Phosphorus availability in the Paraná floodplain lakes (Argentina): Influence of pH and phosphate buffering by fluvial sediments
Limnol. Oceanogr., 44(6), 1999, 1540-1548 | DOI: 10.4319/lo.19220.127.116.110
ABSTRACT: 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 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 g-1 for the Paraguay River to 1.38 liter g-1 for the Upper Parana´ River. Equilibrium PO4-P ranges from 0.17 µmol liter-1 for the Upper Parana´ River to 2.92 µmol liter-1 for the Bermejo River. Short-term (60 h) desorbable PO4-P ranges from 0.23 µmol g-1 for the Paraguay to 0.77 µmol g-1 for the Upper Parana´. 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 5- to 10-fold increase 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 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 Parana´ 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.