Relationship between antropogenically caused turbidity and phytoplankton production in a clear Amazonian floodplain lake

The present study was performed in Lake Batata, a clear water Amazonian floodplain ecosystem, located on the right bank of the Trombetas river (Pará State, Brazil). For a period of 10 years (1979-1989), the lake received a large quantity of bauxite tailings from a nearby mine. These tailings originally covered about 30 % of the lake sediments, but resuspension and redeposition have resulted in an even wider distribution of the tailings. Phytoplankton production was measured at dìfferent sites in natural and impacted areas at different hydrological phases. Incubations were exposed at 5 light depths using (14)C as tracer. All vertical profiles of primary production exhibited ca. 13 % light inhibition in the surface. The maximum primary production rates (Pmax) were always found near the surface at approximately 30 cm. Production rates ranged from 9 to 664 mgC m^(-3) d^(-1), being the higher rates found during low water and lower rates during high water periods. Transparency measurements were good predictors of phytoplankton attributes. A strong predictive capacity of the turbidity effect was shown. An increase in turbidity of 3.6 times could reduce the carbon production in the lake by 51 %. In areal terms, it was estimated that Lake Batata produces ca. 122 g C m^(-2) per year. Our data reveal that although being poorer in nutrients, clear water systems are as productive as decanted white water systems. The results suggest that phytoplankton production is regulated by solar radiation and not nutrient. Despite the turbidity increase caused by resuspension of tailings, phytoplankton production remains relatively high and still plays a central role to the carbon flux in Lake Batata, especially during low water hydrological periods