THE CYCLE OF PHOSPHORUS IN AN ATOLL ECOSYSTEM

The major subsystems of an atoll relevant to the phosphorus cycle are the sea (0.015 g/t P) and atmosphere, the marine biosphere (approximat ely 103 g/t P), the supramarine portion of the atoll's carbonate subst rate including rock and soils (approximately 10(3) g/t P), the freshwa ter lens contained in that substrate (approximately 7 x 10(2) g/t P), and terrestrial flora and fauna. By way of example, among low islands of Tuvalu, vegetation, principally coconut palms, contains approximate ly 0.5 t/ha P, while people, pigs and minor fowl, account for about 2. 5-3 kg/ha P. The sea is the primary source and sink for phosphorus in an atoll ecosystem with the primary concentration taking place in the marine biosphere (1-2 kg/ha.m/a P) and subsequent transference to the land by physical geological processes (10(4) g/t/a P) and terrestrial predators (< 1 kg/ha/a P). The freshwater lens has a central role in r edistributing phosphate received from atmospheric fallout, corrosion o f bioclasts and breakdown of surface debris. This phosphorus is transp orted through the calcareous substrate as a humate complex that breaks down in or near the vadose zone. Excess free phosphate is precipitate d as dahllite, carbonate hydroxyapatite, removing it from further part icipation in the atoll's geochemical cycle. Phosphorus in the fresh gr oundwater is recycled in the terrestrial flora (approximately 0.1 t/ha /a P) or lost in edge discharge. The cycle of phosphorus offers a mode l for evaluating environmental impacts but data is presently insuffici ent to determine precise quantities being transferred within and betwe en all the component subsystems of the atoll. No satisfactory budget c an be constructed detailing net losses and net inputs between differen t subsystems and, hence, likely effects of present land management pra ctices, thriving populations, pollution and environmental changes. The atoll is a highly dynamic system able to recover from short term envi ronmental disruptions but its resilience in this regard needs to be ca refully monitored, quantified and nurtured.