NITROGEN AND CARBON ISOTOPIC SYSTEMATICS OF THE FLORIDA REEF TRACT

The nitrogen isotopic composition of a variety of organisms in coral reefs has been used extensively, not only to study nitrogen dynamics, but also as a tracer for the input of anthropogenic nitrogen. However, the interplay between isotopic fractionation associated with the internal nitrogen cycling and variations in the absolute values supplied by external source signatures is poorly understood. Here we report the delta N-15 and delta C-13 of algae, sponges, and fishes, the delta N-15 and delta O-18 of dissolved NO3-, and the delta N-15 of dissolved organic nitrogen in samples collected from the Florida reef tract over a 2.5-yr period (2003-2005). Our data are synthesized with results from previous studies of the delta N-15 of particulate organic material and coral tissue and zooxanthellae from the same area to provide a more detailed understanding of factors controlling coral reef delta N-15 and variation among biogenic components. These data show that during the study period there were (1) no clear spatial patterns in the delta N-15 of biogenic components related to proximity to the Florida Reef tract, and (2) no temporal patterns related to the wet or dry seasons. The range of delta N-15 and delta C-13 in the particulate organic material could be best explained as a mixture of material derived from seagrass, algae, mangroves, and fishes. The delta N-15 and delta O-18 of NO3- support a model in which variations in nitrogen isotopic composition are derived mainly from the isotopic effects associated with the nitrification of NH4+ to NO3- and subsequent assimilation by primary producers rather than through the input of isotopically distinct NO3- from external sources.