Phosphorus transport by the largest Amazon tributary (Madeira River, Brazil) and its sensitivity to precipitation and damming

Originating in the Bolivian and Peruvian Andes, the Madeira River is the largest tributary of the Amazon River in terms of discharge. Andean rivers transport large quantities of nutrient-rich suspended sediments and are the main source of phosphorus (P) to the Amazon basin. Here, we show the seasonal variability in concentrations and loads of different P forms (total, particulate, dissolved, and soluble reactive P) in the Madeira River through 8 field campaigns between 2009 and 2011. At our sampling reach in Porto Velho, Brazil, the Madeira River transports ~177–247 Gg yr−1 of P, mostly linked to particles (~85%). Concentrations and loads of all P forms have a maximum at rising waters and a minimum at low waters. Total P concentrations were substantially higher at a given discharge at rising water than at a similar discharge at falling water. The peak of P concentrations matched the peak of rainfall in the upper basin, suggesting an influence of precipitation-driven erosion. Projected precipitation increase in the eastern slopes of the Andes could enhance sediment yield and hence the P transport in the Madeira River. Because most of the P is particulate, however, we hypothesize that the planned proliferation of hydropower dams in the Madeira basin has the potential to reduce P loads substantially, possibly counteracting any precipitation-related increases. In the long term, this could be detrimental to highly productive downstream floodplain forests that are seasonally fertilized with P-rich deposits.