Fósforo no rio Madeira (Amazônia Ocidental): formas, concentrações, transporte e projeções de futuras alterações ambientais

Abstract

Amazonian rivers originating in the Andes (White-water rivers) transport large amounts of phosphorus-rich sediments. Originating in the Bolivian and Peruvian Andes, the Madeira River is the largest tributary of the Solimões/Amazon River. The transport and deposition of nutrients, such as phosphorus (P), associated with Andean-derived sediments are of crucial importance to support high productivity in the Madeira River wetlands, which span over 210,000 km2. Despite the great ecological significance of the Madeira River, there is a lack of studies showing its biogeochemical characteristics, especially with respect to P. The aim of this study was to quantify the concentrations and transport of different P forms by the Madeira River, and how they vary at seasonal and interannual scales. In the study reach, it was estimated a transport of 182 Gg P yr-1, which is equivalent to 65% of the amount that the Amazon River delivers to the sea. This suggests the Madeira River plays a key role in supplying P to the Amazon floodplain. The Andean precipitation strongly explained P concentrations in the Madeira River, suggesting that the increase in precipitation projected to the eastern Andes slopes tend to enhance the Madeira River’s P transport. On the other hand, as P is mainly particulate and exhibits a high tendency to sink under low turbulence, the proliferation of hydroelectric reservoirs throughout the basin tends to trap P upstream and, consequently, reduce the downstream transport. Overall, the Madeira River may transport 26% less P in the future. As downstream floodplain forests and lakes highly depend on the input of Andean-derived P, there will be a likely reduction in primary production and, ultimately, in the Amazon lowland’s carbon sequestration.