Iron as a Limiting Factor in Oceanic Productivity

Abstract

Iron has been hypothesized to be a factor limiting phytoplankton standing crop in the ocean for decades. For example, in 1931, Gran suggested that, “If the productivity of the coastal waters is dependent on any factor of a chemical nature acting as a minimum factor, it must be an element which in its circulation does not follow the nitrates and phosphates accumulating in solution in the deep sea and reaching the surface again by vertical circulation of any kind. If such minimum stuffs exist, they must irreversibly go out of circulation in the sea, so that they can only be renewed from the land.” Based on growth in culture solution, Gran (1931, p.41) concluded that lack or low concentration of iron probably limited plant growth at times and in areas of the sea where it was not replenished by land drainage. Previous data, and those obtained by Braarud & Klem (1931) off the Norwegian coast at his instigation, showed the iron content of sea water to be very small, ranging from 3 to 21 mg. Fe per m3 (Harvey, 1938, p.205)