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

Abstract

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^–1 y^–1. 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, ¹⁵N labelling and ¹⁵N natural abundance (δ¹⁵N) techniques. Deposited guano-derived uric acid is hydrolysed to ammonium (NH₄⁺) and gaseous ammonia (NH₃). Ammonium undergoes nitrification, and nitrate (NO₃^–) and NH₄⁺ were the main forms of soluble N in the soil. Plants from seabird rookeries have a high capacity to take up and assimilate NH₄⁺, are able to metabolise uric acid, but have low rates of NO₃^– uptake and assimilation. We concluded that NH₄⁺ is the principal source of N for plants growing at seabird rookeries, and that the presence of NH₄⁺ in soil and gaseous NH₃ in the atmosphere inhibits assimilation of NO₃^–, although NO₃^– is taken up and stored. Seabird guano, Pisonia forest soil and vegetation were similarly enriched in ¹⁵N suggesting that the isotopic enrichment of guano (δ¹⁵N 9.9‰) carries through the forest ecosystem. Soil and plants from woodland and beach environments had lower δ¹⁵N (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 ¹⁵N-enriched NO₃^– (δ¹⁵N 7.9‰). Macroalgae from reefs with and without seabirds had similar δ¹⁵N values of 2.0–3.9‰ suggesting that reef macroalgae do not utilise ¹⁵N-enriched seabird-derived N as a main source of N. At a site beyond the Heron Reef Crest, macroalgae had elevated δ¹⁵N 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.