Koen Parmentier, chair of the Marine Chemistry Working Group (MCWG)
'Primary production is the building of organic molecules starting from carbon dioxide and water. Through photosynthesis sugar is produced. This in turn is converted to a wide variety of other organic compounds which arethe building blocks for the living cells.
In the marine environment, this is done by marine algae, which we call autotrophic organisms. But they also rely on other compounds to build up their biology, and these we call nutrients. All living organisms require a nitrogen source, which can be nitrate, nitrite, ammonia, and amines (nitrogen-containing organic molecules). Phosphate is the usual source of phosphorus, and a lot of algae build a silicon shell and therefor need silicate.
Moreover they need a number of micronutrients, like sulphate and metal ions like Zn2+ (zinc), Cu2+ (copper), Mg2+ (magnesium), Ca2+ (calcium), and Fe3+ (iron). As seawater is usually rich in minerals, these minerals rarely limit the growth of phytoplankton, although in open ocean this might occur.
Most often, the phytoplankton growth limiting factor is a nitrogen source, phosphate or silicate. Since they mainly originate from domestic wastewater, agriculture (fertilizers), and industry, it is the input of these nutrients we have to reduce in order to avoid massive algal blooms. Blooms can be annoying, (giving rise to a dirty foam carpet), but also harmful (species such as phaeocystis), and even really dangerous (toxin-producing algae).'