Influência da disponibilidade de luz na dinâmica do mercúrio em pequenos tributários do sistema fluvial do rio Jaú, Amazônia Central

Abstract

Amazortian soils have accumulated naturally relatively high concentrations of mercury (Hg), which is slowly exported to tlie fluvial system and may bioaccumulate in aquatic food chains. The solar radiation can reduce Hg+2 to Hg^, a highly volatile form of the element, and its formation favors the removal of Hg from the aquatic system. This mechanism is called photoreduction and can diminish Hg concentrations on the aquatic system. This work investigated the influence of light availability on the leveis of dissolved gaseous mercury(DGM) and total Hg in water and scdimcnts of 30 small black water tributaries of the Jaú River, central Amazônia. Light intcnsity was asscsscd by using the proportion of canopy opening(% CO) above the sampled tributary, in addition to the light intensity relativo to the hour of sampling. The effects of dissolved organic carbon (DOC)leveis and water pH were also evaluated. Multiple regressions were used for data analysis. Results showed that tributaries with a higher% CO had lower total Hg concentrations in water and sediments, suggesting the occurrence of photoreduction. There may exist a bottleneck effect on total Hg leveis in water that occurs when CO above tributaries ícaches 20 %.The light availability may bc cnough to reduce Hg to its volatile form. A relationship between%CO and DGM leveis was not observed because (i) highcst Hg"production in water only occurs at ccrtain pcriods ofthe day,when hght is less intenso,(ii) outflow in this system is quick and thus, avoids OGM accumulation on superficial waters, and (iii) there may be is a limitation of substrate available to photoreductif)n, as indicated by positively correlations between DGM with the possiblc water and sedimcnt substratcs. DOC and pH corrclatcd positively with Hg leveis in water. Results indicated that the common black water high DOC and low pH values may increase the residence time ofHg in the water column in detriment ofthe photoreduction and HgO outflow.