MEPS 296:155-163 (2005)  -  doi:10.3354/meps296155

Reconstruction of nitrogen sources on coral reefs: δ15N and δ13C in gorgonians from Florida Reef Tract

Christine A. Ward-Paige1,2,*, Michael J. Risk1, Owen A. Sherwood2

1School of Geography and Geology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
2Department of Earth Sciences, Room 3006, Life Sciences Centre, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
*Present address: Dalhousie University. Email:

ABSTRACT: The gorgonians Plexaura spp. occur throughout the Florida Reef Tract, and lay down annual bands of a tough protein, gorgonin, in their skeletons. We analyzed stable isotopes of nitrogen (δ15N) and carbon (δ13C) from individual annual bands in skeletons of Plexaura homomalla and P. flexuosa from 10 locations on the Florida Reef Tract, producing a proxy record of nutrient inputs dating from the mid-1970s to 2002. Isotope data were compared with tissue from offshore Belize reefs, and inshore reefs from Xel-Ha, Mexico (Yucatan Peninsula), collected in 2003. Based on previous nutrient work, Florida sites were classified as ‘clean’ (relatively low nutrient levels) and ‘dirty’ (elevated nutrient levels). Samples from clean and dirty sites had significantly different nitrogen isotope values, which were correlated with the average total nitrogen of the seawater. Dirty sites (3.9 ± 0.2‰ δ15N) were isotopically enriched over clean sites (2.9 ± 0.1‰) throughout the entire duration of the study. Dirty sites had an enrichment of 0.8‰ δ15N throughout the study period, with the greatest increase in the 10 yr from 1993 to 2002 inclusive. Clean sites showed no significant change since 1974. Levels and trends in δ13C showed a similar picture of high and increasing eutrophication stress. Clean sites had lower δ13C values, consistent with predominantly autotrophic nutrition; dirty sites had higher values, suggesting a shift to a more heterotrophic mode of feeding. The methods outlined herein suggest that long-term records of organic pollution on coral reefs may be retrieved from gorgonian skeletons.

KEY WORDS: Florida Keys · Belize · Plexaura spp. · Gorgonian · δ15N · δ13C · Sewage · Water quality

Full text in pdf format