Atmospheric concentrations of ammonia and nitrogen dioxide at a tropical coral cay with high seabird density

Abstract

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 µg 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 µg NO₂ m⁻³) comparable to traffic-intense urban areas elsewhere. Increasing average NH₃ concentrations from 0.7 to 17 µg 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 µg 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.