Trace elements in marine mammal bone from the Río de la Plata and adjacent Atlantic Ocean: habitat, species and trophic effects

The Río de la Plata estuary is a heavily impacted coastal system that receives substantial fluvial and urban inputs. We quantified 12 trace elements in skull bone from seven marine mammal species stranded or incidentally taken as bycatch along the Uruguay coast between 1992 and 2015 (n = 73): Otaria flavescens, Arctocephalus australis, Pontoporia blainvillei, Phocoena spinipinnis, Delphinus delphis, Pseudorca crassidens and Lagenodelphis hosei. Concentrations of Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sr and Zn were determined by ICP-MS/ICP-OES and integrated with bone stable isotope ratios of carbon, nitrogen and oxygen to assess trophic and habitat influences. Mean concentrations decreased in the order Zn > Fe > Al > Sr > Mn > Cu > Pb > Ni > Cr > Hg > As > Cd. No sex-related differences were detected for any element. Nearshore species exhibited higher concentrations of both Fe, Al, Sr and Mn, particulate elements typically originated from the continental soil erosion, and Pb, Cr and Cd, elements typically associated to anthropogenic inputs. Conversely, oceanic species showed higher concentrations of Hg, Ni and Zn, suggesting exposure through pelagic food webs and cephalopod-rich diets. Isotope-element associations supported these patterns: Al, Fe and Pb were negatively correlated with δ¹⁸O, values indicating stronger freshwater influence; Cd and Ni were negatively correlated with δ¹⁵N values, suggesting biodilution; Zn correlated negatively with δ¹³C and δ¹⁵N but positively with δ¹⁸O values. Overall, the bone proved to be a conservative archive of trace element exposure. The integration of elemental and isotopic data provided valuable insight into habitat-specific contamination patterns. The study highlights the need for continued pollution control in the estuary and reinforces the value of marine mammals as bioindicators of trace element inputs.