Phosphorus availability in the Paraná floodplain lakes (Argentina): Influence of pH andphosphate 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 Paraná) are compared in order to explain the abundance of dissolved PO 4 −P in the Paraná floodplain lakes. The rivers show distinctive P‐buffering capacities related to the chemical composition of their SPM. At natural pH, the linear adsorption coefficient of PO 4 −P ( K d ) ranges from 0.25 liter g −1 for the Paraguay River to 1.38 liter g −1 for the Upper Paraná River. Equilibrium PO 4 −P ranges from 0.17 μmol liter −1 for the Upper Paraná River to 2.92 μmol liter −1 for the Bermejo River. Short‐term (60 h) desorbable PO 4 −P ranges from 0.23 μmol g −1 for the Paraguay to 0.77 μmol g −1 for the Upper Paraná. These values increase substantially 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 units causes a 10‐fold increase in equilibrium PO 4 −P and a 5− to 10−fold increase in desorbable PO 4 −P, and it is associated with the release of Ca. In the Upper Paraná, a similar decrease in pH has opposite effects that are consistent with a stronger complexation of PO 4 −P by hydrous iron oxides at low pH. Compared to the parent riverine SPM, floodplain lake sediments are depleted 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 Paraná floodplain lakes, most of the available P originates from the acid dissolution 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 PO 4 −P in these waters.