<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> <P> Nitrate is depleted in near - surface water except in upwelling regions . Coastal upwelling regions usually have high nitrate and chlorophyll levels as a result of the increased production . However, there are regions of high surface nitrate but low chlorophyll that are referred to as HNLC (high nitrogen, low chlorophyll) regions . The best explanation for HNLC regions relates to iron scarcity in the ocean, which may play an important part in ocean dynamics and nutrient cycles . The input of iron varies by region and is delivered to the ocean by dust (from dust storms) and leached out of rocks . Iron is under consideration as the true limiting element to ecosystem productivity in the ocean . </P> <P> Ammonium and nitrite show a maximum concentration at 50--80 m (lower end of the euphotic zone) with decreasing concentration below that depth . This distribution can be accounted for by the fact that nitrite and ammonium are intermediate species . They are both rapidly produced and consumed through the water column . The amount of ammonium in the ocean is about 3 orders of magnitude less than nitrate . Between ammonium, nitrite, and nitrate, nitrite has the fastest turnover rate . It can be produced during nitrate assimilation, nitrification, and denitrification; however, it is immediately consumed again . </P>

Briefly describe nitrogen cycle with well labeled diagram