Nitrogen fixation by Trichodesmium spp. in the Central and Northern Great Barrier Reef Lagoon:relative importance of the fixed-nitrogen load

Abstract

Studies in the Great Barrier Reef Lagoon (GBRL) show that Trichodesmium (OsciUatoria) spp. fix significant amounts of atmospheric nitrogen and that the loads of 'new' nitrogen introduced by 7'richodesm.um spp. are at least of the same order as those entering via riverine discharge. The elevated growth of other genera of phytoplankton and hence eutrophication in some sections of the GBRL could now be largely driven by the in situ production of this 'new' nitrogen. 7'richodesmium is more prolific in the Central GBRL than in the Northern GBRL and there is evidence that the concentrations of Trichodesmium spp. have increased since the 1928-29 Great Barrier Reef Expedition to Low Isles. It is hypothesised that this increase has resulted from increases in river borne nutrients that would promote N fixation (e.g. phosphorus, iron and dissolved organic matter). It is estimated that the loads of such nutrients have increased several fold since the development of the coastal catchments of Queensland.