Potential of 14N isotope enrichment to resolve ambiguities in coastal trophic relationships

Abstract

Isotopic tracer additions were used in a field setting to differentially label primary producers within seagrass communities and trace the flow of nitrogen to consumers. Tracer experiments were coupled with fertilisation treatments, as the tracer was added to the system via a 15N-depleted (i.e. 14N-enriched) fertiliser. 15N-depleted fertiliser was added to the sediments of Halodule wrightii beds to target the response of the seagrass alone, while addition to the water-column targeted the response of the seagrass/epiphyte complex. After 21 d of exposure to either fertilisation strategy, epiphytes and seagrass tissues in H. wrightii beds were strongly labeled with the tracer, but the mean δ15N value of epiphytes (-78‰) was only significantly lower than that of seagrass (-31‰) in beds exposed to water-column fertilisation. Isotopic label was also detected in individuals of Tozeuma carolinense, Farfantepenaeus aztecus, and Palaemonetes pugio. The ability to both generate differential labeling in H. wrightii and its epiphytes and trace that label to consumers suggests that water-column fertilisation experiments incorporating 15N tracers may be an excellent tool for resolving trophic relationships within aquatic communities and determining how they respond to eutrophication.