Effects of land-use change on solute fluxes to floodplain lakes of the central Amazon

A time-series analysis of airborne photographs and Landsat thematic mapper (TM and ETM+) images and hydrochemical data were used to examine the effects of land-use change from 1930 to 2001 on solute inputs to Lake Calado, a floodplain lake in the central Amazon. Deforestation from slash-and-burn agricultural activities has dramatically decreased the amount of primary growth upland and flooded forests in the basin. The increasing area that is converted to agricultural plots and pasture in the Lake Calado basin has increased solute loading to the lake from upland tributaries (storm and base flow), bank seepage and overland flow, and decreased throughfall inputs. Whereas solute concentrations in stream water were generally higher in 1992 than 1930, Na+ and Cl- concentrations were also considerably higher in 2001 than 1992, likely because of an increase in the number of humans and cattle in the watershed. Estimates of solute inputs to Lake Calado via throughfall indicate that the mass transfer of some major solutes in the throughfall of undisturbed flooded forests can be larger than that from a combination of all other sources in areas that do not have a strong influence from the Solimoes River. Chemical gains in rain as it passed through the forest canopy occurred for most major ions and relatively large gains were observed for PO43- and Ca2+. Although often neglected in studies of tropical forest ecosystems, throughfall can be an important source of solutes to relatively undisturbed lake environments in the central Amazon.

Keywords: Amazon, floodplain lakes, land use, rain, solutes, throughfall, southwestern brazilian amazon, boundary-layer experiment, rain-forest, catchment, major ionic solutes, river floodplain, negro river, , deposition, throughfall, water, basin