Cycling of phosphorus maintains the production of microphytobenthic communities in carbonate sediments of a coral reef

The phosphorus (P) cycle that maintains the production of microphytobenthic communities in carbonate sediments was investigated in the fringing Shiraho coral reef system located off the east coast of Ishigaki Island, Japan. Production rates of the diatom-dominated microphytobenthic communities and the P flux at the sediment-water interface were determined by in situ benthic enclosure experiments. The daily net production (6.9-24 mmol C mm(-2) d(-1)) by microphytobenthic communities in carbonate sediments accounted for 14-27% of organic carbon excess production by the bulk reef communities. Despite the observed active photosynthesis, the exchange of P across the sediment-water interface was negligible. This implies that P in seawater is a minor source for the production of microphytobenthic communities. In situ bottle incubation experiments also demonstrated that P in seawater is a minor source and that P adsorbed on sediment particles is the significant source for production. Laboratory experiments led to an estimated concentration of adsorbed P in the carbonate sediments of similar to0.39 mumol P g(-1) dry wt, which corresponds to 5.5% of total P This source can be rapidly released into seawater as dissolved reactive P, the most available P form. Thus, high primary productivity rates of the microphytobenthic communities is largely maintained by this sedimentary pool of P. Furthermore, the communities play an important role in retaining P in the carbonate sediments through uptake at the sediment-water interface before it can diffuse away.