15N and the assimilation of nitrogen by marine phytoplankton: the past, present and future?

The World Ocean, covering approximately 70% of the surface of the planet, is the largest individual ecosystem on earth. The surface illuminated waters of this system, extending to a maximum of approximately 100 m, but more typically to less than 50 m, contributes about half of the global photoautotrophic-derived organic production. In common with terrestrial systems, nitrogen occurs in marine systems in both organic and inorganic, particulate and dissolved forms. Dissolved organic nitrogen usually dominates, typically representing over 95% of the total nitrogen pool in surface ocean waters, but particulate organic nitrogen, in the form of living, or recently living, biomass may also represent a dominant fraction on occasions. Sampling from large oceanographic research ships is almost entirely carried out using conductivity, temperature, depth sensor packages fitted with an array of sampling bottles.