Phosphorus metabolism in Coral Reef Communities: Dynamics in the Water Column

Abstract

Phosphorus dynamics were studied in the coral reef waters of Heron and Wistari Reefs in the Capricornia group, Great Barrier Reef, Australia, using radioisotopic techniques. All studies were made during August-October 1986. Labelled phosphate (PO₄ -³³P) was used to measure total uptake as well as uptake by bacterioplankton alone. Rates of total PO4-P uptake by microplankton varied between 20 and 120 ng P L^-1 h^-1 at water temperatures of 20-24º C. The daily (24 h) uptake rate of PO₄-P was 0.5-3.0 µg L^-1. The ambient PO₄-P content of water over the reefs was low (0.03- 0.3 µmol L^-1, or 1-10 µg P L^-1). Turnover time of PO₄-P in the water column of these reef areas is therefore very short, being in the range of 2-8 days, even when its possible net consumption by bottom biota is ignored. Bacterioplankton, not phytoplankton, was found to be the major consumer of inorganic phosphorus in these waters. The percentage of total PO₄-P uptake by bacteria varied from 50 to 90%. The results of these estimations are discussed in terms of the importance of active phosphorus dynamics in the water column to the nutrient budget of the coral reef ecosystem.