ISOTOPIC AND ELEMENTAL VARIATIONS OF CARBON AND NITROGEN IN A MANGROVE ESTUARY

Variations in elemental and isotopic ratios of suspended particulate m atter (SPM) were investigated in the Guayas River Estuary Ecosystem (G REE) that empties into the Gulf of Guayaquil, Ecuador. Detritus in the system was identified on the basis of extremely high carbon:chlorophy ll a ratios (>1000). This material had mean delta(13)C of -26.4 +/- 0. 3, delta(15)N of +4.8 +/- 0.2, and (C:N)(atomic) of 14.1 +/- 0.9. The isotopic data were comparable to measurements reported for fresh and d egrading mangrove leaves, whereas the elemental ratio was comparativel y enriched in nitrogen. Isotope measurements of SPM throughout the GRE E were more similar to values for riverine material and detritus compa red with that for the coastal end-member. Values indicative of in situ produced algae, sewage and shrimp pond effluent were only found at se lected sites. Bacterial bioassays, which were used to document potenti al sources of dissolved organic matter in the GREE, were isotopically similar to SPM. This correspondence coupled with the relatively low (C :N), of SPM could be explained by bacterial immobolization of nitrogen onto detritus. Finally, tidal variations of (C:N), and delta(13)C at a brackish mangrove site were similar in magnitude to spatial variatio ns encountered throughout the GREE. Based on these results, the author s caution that care must be taken when samples are taken for food-web studies in these systems. (C) 1996 Academic Press Limited