Cross-continental shelf trends in coral δ15N on the Great Barrier Reef:: further consideration of the reef nutrient paradox

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 1-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 delta(15)N signature in tissue extracts (with zooxanthellae; n = 46). The response curve of the delta(15)N was found to be curvilinear, yielding a highly significant parabolic relationship with distance from shore (p < 0.001, second-order least-squares polynomial regression). Highest values of delta(15)N were observed inshore (5.0 to 5.5 parts per thousand), lowest values at the mid-shelf (similar to 3.8 parts per thousand), and high values again offshore (5.2 parts per thousand). 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 delta(15)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.