River-backwater pond interactions in the lower basin of the Salado River (Buenos Aires, Argentina).

Abstract

The relationships between two sectors of the Salado River, in its lower basin, and a backwater pond were investigated during different phases of the hydrological cycle from May 1995 to June 1997. Samples were analysed for physical and chemical parameters and plankton from three sampling stations. Three main hydrological conditions were identified in relation to the discharge regime and degree of connectivity: 1) flood events characterized by low conductivity values, phosphorus influx to the system and decreased plankton density, with similar composition at all three sites; 2) a steady state condition with declining conductivity values, SRP concentrations, and phytoplankton densities, with peaks of zooplankton in the backwater pond; 3) dry periods characterized by high conductivity values, the isolation of the backwater pond, the river with lentic conditions plus peaks of phytoplankton and minimum zooplankton density in the backwater pond. Internal biological processes prevailed in the backwater pond when the connectivity was low or absent. Combinations of geomorphological features (flush and backwater ponds), the hydrological regime and conductivity had different degrees of control on the plankton, according to their incidence in the community as determined by their transport from the upper basin, the exchange with storage zones and the importance of autogenic processes.