The processes in tropical floodplain lakes enable maintaining phytoplankton nutrient requirements over a hydrological year. The nutrients such as nitrogen, phosphorus and carbon compounds play an essential role in phytoplankton growth. However, the way that nutrients and phytoplankton interact and how this relationship varies seasonally in tropical freshwater ecosystems is not clear. In this study, we evaluate the relationship between phytoplankton–nutrients over the hydrological cycle in Amazonian floodplain lakes and verify if this relationship influences the biomass of cyanobacteria. We also check what factors linked to nutrients act in structuring phytoplankton community. Using the phytoplankton functional approach, we verified how their ability to respond to hydrological and environmental variations reflects the ecological conditions and investigated how these interactions work. The results show that the Amazonian floodplain lakes could maintain long-term nutrient enrichment status. The nutrients input conduces to cyanobacteria dominance, that allied to other factors, play an essential role in supporting the stability of the phytoplankton–nutrients relationship over the hydrological cycle.

This study evaluates the use of mercury (Hg) concentrations in fish muscle tissue to determine a species’ trophic position (TP) in its environment. A campaign conducted in 2019 along 375 km in the middle Araguaia River basin, Brazil, resulted in 239 organisms from 20 species collected. The highest total mercury (THg) concentrations were found in Pellonacastelnaeana (6.93 µg·g⁻¹, wet weight) and in Triportheus elongatus (3.18 µg·g⁻¹, wet weight), whose TPs were different according to the FishBase database. However, they occupied the same trophic level in this study. The intra-specific comparison showed a difference in Hg concentrations between individuals captured in distinct sites. The study of the biota–sediment accumulation factor (BSAF) showed that spatiality interferes with a species’ TP. Statistical analyses revealed that when we used a predicted species’ TP based on each individual’s size, it explained 72% of the variability in THg concentration across all fish species. Multiple regression analysis confirmed that standard length and FishBase values are positively associated with THg (R² = 0.943). These results point to Hg as a viable indicator of a fish species’ TP since it reflects regional, biological, and environmental factors, as demonstrated here for the middle Araguaia River.

In order to assess the influencing factors of the presence of mercury in a river within the Savanna biome (Cerrado), we surveyed total mercury (THg) in bottom sediment from 50 lakes along 750 km of the Middle Araguaia floodplain. The sampling sites included non-urban and urban surroundings over three distinct geomorphologies. We measured water physicochemical parameters at each site and tested statistically if land use nested within the geological formation influenced the THg concentration in bottom sediments and related water parameters. Multivariate results indicate that the interaction between geological groups and land use is statistically significant (p < 0.05). Nested ANOVA and Tukey HSD tests confirmed that the geological formation with its nested land use influences the THg, pH, DO, conductivity, and TDS (p < 0.05). THg was significantly lower in Quaternary terrains (p < 0.05) and differed significantly between non-urban and urban areas in Neoproterozoic terrains (p = 0.02). The spatial projections of the THg eigenvector on the main axes with the scoring factors of the Neoproterozoic/Paleoproterozoic terrains, and urban/non-urban, confirmed the spatial correlations. These results indicate that the association of land use and geology could be the main driver of THg in the bottom sediments of lakes from the Middle Araguaia floodplain.