Physical speciation and solubility of iron from baleen whale faecal material

Primary productivity in large areas of the Southern Ocean is limited by the availability of a key micronutrient—iron (Fe). Recently it has been suggested that marine animals could play an important role in recycling Fe through their diet and subsequent defecation, however there is no information on the relative bioavailability of faecal Fe for uptake. The bioavailability of Fe in seawater is controlled by a number of complex interactions. The physical separation between the dissolved (< 0.2 μm) and particulate (> 0.2 μm) fractions is one common measure used to determine element bioavailability. Here, the size fractionation of Fe from 3 whale faecal samples in 4 different size classes (< 0.2 μm, 0.2–10 μm, 10–60 μm and > 60 μm) was investigated, along with the leaching of particulate Fe over time. Although the total particulate fraction (> 0.2 μm, 5026–22,526 nmol L⁻¹) dominated the total Fe pool, the concentrations of dissolved Fe in whale faecal samples (186–754 nmol L⁻¹) were three order of magnitude higher than published Southern Ocean surface seawater concentrations. Furthermore, results from the leaching experiment suggest that Fe is continually leached from faecal particles over an initial 12-hour period, thus increasing the concentration of bioavailable Fe in surface seawater. Although the concentrations measured here are some of the highest reported in the literature, the true supply of Fe back to surface seawater will be controlled by processes such as organic complexation, scavenging and sinking by particles, remineralisation, and vertical transport, not measured in this study.