Inputs of inorganic nutnents in 1992 from seablrds (wh~ te-capped noddy and wedgetail shearwate~, maximum of ca 80 000 breeding palrs) are compared to inputs from humans (97 700 person days) on Heron Island at the southern end of the Great Barr~ er Reef, Australia We estimated that noddies deposited ca 107 t and shearwaters ca 22 t, of fresh guano The composition of fresh noddy guano was 7 3% nitrogen 1 5% phosphorus. 60% molsture and 31 2% other substances Assum~ ng a similar compos~ t~ on for shearwaters the total annual deposit~ on of guano contained 9 4 t nitrogen and 1 9 t phosphorus Exper~ ments examlnlng effects of ageing of guano indicated a large decrease In total nitrogen In guano under humld cond~ tions over 4 d as a result of volatilisat~ on of NH, Under natural conditions most of the depos~ ted nitrogen is likely to be lost as NH, and a relatively small fraction ot the soluble component leached into the cay, but the precise dynam~ cs w~ ll depend on ramfall and\mnd patterns Phosphorus was not\~ oldtilised from guano undei any expenmental conditions Although inputs of nltrogen and phosphorus from human sewage into the cay system (ca 0 3 t of each) were much less than that from b~ rds, all nutrlents from humans are released in liquid form and percolate directly into the cay Significant seasonal and tidal variations in standing concentratlons of NH? oxid~ sed forms of nitrogen (NO,+ NO,) and PO, were detected in the water coluinn around the island Trends of higher mean concentratlons of all nutrlents In summer than In winter, and higher concentrations at low tide than at high tide except at sites close to the Island where nutrient levels were high independent of tide suggest that nutnents may be transported from guano on Heron Island into the water immediately surrounding the island The exact fate and n~ echanisms of transport of all nutnents require further attention