RESUMO
In this study, nitrogen stable isotope (δ(15) N) and total mercury (THg) analyses were conducted on algae, submersed and emergent macrophytes, shrubs and trees, Macrobrachium sp. and Paracheirodon axelrodi collected in three streams that drain a large interfluvial region in the middle Rio Negro, Amazonas State, Brazil. Samples were collected during different hydrological periods over 12 months in lower stream reaches and their headwaters; the latter being characterized by shallow, open-canopy swamps. Additionally, δ(15) N values and mercury concentrations of Paracheirodon simulans and Cichla spp. from the middle Rio Negro were analysed to demonstrate THg biomagnification in the food web. The highest mercury levels of P. axelrodi were found in small individuals, which were collected principally in the low water period. The log10 THg-δ(15) N relationship of vascular plants and algae, Macrobrachium sp., Paracheirodon spp. and Cichla spp. showed significant mercury biomagnification among trophic levels, with regression slopes of 0·15 and 0·25 for the entire food web and heterotrophs-only food web, respectively. The mean ± s.d. THg concentrations for Macrobrachium sp., P. axelrodi, P. simulans and Cichla spp. were 63·6 ± 23·7, 104·5 ± 40·0, 112·3 ± 31·4 and 418·5 ± 188·1 ng g(-1) wet mass, respectively. Elevated levels of mercury found in Paracheirodon spp. and top predators such as Cichla spp. in a remote area far from anthropogenic inputs provide evidence that high mercury concentrations occur naturally in Rio Negro aquatic food webs.
