Limnological characteristics and trophic state of an inland reservoir in Argentina

Abstract

The current trophic status of Paso de las Piedras Reser voir was evaluated by analysing different physicochemical, biological and environmental variables, in relation to its water quality, and application of two different classification criteria. Water quality sampling was conducted at four sampling sites from June 2004 to June 2005. During this study, 183 phytoplankton taxa were identified. The phytoplankton abundance exhibited a maximum of 368.037 × 10 3 cells.mL –1 in Februar y 2005, and at least 1.133 × 10 3 cells.mL –1 in October 2004. An almost exclusive dominance of Cyanobacteria was obser ved between December 2004 and May 2005, the product of a large relative abundance of Anabaena circinalis and Microcystis natans which, together with other companions, were the reason for an algal bloom characterized by an average density of 133.05 × 10 3 cells.mL –1 , and an average chlorophyll concentration of 28.7 mg.m –3 . These study results indicate that the seasonal variations of physical, chemical and biological parameters in the waters of this reser voir were essentially a consequence of environmental and hydrological conditions in the dam area. In contrast, the spatial variations inside the lake were the result of the characteristics of the water inflow provided by its two main tributaries. The N:P ratio suggests neither nutrient is a limiting factor for maximum algal biomass in the lake, indicating that variations in the phytoplankton community structure, and development of phytoplankton blooms, would be more constrained by environmental and hydrological conditions than nutrient competition. The high concentration of measured nutrients could be attributed to the concurrence of various non-point sources. The phytoplankton species richness was high, exhibiting values even higher than those mentioned in previous studies. Considering the two trophic classification systems, and based on total phosphorus data, the reser voir is classified within the hypertrophic categor y. In contrast, considering only the chlorophyll and turbidity data, the lake would be classified within the eutrophic category.