In this study, we investigate potential sources of nitrogen for the scleractinian coral Porites lobata in a transect across the central region of the Great Barrier Reef, Australia. The experiment followed a l-way, Model I, nested ANOVA design. We sampled colonies of P. lobata from 12 reefs spanning the 110 km wide continental shelf at 5 m depth, and determined the 615N signature in tissue extracts (with zooxanthellae; n = 46). The response curve of the 6 "~ was found to be curvihnear, yielding a highly significant parabolic relationshp w~t h distance from shore (p < 0.001, second-order leastsquares polynonlial regression). Highest values of 615N were observed inshore (5.0 to 5.5%), lowest values at the mid-shelf (-3.8%0), and high values again offshore (5.2%0). We suggest the following causal factors, based on environmental characteristics and phenomena known to occur in this region:(1) inshore corals may be receiving much of their nitrogen from terrigenous sources; (2) mid-shelf corals may be receiving at least some of their nitrogen from associated algal mats known to possess high rates of nitrogen-fixation in this region, which in turn could lower 6"N values; and (3) offshore corals may be receiving their nitrogen from seasonal, nutrient-rich, cold-water intrusions or upwellings, documented to occur in this area.

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