Seasonal nitrogen fixation in the sediment of an Amazonian lake impacted by bauxite tailings (Batata Lake-Pará)

Batata Lake is an Amazonian clear water lake that undergoes large seasonal fluctuations in water level. For a period of 10 years (1979-1989), the northern end of the lake received a total of 50,000 m³d^1 of bauxite tailings. As a consequence spelling aproximately 30 % of its sediments are covered by tailings. The principal goal of this research was to estimate rates of nitrogen fixation in the impacted and non-impacted sediment in the different hydroperiods that occur in this ecosystem (drawdown, drying, fìlling and flooding). Nitrogen fìxation was estimated using the acetylene reduction method. The highest rates of nitrogen fixation were observed to occur during the drying period and appear to be directly related to an increase in primary production by phytoplankton. Decreased rates of nitrogen fixation occurred during the hydroperiods of filling, flooding and drawdown with the greatest reductions occuring in the impacted area of the lake. In the impacted area of the lake, bauxite tailings have reduced primary production in the water column, decreased labile authoctonous carbon availability to heterotrophic bacteria in the sediments, and decreased nitrogen fixing activity of organisms present the sediments