Enewetak (Eniwetok) Atoll: Aspects of the nitrogen cycle on a coral reef 1

Changes in the concentrations of dissolved and particulate nitrogen in seawater crossing two interisland reef transects on Enewetak Atoll were measured. The upstream half of transect II consisted largely of algal pavement; the downstream half was visually dominated by corals. The other community, transect III, consisted of benthic algal covered pavement in its entirety, with only scattered small corals downstream. Both transects showed a significant net export of combined nitrogen, implying a large input of nitrogen into the reef system from a source other than the combined nitrogen in overlying waters. High rates of gaseous nitrogen fixation were found. There was a net uptake of nitrate by the community dominated by algae and a net export of nitrate from the transect dominated jointly by algae and corals. Although nitrification is of central importance in the marine nitrogen cycle, this coral-algal community at Enewetak appears to have been the first marine site at which this process has been measured directly.