Mercury distribution and exchanges between the Amazon River and connected floodplain lakes.

Mercury transport and distribution in river-floodplain systems remain poorly understood, especially in tropical regions. Prior research has shown that floodplains can influence mercury speciation and retention. However, no prior work had resolved how seasonal hydrological changes affect mercury partitioning in Amazonian lakes. This gap motivated a closer examination of how flood cycles interact with mercury dynamics. The Amazon River system is known for high mercury levels, but the mechanisms behind these concentrations are unclear. Local rainfall and sediment resuspension may play roles, but their exact contributions are uncertain. Floodplain lakes are suspected to act as mercury sinks, but evidence is limited. This study addresses these uncertainties by analyzing mercury in both dissolved and particulate forms. The findings may clarify how seasonal flooding affects mercury retention in tropical wetlands.

The study aimed to evaluate how mercury is distributed and exchanged between the Amazon River and its connected floodplain lakes. Researchers focused on the Curuai floodplain system to understand seasonal variations in mercury concentrations. They measured total filtered mercury and total particulate mercury in different water types. The goal was to determine how hydrological conditions influence mercury partitioning. They also sought to identify the geochemical factors controlling mercury retention. The study examined both white water and black water lakes for differences in mercury behavior. By comparing dry and flood seasons, the team aimed to assess mercury storage patterns. The results could help explain how floodplain systems influence mercury cycling in the Amazon.

Researchers collected water samples from Curuai floodplain lakes during different hydrological periods. They measured total filtered mercury and total particulate mercury in both white and black water lakes. The team used high-resolution spectrometry to quantify mercury concentrations. They also analyzed total suspended solids and redox conditions in the water. The study compared mercury partitioning in isolated and flooded lakes. Researchers calculated the mercury partition coefficient to assess transport mechanisms. They evaluated correlations between mercury and other elements like iron and manganese. The team estimated mercury mass budgets to determine net retention in the system.

Total filtered mercury concentrations in floodplain lakes ranged from 3 to 52 pmol L(-1). The highest values coincided with Amazon River concentrations during the flooding period. Total particulate mercury in lakes reached 47 to 478 pmol L(-1) during the dry season. Mercury partitioning was dominated by the particulate phase, with a Kd(Hg) between 4.77 and 5.83 L kg(-1). White water lakes showed higher TSS and oxidative conditions, linking T-PHg to organic matter. Black water lakes had reductive pH and low TSS, showing a positive P-iron and T-PHg relationship. In these lakes, redox conditions favored mercury desorption to the filtered phase. The study estimated a net particulate mercury storage of 150 kg PHg year(-1) in the floodplain system.

The study suggests that the Curuai floodplain system acts as a particulate mercury trap. Mercury retention is highest during the dry season when lakes are isolated. Seasonal flooding influences mercury partitioning between dissolved and particulate phases. White water lakes retain mercury through organic matter association during dry periods. Black water lakes show mercury desorption under reductive conditions. The mercury mass budget confirms significant annual storage in the floodplain. These findings align with the observed geochemical differences between white and black water lakes. The results support the idea that floodplain dynamics regulate mercury cycling in the Amazon.