MEPS 180:131-138 (1999)  -  doi:10.3354/meps180131

ABSTRACT: 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 δ¹⁵N signature in tissue extracts (with zooxanthellae; n = 46). The response curve of the δ¹⁵N 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 δ¹⁵N were observed inshore (5.0 to 5.5o/oo), lowest values at the mid-shelf (~3.8o/oo), and high values again offshore (5.2o/oo). 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 δ¹⁵N 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.

KEY WORDS: δ¹⁵N · Corals · Upwelling · Nutrient sources · Trophic shifts