Limnol. Oceanogr., 44(6), 1999, 1540–1548

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 PO4-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 PO4-P (Kd) ranges from 0.25 liter g21 for the Paraguay River to 1.38 liter g21 for the Upper Paraná River. Equilibrium PO4-P ranges from 0.17 mmol liter21 for the Upper Paraná River to 2.92 mmol liter21 for the Bermejo River. Short-term (60 h) desorbable PO4-P ranges from 0.23 mmol g21 for the Paraguay to 0.77 mmol g21 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 PO4-P and a 5to 10-fold increase in desorbable PO4-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 PO4-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 PO4-P in these waters. Phosphate exchange reactions taking place between the liquid and solid phases influence the concentration of dissolved phosphate in waters having a high SPM concentration (e.g., Mayer and Gloss 1980). These reactions may involve adsorption, desorption, precipitation, and dissolution, and they are often collectively referred to as phosphate ‘‘buffering’’ (Froelich 1988). They are influenced by pH, dissolved phosphate concentration (hereafter expressed as PO4P), competing anions, and solid-phase composition. Phosphate buffering can play an important role in the P economy of aquatic systems. Algal uptake of PO4-P can be balanced by P desorption from bottom and suspended sediments, thus enabling the sustained productivity of streams (Ellis and Stanford 1988) and floodplain lakes (Engle and Sarnelle 1990). Phosphate released from fluvial SPM is an important source of P to estuaries (e.g., Chase and Sayles 1980; Fox et al. 1986). In most freshwaters, dissolved phosphate concentrations are usually undetectable or low (,0.1 mmol liter21) and limit biological productivity. This generality does not always apply to floodplain lakes, however, where rivers can supply large amounts of P during floods. In the floodplain lakes of