Aside from light, phytoplankton are also crucially dependent on the availability of nutrients for growth. These are primarily macronutrients such as nitrate, phosphate or silicic acid, whose availability is governed by the balance between the so-called biological pump and upwelling of deep, nutrient-rich waters. However, across large regions of the World Ocean such as the Southern Ocean, phytoplankton are also limited by the availability of the micronutrient iron. This has led to some scientists advocating iron fertilization as a means to counteract the accumulation of anthropogenic carbon dioxide (CO2) in the atmosphere.