Iron plays an essential role in marine systems and can act as a limiting nutrient for planktonic activity. Because of this, too much of a decrease in iron may lead to a decrease in growth rates in phytoplanktonic organisms such as diatoms. Iron can also be oxidized by marine microbes under conditions that are high in iron and low in oxygen.
Iron can enter marine systems through adjoining rivers and directly from the atmosphere. Once iron enters the ocean, it can be distributed throughout the water column through ocean mixing and through recycling on the cellular level. In the arctic, sea ice plays a major role in the store and distribution of iron in the ocean, depleting oceanic iron as it freezes in the winter and releasing it back into the water when thawing occurs in the summer. The iron cycle can fluctuate the forms of iron from aqueous to particle forms altering the availability of iron to primary producers. Increased light and warmth increases the amount of iron that is in forms that are usable by primary producers.