Iron and manganese recycling in the Southern Ocean: the relative influence of the “ferrous wheel” and the “spinning manga”

Iron is a key component of many enzymes and thus helps set the magnitude of primary production and nitrogen fixation. Other trace metals have essential metabolic functions but probably play lesser roles relative to iron. Studies report that manganese can, in certain ocean provinces and during some seasons, play an influential role in regulating phytoplankton metabolism. Evidence is based on nutrient amendment in shipboard incubations or manganese/iron stoichiometric patterns in resupply from depth. Iron recycling underpins primary production in High Nutrient Low Chlorophyll regions over most of the year. In contrast, less is known about manganese recycling. Here we compare Southern Ocean iron and manganese recycling using combined approaches to construct an ecological stoichiometric framework of the epipelagic (0–200 m depth) ocean. Our findings point to important differences in the stoichiometry of resupply of iron versus manganese, with manganese likely more efficiently recycled and hence retained in the upper ocean. In particular, the export of iron-rich copepod faecal pellets drives less retentive recycling by the “ferrous wheel” relative to the “spinning manga”. Such preferential retention of recycled manganese may offset observed differences in the stoichiometric supply of new iron compared with manganese and contribute to observed patterns of iron versus manganese limitation in the Southern Ocean.