<P> Though nitrogen fixation is the primary source of plant - available nitrogen in most ecosystems, in areas with nitrogen - rich bedrock, the breakdown of this rock also serves as a nitrogen source . </P> <P> The nitrogen cycle is an important process in the ocean as well . While the overall cycle is similar, there are different players and modes of transfer for nitrogen in the ocean . Nitrogen enters the water through the precipitation, runoff, or as N from the atmosphere . Nitrogen cannot be utilized by phytoplankton as N so it must undergo nitrogen fixation which is performed predominately by cyanobacteria . Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years . Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter . Ammonia and urea are released into the water by excretion from plankton . Nitrogen sources are removed from the euphotic zone by the downward movement of the organic matter . This can occur from sinking of phytoplankton, vertical mixing, or sinking of waste of vertical migrators . The sinking results in ammonia being introduced at lower depths below the euphotic zone . Bacteria are able to convert ammonia to nitrite and nitrate but they are inhibited by light so this must occur below the euphotic zone . Ammonification or Mineralization is performed by bacteria to convert organic nitrogen to ammonia . Nitrification can then occur to convert the ammonium to nitrite and nitrate . Nitrate can be returned to the euphotic zone by vertical mixing and upwelling where it can be taken up by phytoplankton to continue the cycle . N can be returned to the atmosphere through denitrification . </P> <P> Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve a redox reaction and therefore requires little energy . Nitrate requires a redox reaction for assimilation but is more abundant so most phytoplankton have adapted to have the enzymes necessary to undertake this reduction (nitrate reductase). There are a few notable and well - known exceptions that include Prochlorococcus and some Synechococcus . These species can only take up nitrogen as ammonium . </P> <P> The nutrients in the ocean are not uniformly distributed . Areas of upwelling provide supplies of nitrogen from below the euphotic zone . Coastal zones provide nitrogen from runoff and upwelling occurs readily along the coast . However, the rate at which nitrogen can be taken up by phytoplankton is decreased in oligotrophic waters year - round and temperate water in the summer resulting in lower primary production . The distribution of the different forms of nitrogen varies throughout the oceans as well . </P>

Draw a well labelled diagram of nitrogen cycle