SEASONAL NITROGEN-FIXATION IN THE SEDIMENT OF AN AMAZONIAN LAKE IMPACTED BY BAUXITE TAILINGS (BATATA LAKE PARA)

Batata Lake is an Amazonian clear water lake that undergoes large seas onal fluctuations in water level. For a period of 10 years (1979-1989) , the northern end of the lake received a total of 50.000 m(3)d(-1) of bauxite tailings. As a consequence spelling aproximately 30% of its s ediments are covered by tailings. The principal goal of this research was to estimate rates of nitrogen fixation in the impacted and non-imp acted sediment in the different hydroperiods that occur in this ecosys tem (drawdown, drying, filling and flooding). Nitrogen fixation was es timated using the acetylene reduction method. The highest rates of nit rogen fixation were observed to occur during the drying period and app ear to be directly related to an increase in primary production by phy toplankton. Decreased rates of nitrogen fixation occurred during the h ydroperiods of filling, flooding and drawdown with the greatest reduct ions occuring in the impacted area of the lake. In the impacted area o f the lake, bauxite tailings have reduced primary production in the wa ter column, decreased labile authoctonous carbon availability to herer otrophic bacteria in the sediments, and decreased nitrogen Fixing acti vity of organisms present the sediments.