Effects of seasonal contaminant remobilization on the community trophic dynamics in a Brazilian tropical estuary.

In tropical estuaries, wet seasons are responsible for the downstream transport of allochthonous material from the upper basin and flooded plains. Although allochthonous matter is commonly associated to nutrient and detritus input, pollutants are also transported throughout the basin or suspended from the river bottom via strong streamflow remobilization and rainfall dynamics. We assessed community and population trophic niche-based patterns using organisms' stable isotopes signatures in the wet and the dry seasons to test if estuarine trophic diversity is affected by remobilization of metal-contaminated material from a mining dam collapse that occurred in the Doce river basin, Brazil. Trophic depletion was detected community-wide and in a key consumer group (bottom-dwelling fishes) at the end of the wet season in the impacted Doce river estuary (DRE). Conversely, higher trophic diversity values were recorded in a well-preserved estuary used as control site. Stable isotopes mixing models indicated in the DRE that G. genidens, a predator fish species, presented poor-quality diet based on pollutant-tolerant tiny organisms, a finding that strongly contrasts from diet described in other, little-impacted Brazilian estuaries. Although wet seasons are expected to increase trophic, functional and taxonomic diversity in tropical estuaries, in the DRE the rainfall-driven dynamics poses a threat to the community due to the presence of ore tailings.