Phosphorus availability in the Parana floodplain lakes (Argentina): Influence of pH and phosphate buffering by fluvial sediments

The phosphate-buffering properties and P chemistry of suspended particulate matter (SPM) collected in three geologically contrasting rivers (Bermejo, Paraguay, and Upper Parana) are compared in order to explain the abundance of dissolved PO4-P in the Parana floodplain lakes. The rivers show distinct ive P-buffering capacities related to the chemical composition of their SPM . At natural pH, the linear adsorption coefficient of PO4-P (K-d) ranges fr om 0.25 liter g(-1) for the Paraguay River to 1.38 liter g(-1) for the Uppe r Parana River. Equilibrium PO4-P ranges from 0.17 mu mol liter(-1) for the Upper Parana River to 2.92 mu mol liter(-1) for the Bermejo River. Short-t erm (60 h) desorbable PO4-P ranges from 0.23 mu mol g(-1) for the Paraguay to 0.77 mu mol g(-1) for the Upper Parana. These values increase substantia lly with a small decrease in pH representative of conditions experienced by riverine SPM when it enters the floodplain environment. For the carbonate- bearing SPM of the Bermejo and Paraguay Rivers, a pH decrease of 1-1.5 unit s causes a 10-fold increase in equilibrium PO4-P and a 5- to 10-fold increa se in desorbable PO4-P, and it is associated with the release of Ca. In the Upper Parana, a similar decrease in pH has opposite effects that are consi stent with a stronger complexation of PO4-P by hydrous iron oxides at low p H. Compared to the parent riverine SPM, floodplain lake sediments are deple ted in Ca-bound P and enriched in Fe-bound P. Although Ca-bound P is often considered biologically unavailable, our observations suggest that in the P arana floodplain lakes, most of the available P originates from the acid di ssolution of Ca-bound P. Loading of fluvial sediments to the lakes and the acidic dissolution of Ca-bound P appear sufficient to explain the permanent excess of dissolved PO4-P in these waters.