Bio-available nitrogen occurs in the ocean in several forms, including simple ionic forms such as nitrate, nitrite and ammonium, and more complex organic forms such as urea. These forms are utilised by autotrophicphytoplankton to synthesise organic molecules such as amino acids. Grazing of phytoplankton by zooplankton and larger organisms transfers this organic nitrogen up the food chain and throughout the marinefood-web. When nitrogenous organic molecules are ultimately metabolised by organisms, they are returned to the water column as ammonium. This is known as regeneration, since the ammonium can be used by phytoplankton, and again enter the food-web. Primary production fuelled by ammonium in this way is thus referred to as regenerated production. However, ammonium can also be oxidised to nitrate, by the process of nitrification. This is performed by different bacteria in two stages : NH3 + O2 → NO2− + 3H+ + 2e− NO2− + H2O → NO3− + 2H+ + 2e− Crucially, this process is believed to only occur in the absence of light. In the ocean, this leads to a vertical separation of nitrification from primary production, and confines it to the aphotic zone. This leads to the situation whereby any nitrate in the water column must be from the aphotic zone, and must have originated from organic materialtransported there by sinking. Primary production fuelled by nitrate is, therefore, making use of a "fresh" nutrient source rather than a regenerated one. Production by nitrate is thus referred to as new production. To sum up, production based on nitrate is using nutrient molecules newly arrived from outside the productive layer, it is termed new production. The rate of nitrate utilization remains a good measure of the new production. While if the organic matter is then eaten, respired and the nitrogen excreted as ammonia, its subsequent uptake and re-incorporation in organic matter by phytoplankton is termed recycled production. The rate of ammonia utilization is, in the same sense, a measure of recycled production. The use of 15N-compounds makes it possible to measure the fractions of new nitrogen and regenerated nitrogen associated with the primary production in the sea.
