Résumé
Ecosystems with high seabird densities can receive extremely high inputs of nitrogen (N) from bird guano. Seabirds deposit up to 1000 kg N ha(-1) y(-1) on Heron Island, a tropical coral cay of the Great Barrier Reef. We quantified atmospheric concentrations of ammonia (NH3) and nitrogen dioxide (NO2) with passive air samplers at beach, woodland and forest along a gradient of low, intermediate and high bird densities, respectively. NO2 concentrations at all studied sites were generally low (average 0.2-2.3 mu g NO2 m(-3)) and similar to other ecosystems. An exception was the main traffic zone of helicopter and barge traffic which had elevated concentrations (average 6.2, maximum 25 mu g NO2 m(-3)) comparable to traffic-intense urban areas elsewhere. Increasing average NH3 concentrations from 0.7 to 17 mu g NH3 m(-3) was associated with greater seabird nesting density. In areas of intermediate and high bird density, NH3 concentrations were substantially higher than those typically detected in natural and agricultural systems, supporting the notion that seabird guano is a major source of NH3. The steep decline of NH3 concentrations in areas with low bird density indicates that trans-island transport of NH3 is low. NH3 may not only be re-deposited in close vicinity of the source but is also transported vertically as concentrations above the tree canopy averaged 7.5 mu g NH3 m(-3). How much guano-derived NH3 contributes to reefal waters via the possible transfer path water -> land -> water remains to be established. We discuss atmospheric concentrations of NH3 and NO2 in context of N-based gaseous pollutants and effects on vegetation.