Understanding the variability in the iron concentration of Antarctic krill

Antarctic krill may play a significant role in the Southern Ocean iron cycle. However, understanding the control on iron budgets by Antarctic krill is hampered by the large range in the reported iron concentration of krill. The aim of this study was to investigate the causes of the large range of iron concentrations in krill reported in the literature (6–190 mg kg⁻¹). Antarctic krill samples were collected from three research voyages to Pyrdz Bay, Antarctica, and analysed individually. Iron concentrations were measured using sector field inductively coupled plasma mass spectrometry in whole krill specimens and in the isolated stomach, digestive gland, muscle, body (whole krill excluding stomach and digestive gland), exoskeleton and faecal pellets. Iron concentrations in stomach (6–98 mg/kg), digestive gland (14–82 mg kg⁻¹), and faecal pellet (683–1039 mg kg⁻¹) were higher compared to muscle (4–7 mg kg⁻¹), exoskeleton (6–15 mg kg⁻¹), and body (4–18 mg kg⁻¹) indicating that krill may ingest more iron than they require for physiological processes. Iron concentrations in whole krill from March 2012 (10 ± 3 mg kg⁻¹) were significantly lower compared to February 2003 (19 ± 7 mg kg⁻¹) and February 2015 (18 ± 12 mg kg⁻¹). Overall, the iron concentrations in krill from this study were consistently at the lower end of the published range. We propose that the large range in reported whole iron concentrations of krill can be accounted for by a combination of seasonal and regional differences in sampling, reflecting differences in the quantity and quality of their diet.