Nitrogen budgets were constructed to compare the relative importance of ammonium- and DFAA-uptake systems in providing nitrogen for Pocillopora damicornis (Table 2). The nitrogen demand of the brown morph of P. damicornis was 23.80 and 21.09 nmol N cm)2 d)1 in the control and N microatolls at One Tree Island, respectively. The rate at which N atoms could be supplied by the inward transport of amino acids was 2.7 and 1.8 nmol N cm)2 d)1 (control and N corals respectively, Table 2) at the highest ambient concentration of DFAAs (100 nM). This represented 11.3 and 8.5% of the total nitrogen demand of P. damicornis in control and N microatolls. The concentration of DFAAs that would be able to satisfy the nitrogen demand of P. damicornis ranged between 0.89 and 1.18 lM (Table 2). The concentration at which the uptake of ammonium would satisfy the nitrogen needs of brown P. damicornis was 0.13 and 0.11 lM for control and N corals, respectively (Table 2). P. damicornis was '7 to 10 times more eective at acquiring N via inward transport of ammonium ions as opposed to DFAAs