This study aimed to determine whether fish species occupy different mesohabitats, as defined by specific criteria of substrate types in Atlantic Forest blackwater streams. We sampled fourteen coastal blackwater rivers along the coast of São Paulo State, Brazil, during the low-flow season (June-September/2016). For each stream, we selected three mesohabitat types (sand, leaf-litter and trunks) in a 100 m river stretch. We sampled 41 mesohabitats, 31 of which contained fish, resulting in 15 species. When multivariate analysis of variance (MANOVA) was applied to the mesohabitat abiotic variables, no significant differences were indicated between them. ANOSIM for species density and biomass and Redundancy Analyses (RDAs) for species density, biomass and fish community indexes showed similar patterns of community structure among mesohabitats. Nevertheless, fish biomass, diversity and richness were associated with such unique environmental features as low shading and shallow habitats. The observed dispersion among mesohabitats suggests that these communities are subject to some level of disturbance, which may affect the value of patches as refugia. Thus, the sharing of these mesohabitat patches among species may represent an opportunistic strategy to maximize the use of available resources for these low density populations living in these unique environments.

Total mercury (Hg) levels in water, zooplankton and in the invading fish Platanichthys platana were studied in the Billings Complex, in order to understand the transfer of the total Hg to higher trophic levels. Samples were collected at four sites with different levels of eutrophication, at the dry and rainy seasons of 2014. A total of 93 specimens of P. platana were analyzed. During the rainy season the Grande River site was the only one that showed total Hg values above the legal standards (0,004 mg.L-1) in the water, while in the zooplankton, it was detectable only in the Central Body in the same season. Total Hg observed in fish varied between 0,04 e 0,29 μg.g-1, and highest values in fish were found at the Grande River site. The analysis of covariance (ANCOVA) showed an influence of the sites (p=0,000) and total length (p = 0.008) on Hg concentrations, suggesting an influence of the environmental conditions on Hg bioaccumulation in the fishes. The successful invasion of P. platana in the Billings Complex suggests that this species is acting as an additional element for the transfer of the total Hg to higher trophic levels.

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