The Fecal Iron Pump: Animals and Marine Particles Fe Stoichiometry

The impact of marine animals on the iron (Fe) cycle is mostly considered through their supply of dissolved Fe to primary producers, via their grazing activity, metabolic excretion and the remineralization of exported fecal matter. However, the transformation of ingested particulate Fe into fecal pellets by animals can also alter the vertical distribution of particulate Fe in the water column, a process that has received little attention. Furthermore, there has been very little consideration of how the iron contents of particles might influence nutrition for animals through coprophagy. Here we show that the iron to carbon ratio (Fe:C) of fecal pellets of various marine animals is consistently enriched compared to their food, by up to an order of magnitude. This leads to some general expectations for how animals will influence the sinking flux of Fe, relative to carbon, which we illustrate for a range of possible fecal pellet abundances. Our results have implications both for global iron cycling and for the iron nutrition of mesopelagic communities.