Stable isotopes in bivalves as indicators of nutrient source in coastal waters in the Bocas del Toro Archipelago, Panama

To examine N-isotope ratios ( 15 N/ 14 N) in tissues and shell organic matrix of bivalves as a proxy for natural and anthropogenic nutrient fluxes in coastal environments, Pinctada imbricata , Isognomon alatus , and Brachidontes exustus bivalves were live-collected and analyzed from eight sites in Bocas del Toro, Panama. Sites represent a variety of coastal environments, including more urbanized, uninhabited, riverine, and oceanic sites. Growth under differing environmental conditions is confirmed by δ 18 O values, with open ocean Escudo de Veraguas shells yielding the highest average δ 18 O (−1.0‰) value and freshwater endmember Rio Guarumo the lowest (−1.7‰). At all sites there is no single dominant source of organic matter contributing to bivalve δ 15 N and δ 13 C values. Bivalve δ 15 N and δ 13 C values likely represent a mixture of mangrove and seagrass N and C, although terrestrial sources cannot be ruled out. Despite hydrographic differences between end-members, we see minimal δ 15 N and δ 13 C difference between bivalves from the river-influenced Rio Guarumo site and those from the oceanic Escudo de Veraguas site, with no evidence for N from open-ocean phytoplankton in the latter. Populated sites yield relative 15 N enrichments suggestive of anthropogenic nutrient input, but low δ 15 N values overall make this interpretation equivocal. Lastly, δ 15 N values of tissue and shell organic matrix correlate significantly for pterioideans P. imbricata and I. alatus . Thus for these species, N isotope studies of historical and fossil shells should provide records of ecology of past environments.