Although iron and light are understood to regulate the Southern Ocean
biological carbon pump, observations have also indicated a possible role
for manganese. Low concentrations in Southern Ocean surface waters
suggest manganese limitation is possible, but its spatial extent remains
poorly constrained and direct manganese limitation of the marine carbon
cycle has been neglected by ocean models. Here, using available
observations, we develop a new global biogeochemical model and find that
phytoplankton in over half of the Southern Ocean cannot attain maximal
growth rates because of manganese deficiency. Manganese limitation is
most extensive in austral spring and depends on phytoplankton traits
related to the size of photosynthetic antennae and the inhibition of
manganese uptake by high zinc in Antarctic waters. Importantly,
manganese limitation expands under the increased iron supply of past
glacial periods, reducing the response of the biological carbon pump.
Overall, these model experiments describe a mosaic of controls on
Southern Ocean productivity that emerge from the interplay of light,
iron, manganese and zinc, shaping the evolution of Antarctic
phytoplankton since the opening of the Drake Passage.