erosion on Heard Island is sufficient to satisfy previously unmet estimates of phytoplankton demand for the region, and therefore critical to the area’s productivity. As we move into a world facing major ecosystem shifts under a changing climate, it is important to understand those ecosystem services that may change into the future. At the current rate of glacier retreat, this ecosystem service of glacial erosion and Fe supply to coastal waters will cease with the eventual loss of glacial cover with direct impacts for this historically highly productive region.
Highlights
The Southern Ocean maintains relatively little phytoplankton growth, despite a wealth of macronutrients, due to a lack of micronutrients, principally the trace element iron (Fe) (de Baar et al, 2005; Boyd and Ellwood, 2010)
While total particulate Fe, aluminium (Al), Mn, and Ti were all higher at the stations sampled around McDonald Island (Figure 4), the fraction of Fe that was labile was significantly higher at Heard Island compared to McDonald Island
It is likely that the Fe oxyhydroxides that we observed within glacial outflow near Heard Island were relatively fresh, leading to the higher chemical lability we observed, in comparison to highly refractory minerals such as ilmenite collected ubiquitously around both islands
Summary
The Southern Ocean maintains relatively little phytoplankton growth, despite a wealth of macronutrients (phosphate, nitrate + nitrite), due to a lack of micronutrients, principally the trace element iron (Fe) (de Baar et al, 2005; Boyd and Ellwood, 2010). Iron is supplied to the ocean through various forms of rock weathering and its physical and chemical form dictate whether phytoplankton and bacteria can utilise and recycle it (Boyd et al, 2017). Each spring and into summer, a large phytoplankton bloom extends from the Kerguelen Plateau thousands of kilometres into the Southern Ocean (Schallenberg et al, 2018). Previous work has shown that weathering of Kerguelen Island, ∼500 km to the north (and north of the Polar Front), and the shallow plateaus surrounding it, supply the micronutrient Fe, temporarily alleviating Fe limitation and allowing the development of this bloom (Blain et al, 2007; Queroue et al, 2015; van der Merwe et al, 2015). The processes that supply Fe into waters surrounding Heard and McDonald Islands (HIMI), south of the Polar front, situated in significantly colder waters, are not well understood in terms of their magnitude and lability of the Fe supplied
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