Light-mediated ^sup 15^N fractionation in Caribbean gorgonian octocorals: implications for pollution monitoring

• Published in: Coral reefs 2011-09, Vol.30 (3), p.709
• Main Authors: Baker, D M, Kim, K, Andras, J P, Sparks, J P
• Format: Article
• Language: English
• Subjects: Anthropogenic factors; Coral reefs; Environmental monitoring; Fractionation; Isotopes; Marine ecosystems; Marine pollution; Nitrogen; Photosynthesis; Pollution monitoring; Pollution sources; Turbidity; Water pollution
• ISSN: 0722-4028; 1432-0975
• DOI: 10.1007/s00338-011-0759-x

The stable nitrogen isotope ratio (δ ^sup 15^N) of coral tissue is a useful recorder of anthropogenic pollution in tropical marine ecosystems. However, little is known of the natural environmentally induced fractionations that affect our interpretation of coral δ ^sup 15^N values. In symbiotic scleractinians, light affects metabolic fractionation of N during photosynthesis, which may confound the identification of N pollution between sites of varied depth or turbidity. Given the superiority of octocorals for δ ^sup 15^N studies, our goal was to quantify the effect of light on gorgonian δ ^sup 15^N in the context of monitoring N pollution sources. Using field collections, we show that δ ^sup 15^N declined by 1.4[per thousand] over 20 m depth in two species of gorgonians, the common sea fan, Gorgonia ventalina, and the slimy sea plume, Pseudopterogorgia americana. An 8-week laboratory experiment with P. americana showed that light, not temperature causes this variation, whereby the lowest fractionation of the N source was observed in the highest light treatment. Finally, we used a yearlong reciprocal depth transplant experiment to quantify the time frame over which δ ^sup 15^N changes in G. ventalina as a function of light regime. Over the year, δ ^sup 15^N was unchanged and increased slightly in the deep control colonies and shallow colonies transplanted to the deep site, respectively. Within 6 months, colonies transplanted from deep to shallow became enriched by 0.8[per thousand], mirroring the enrichment observed in the shallow controls, which was likely due to the combined effect of an increase in the source δ ^sup 15^N and reduced fractionation. We conclude that light affects gorgonian δ ^sup 15^N fractionation and should be considered in sampling designs for N pollution monitoring. However, these fractionations are small relative to differences observed between natural and anthropogenic N sources.[PUBLICATION ABSTRACT]