Mercury and selenium in seston, marine plankton and fish (Sardinella brasiliensis) as a tool for understanding a tropical food web

• Hg and Se distributions were uniform in the sampling portion of the bay.
• Hg is not biomagnifying in the base of the Ilha Grande bay food web.
• Hg and Se increased with increases in the size class of the analyzed plankton.
• Amplification of Hg and Se concentrations from plankton to planktivorous fish
• Hg and Se can be used as tool for understanding a tropical food web.

Mercury (Hg) and selenium (Se) concentrations were evaluated in a planktivorous fish and four size classes of organisms (FSCO), collected at an oligotrophic bay in the Southeastern Brazilian coast. No significant spatial differences between Hg and Se were found in the FSCO within the five sampling points in the bay. Hg and Se concentrations increased with successive increases in the size class of the analyzed plankton, i.e. approximately 3-and 2-fold, respectively, from microplankton to macroplankton. Hg and Se biomagnified throughout the planktonic food web. The smallest size class of organism, seston, composed of both biotic and abiotic portions, and fish showed the highest Hg concentrations. This indicates that Hg is not biomagnifying in the base of the bay food web. Selenium concentrations in fish were approximately 5.9 times higher than those in seston. Hg and Se concentrations in fish were approximately 3.5 and 14.6 times higher than those found in the plankton, respectively.