Iron availability is not the only regulator of phytoplankton productivity and biomass. In the Southern Ocean, prevailing low temperatures are believed to have a negative impact on phytoplankton growth rates. Phytoplankton growth rate is very intense and short lived in open areas surrounded by sea ice and permanent sea-ice zones. Grazing by herbivores such as krill, copepods and salps is believed to suppress phytoplankton standing stock. Unlike the open waters of the Southern Ocean, grazing along continental shelf margins is low, so most phytoplankton that are not consumed sink to the sea floor which provides nutrients to benthic organisms.