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

Abstract

To examine N-isotope ratios (15N/14N) 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 δ18O values, with open ocean Escudo de Veraguas shells yielding the highest average δ18O (-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 δ15N and δ13C values. Bivalve δ15N and δ13C values likely represent a mixture of mangrove and seagrassNand C, although terrestrial sources cannot be ruled out. Despite hydrographic differences between end-members, we see minimal δ15N and δ13C 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 15N enrichments suggestive of anthropogenic nutrient input, but low δ15N values overall make this interpretation equivocal. Lastly, δ15N 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.