From the Origin of Feces - The Impact of Krill and Salp Fecal Pellets on Iron Chemistry and Iron Bioavailability to Southern Ocean Phytoplankton

The Southern Ocean is considered to be a major player in the climate system of our planet while being extremely sensitive to climate change itself. The pelagic Southern Ocean is limited by the bioavailability of iron. Zooplankton has a large impact on the remineralization of iron in the water column and thereby an important influence on primary production. Indications exist that due to increasing water temperatures in the course of climate change, vast areas of the Southern Ocean might shift from a krill to a salp-dominated community. Since the degree of iron remineralization is dependent on the taxonomic group of zooplankton, we investigated the different impacts that salp and krill fecal pellets have on iron chemistry and its bioavailability to Southern Ocean phytoplankton, during a Polarstern cruise in spring 2018. We incubated salp and krill fecal pellet material in Antarctic low-iron water without phytoplankton. In a second step, a concentrated natural phytoplankton community was added into the thusly preconditioned water and for the first time ever the iron uptake into the living cells, in respect to the fecal pellet type that acted as an iron source, was determined. Our results indicate that iron released from salp fecal pellets into the seawater was significantly more bioavailable to phytoplankton than iron from krill fecal pellets, since phytoplankton picked up 0.28 nM Fe d^-1 from water treated with salp fecal pellets and 0.16 nM Fe d^-1 from water treated with krill fecal pellets. These results demonstrate that salps might actually play a role in stimulating phytoplankton growth in the Southern Ocean, thusly influencing the biological carbon pump.