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Pack-ice seals contribute to biological transfers of iron in the Southern Ocean

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The contribution of animals to biological transfers of essential nutrients in ecosystems is increasingly recognised as a significant component of ecosystem functioning. In the Southern Ocean (SO), primary productivity is primarily limited by the availability of iron in the euphotic zone, which makes animals locally releasing iron-rich faeces potential fertilizers of the SO food web. We quantified the amounts of iron released by four species of Antarctic pack-ice seals using a bioenergetic model set up with best available data on species abundance, energetics, diets and prey composition. We estimated that leopard, crabeater, Weddell and Ross seals together release 208 tonnes of iron per year (95% confidence interval [104–378]). This is equivalent to the current contribution of SO humpback whales and four times that of SO sperm whales. At the population level, crabeater seals are the major contributors (73%), followed by Weddell (21%), leopard (4%) and Ross seals (1%). Locally, each species shows different daily individual iron release rates, suggesting the patchy and transient impact of these iron releases on primary producers might differ according to species. Beyond quantitative aspects, pack-ice seals’ contribution to horizontal, vertical and trophic transfers of iron depends on their habitat preferences, on their ecology and behaviours at sea and on the ice. Although their role as iron vectors has been mostly overlooked so far, our results place pack-ice seals alongside whales and penguins as significant components of the SO ecosystem biological iron cycling, thus contributing substantially to its productivity and functioning.

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Data and code availability

No original data were used nor generated for this study which mobilised data from the literature for the setting of parameters in our bioenergetic model. References are provided in the manuscript (Methods and Tables 1 and 2). Statistics of estimates used to generate Fig. 1 are provided in Table 1 and ESM 1, with additional information on estimates for other input or output parameters of the model in the latter. Statistics for Sobol sensivity index reported on Fig. 3 are provided in ESM 1. Figure 4 is made based on Shirihai (2008), with the base map originating from Figure 5 mobilises data from the literature (references in the Discussion) and is made by the authors. Data and code to generate results presented in the present study are available in the public Github repository


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We wish to thank Sarah Kienle and Douglas J. Krause for helpful discussion about the predatory behaviour of leopard seals, and Matthew S. Savoca for kindly providing details of estimates of iron release by Southern Ocean baleen whales.


This work was supported by the French Ministry for Ecology (“Ministère de la Transition Ecologique et Solidaire/Direction de l’Eau et de la Biodiversité—MTES/DEB”).

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LG, TJDD and JS designed the study, LG performed the data analysis with contributions of JS and TJDD on parameter settings, LG, JS and TJDD wrote the manuscript.

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Correspondence to Lola Gilbert.

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The authors disclose no conflict of interest.

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Gilbert, L., Spitz, J. & Jeanniard-du-Dot, T. Pack-ice seals contribute to biological transfers of iron in the Southern Ocean. Polar Biol (2023).

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