The Southern Ocean is the largest HNLC region in the global ocean. The surface waters of the Southern Ocean have been widely identified as being rich in macronutrients despite low phytoplankton stocks. Iron deposited in the North Atlantic is incorporated into North Atlantic Deep Water and is transported to the Southern Ocean via thermohaline circulation. Eventually mixing with the Antarctic Circumpolar Water, upwelling provides iron and macronutrients to the Southern Ocean surface waters. Therefore, iron inputs and primary production in the Southern Ocean are sensitive to iron-rich Saharan dust deposited over the Atlantic. Because of low atmospheric dust inputs directly onto Southern Ocean surface waters, chlorophyll α concentrations are low. Light availability in the Southern Ocean changes dramatically seasonally, but it does not seem to be a significant constraint on phytoplankton growth.