The results of the present study demonstrate that ambient concentrations of dissolved nitrogen can supply the nitrogen requirements of the symbiotic coral Pocillopora damicornis. Of the two potential sources investigated, ammonium was the only resource that appeared to be able to completely satisfy the nitrogen demand of P. damicornis at ®eld concentrations, with the inward ¯ux of ammonium readily supplying the nitrogen
requirements of P. damicornis at ammonium concentrations as low as 100 nM. This is well within the bounds of the concentrations normally found in the water surrounding P. damicornis. The potential of DFAAs to supply signi®cant amounts of nitrogen, however, was an order of magnitude smaller. Even at the rare maximum ambient concentrations of DFAAs measured in the ®eld (100 nM), only 8.5 to 11.3% of the nitrogen requirements of P. damicornis and its symbiotic dino¯agellates could be supplied