Macronutrients present in Southern Ocean surface waters come from upwelled deep water. While micronutrients such as zinc and cobalt may possibly co-limit phytoplankton growth in the Southern Ocean, iron appears to be a critical limiting micronutrient. Some regions of the Southern Ocean experience both adequate bioavailable iron and macronutrient concentrations yet phytoplankton growth is limited. Hydrographic studies and explorations of the Southern Drake Passage region have observed this phenomenon around the Crozet Islands, Kerguelen Islands, and South Georgia and the South Sandwich Islands. These areas are adjacent to shelf regions of Antarctica and islands of the Southern Ocean. The micronutrients required for algal growth are believed to be supplied from the shelves themselves. Except in areas close to the Antarctic shelf, micronutrient deficiency severely limits productivity in the Southern Ocean.