Lotic and lentic environments of the Pampean Plain
Solari, Lía; Mac Donagh, María E.
The Pampean Plain landscape is characterized by a great number of shallow lakes (flushing, backwater, and seepage lakes) with different degrees of interconnection with rivers and wetlands. The Salado River is the major autochthonous water course of the Buenos Aires Province and is a typical lowland river. The first phytoplankton studies in these shallow lakes were published during the middle of the last century, whereas investigations in lotic environments did not start until 1980. Cyanobacteria and Chlorophyta are usually dominant in all water bodies of the region, while Bacillariophyceae are abundant in certain environments. Chlorophyta and Cyanobacteria are mainly represented by euplanktonic species, whereas a high proportion of the diatoms are brought in from the periphytic and benthic communities. Shallow lakes of this region are characterized according to the presence or absence of submerged and emergent macrophytes and by the relative predominance of inorganic and organic suspended solids. These conditions influence the temporal and spatial variability of the phytoplankton structure because of the changes in water transparency, nutrient balance, and the zooplankton structure related to macrophyte development. The shallow lakes connected to the Salado River act as an alluvial valley, exchanging organisms between the river and the lakes, though this influence depends on the direction of the water flow, which can reverse during flooding events. The differences in the structure and dynamics of the phytoplankton of lotic and lentic environments of the Pampean Plain are related to the hydrological regime and geomorphologic-hydrologic features, to the incorporation of inocula from connected systems, and to the colonization of submerged macrophytes. In recent years, several hydraulic constructions have been made, which have modified the natural dynamic system and thus, influenced the normal exchange of phytoplankton assemblages in this complex system.