Phosphate and nitrogen transformations in a large floodplain river.

MUGNI, H.; MAINE, A.; BONETTO, C.

Sevilla, 9-12 September España

Simposio; 4th Symposium on Phosphate in Sediments; 2003

With an area of 3 x106 km2 and a mean discharge of 18,000 m3 s-1 at the mouth, the Paraná River is the second largest hydrographic system in South América, after the Amazon River. The Upper Paraná River joints its main affluent, the Paraguay River, to form the Middle Parana stretch. The Paraguay and the Middle Paraná Rivers are fringed by a 10-50 km wide floodplain. The Bermejo River, the main Paraguay River affluent, drains the Andes Mountains and the Puna Highlands. It has an order of magnitude lower discharge, and an order of magnitude larger suspended matter and total phosphate concentrations than the two others. The Upper Paraná River drains a watershed dominated by lateritic soils developed over old basalts. The contrasting effect of agriculturally induced erosion and sediment settlement within man-made lakes results in low suspended matter, total and dissolved reactive phosphorus concentrations. The Paraguay River, draining the huge Pantanal wetland, has intermediate P and lowest inorganic N concentrations. In the lower Paraná stretch, the floodplain is mainly occupied by marshes covered by emergent macrophytes. Suspended matter, oxygen and nitrate concentrations are depleted in the marsh water. On the contrary, dissolved reactive phosphorus increased with respect to river water. Dissolved reactive phosphorus is released from the river-borne sediments after settlement in the acid and anoxic marsh bottom. Denitrification makes an important contribution to the nitrate depletion within the marshes. Extensive exchange of water between the river and the marshes determine a downstream decrease of suspended matter, total phosphate and nitrate and an increase dissolved reactive phosphorus concentrations along the last 500 km of the Lower Paraná River.