Temporal and spatial variability in streams of a pampean basin

The Argentine pampean streams exist in a traditionally agricultural region, but they also receive a supply of contaminants in the more urban zones. The objectives of this study are: 1. To analyze the physical, chemical and microbiological characteristics of the streams in the Lujan river basin; and 2. To describe the annual variation of some chemical characteristics (primarily nutrients) in one of the streams. Two samplings (winter and summer) were made in 20 streams to determine some physical, chemical and microbiological variables. Topographical variables were also estimated in each stream. Additionally, in one of the streams (Las Flores), monthly measurements of current velocity, flow, and dissolved nitrogen, phosphorus and oxygen were taken during 4 years. The streams are characterized by high nutrient contents. There is no strong heterogeneity in stream chemical characteristics within the same period, however seasonal variations are important . The principal components analysis suggests the existence of two axes of variation within the basin. One reflects a pollution gradient that increases downstream; the other axis shows that waters change from high to low alkalinity and from less to more eutrophic. In Las Flores stream, nutrient concentration varied greatly between years. The concentration of soluble reactive phosphorus was significantly correlated with stream flow (P < 0.05). Though there was no correlation between nitrate levels and flow, some estimates indicated that these ions are in high concentrations in the groundwater. The physiognomical characteristics of pampean streams differ greatly from those described in temperate streams of the northern hemisphere, and it may be assumed that they also exhibit distinct functional characteristics.