Processes governing the supply of iron to phytoplankton in stratified seas

[ 1] The impact of iron speciation on phytoplankton dynamics in the marine environment is currently not well understood. In this study, we modified the existing CIAO model of the Ross Sea to include major aspects of iron chemistry and examined the processes governing the supply of iron to phytoplankton. We suggest that the melting of sea ice and ensuing stratification dictates the speciation and bioavailability of iron. Photoreduction is the key process governing the supply of bioavailable Fe to the resident phytoplankton of the Ross Sea. Access to organically complexed iron is of greater significance in strongly stratified waters dominated by diatoms than in weakly stratified waters dominated by Phaeocystis antarctica. Light may control phytoplankton taxonomic composition in the Ross Sea by impacting both photosynthetic performance and iron speciation. The efficiency with which iron fuels phytoplankton carbon incorporation is greater in seasonal ice zones than in permanently ice-free waters. Therefore, changes in the physical structure of the Southern Ocean could impact iron bioavailability, independent of any change in iron supply.