Nitrogen ecophysiology of Heron Island, a subtropical coral cay of the Great Barrier Reef, Australia.

Coral cays form part of the Australian Great Barrier Reef. Coral cays with high densities of seabirds are areas of extreme nitrogen (N) enrichment with deposition rates of up to 1000 kg N ha y. The ways in which N sources are utilised by coral cay plants, N is distributed within the cay, and whether or not seabird-derived N moves from cay to surrounding marine environments were investigated. We used N metabolite analysis, N labelling and N natural abundance (δN) techniques. Deposited guano-derived uric acid is hydrolysed to ammonium (NH) and gaseous ammonia (NH). Ammonium undergoes nitrification, and nitrate (NO) and NH were the main forms of soluble N in the soil. Plants from seabird rookeries have a high capacity to take up and assimilate NH, are able to metabolise uric acid, but have low rates of NO uptake and assimilation. We concluded that NH is the principal source of N for plants growing at seabird rookeries, and that the presence of NH in soil and gaseous NH in the atmosphere inhibits assimilation of NO, although NO is taken up and stored. Seabird guano, Pisonia forest soil and vegetation were similarly enriched in N suggesting that the isotopic enrichment of guano (δN 9.9‰) carries through the forest ecosystem. Soil and plants from woodland and beach environments had lower δN (average 6.5‰) indicating a lower contribution of bird-derived N to the N nutrition of plants at these sites. The aquifer under the cay receives seabird-derived N leached from the cay and has high concentrations of N-enriched NO (δN7.9‰). Macroalgae from reefs with and without seabirds had similar δN values of 2.0-3.9‰ suggesting that reef macroalgae do not utilise N-enriched seabird-derived N as a main source of N. At a site beyond the Heron Reef Crest, macroalgae had elevated δN of 5.2‰, possibly indicating that there are locations where macroalgae access isotopically enriched aquifer-derived N. Nitrogen relations of Heron Island vegetation are compared with other reef islands and a conceptual model is presented.