Modelling Trichodesmium dynamics and its contribution to the Great Barrier Reef nitrogen budget

Trichodesmium is a nitrogen-fixing marine cyanobacterium, proposed to be an important source of “new” nitrogen (i.e., nitrogen that is not generated from remineralised organic matter) in the Great Barrier Reef. We estimated the contribution of nitrogen fixation by Trichodesmium to the Great Barrier Reef’s annual nitrogen budget using the CSIRO Environmental Modelling Suite (EMS), a coupled hydrodynamic, biogeochemical and sediment modelling system. EMS outputs were improved by optimally parameterising the temperature dependence of Trichodesmium physiological rates to capture influence of ongoing temperature increases associated with climate change. The model estimated that Trichodesmium contributed 0.49 MT to the nitrogen budget between December 2010 and November 2011 in the Great Barrier Reef. During this period, there was record high rainfall causing widespread flooding and record river discharge in most Great Barrier Reef rivers. The estimated fixed-nitrogen load is greater than the mean-annual riverine nitrogen loads in the Great Barrier Reef. Nitrogen fixed from atmospheric N2 by Trichodesmium and other diazotrophs could enhance phytoplankton growth and cause eutrophication. Blooms of Trichodesmium are likely to cause hypoxia and have been shown to contain harmful biotoxins and neurotoxins. Dense Trichodesmium blooms are thus likely to locally reduce water quality and alter marine ecosystems. Reliable quantification of the annual nitrogen load from Trichodesmium nitrogen fixation in the Great Barrier Reef will enable environmental managers to better understand the role of terrestrial nitrogen loads in the Great Barrier Reef and the potential impacts of terrestrial phosphorus and iron loads. Our findings could help to broaden the focus of water quality management programs and support management to improve Great Barrier Reef water quality.