Mercury and selenium biomagnification in a Brazilian coastal food web using nitrogen stable isotope analysis: A case study in an area under the influence of the Paraiba do Sul River plume

•Cetacean and voracious fish displayed similar δ15N values.•Cetacean displayed higher Hg and lower Se than voracious fish.•Hg showed an elevated biomagnification power over the entire tropical food web.•Se biomagnified in lower and intermediate trophic positions.•Five trophic positions were found in relation to primary consumer as baseline. Mercury (Hg), selenium (Se) and nitrogen (δ15N) stable isotope were assessed in a tropical food web of Rio de Janeiro’s north coast. Isotopic data on muscle suggest a difference related to this parameter along the food web; where top-predators (cetacean and voracious fish) displayed heavier δ15N over the entire food web. Both top-predators presented similar δ15N values. Cetacean displayed higher Hg and lower Se than voracious fish. Five trophic positions (TP) were found in relation to primary consumer as baseline, ranging from 2.0 to 4.0. Positive relationships were found between trace-element and δ15N. The slope of regression equations (0.11 for Se and 0.21 for Hg) and food web magnification factors (2.4 for Se and 5.4 for Hg) showed that Hg presented higher rate of increase over the food web. Simultaneous measurements of trace-elements and ecological tracers emphasize the importance of TP into the trophic structure and distribution of Hg and Se throughout the food web.