Phosphorus metabolism in coral reef communities: exchange between the water column and bottom biotopes

Exchange of phosphate between components of the reef bottom and the water column were studied on reefs around Heron Island (Great Barrier Reef), both in aquaria and in in situ enclosures, using radioactive phosphorus (³²P) as a tracer. Living corals, dead corals, coral rubble overgrown with periphyton, and soft sediments of coral sand were used in experiments. In all of these components of bottom reef biotopes, two opposite flows of inorganic phosphate were recorded and measured, i.e. the rate of PO₄-P uptake from water (A_c), and its release (A_e). At ambient PO₄-P concentrations in water of 0.1– 0.3 µmoll^–1, both flows varied in living corals and coral rubble between 10 and 70 µg P kg^–1 h^–1, 3–10 mg P m^–2 day^–1, and in coral sand between 10 and 30 µg P kg^–1 h^–1, or 2–7 mg P m^–2 day^–1. Under the latter concentration range (which is typical for coral reef areas), the reciprocal PO₄-P flows almost balanced each other, so that net uptake (A_t) was very low. Often it approached zero or was positive, showing that a net PO₄-P release had taken place. The uptake flow (A_c) in living coral was much more dependent on the PO₄-P content in overlying water than was the release flow (A_e). The influence of conditions of illumination upon the values of A_c and A_e was comparatively low. The data obtained are used to discuss problems of phosphorus balance and dynamics in coral reef ecosystems.