<p>The effects of several environmental variables on net nitrate uptake by the scleractinian coral Diploria strigosa were investigated under controlled flow conditions. D. strigosa exhibited nitrate uptake rates ranging from 1 to 5 nmol cm(-2)h(-1) at ambient concentrations of 0.1-0.3 mu M that are typical of oligotrophic reefs such as Bermuda. Net uptake ceased at approximately 0.045 mu M. The uptake was positively correlated with concentration up to a saturation concentration of approximately 3 mu M. The uptake was also positively correlated with water velocity at 1 mu M, but not at 6 mu M, suggesting diffusional limitation at low concentrations and kinetic limitation at higher concentrations. Nitrate uptake by D. strigosa was not affected by light intensity or time of day, but was almost completely inhibited by 48 h exposure to ammonium levels found on many reefs.</p>