Nutrient inputs from seabirds and humans on a populated coral cay

Abstract

Inputs of inorganic nutnents in 1992 from seabirds (white-capped noddy and wedgetail shearwater, maximum of ca 80 000 breeding pairs) are compared to inputs from humans (97 700 person days) on Heron Island at the southern end of the Great Barrier Reef, Australia. We estimated that nod- dies 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 % moisture and 31.2 % other substances. Assuming a similar composition for shearwaters the total annual deposition of guano contained 9.4 t nitrogen and 1.9 t phosphorus. Experiments examining effects of ageing of guano indicated a large decrease in total nitrogen in guano under humid conditions over 4 d as a result of volatilisation of NH3. Under natural conditions most of the deposited nitrogen is likely to be lost as NH, and a relatively small fraction ot the soluble component leached into the cay, but the precise dynamics will depend on ramfall and wind patterns. Phosphorus was not volatilised from guano under any expenmental conditions. Although inputs of nitrogen and phosphorus from human sewage into the cay system (ca 0.3 t of each) were much less than that from birds, all nutrients from humans are released in liquid form and percolate directly into the cay. Significant seasonal and tidal variations in standing concentratlons of NH3 oxidised forms of nitrogen (NO2,+NO3), and PO4, were detected in the water column around the island. Trends of higher mean concentrations of all nutrients 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 nutrients may be transported from guano on Heron Island into the water immediately surrounding the island The exact fate and mechanisms of transport of all nutnents require further attention.