No abstract

No abstract

No abstract

Abstract:This study evaluates temporal variations of physicochemical variables and densities of phytoplankton and zooplankton, under different flow conditions. Four samplings were carried out at four sites on the Lower Delta of the Paraná River (Argentina). This first approach was developed in 1995 and included a flood of high amplitude and average duration. Daily data analysis along the year qualified sampling water levels as very high, medium (two opportunities) and low level. Triplicate subsuperficial water samples were taken in the middle of the navigation channel. Dissolved oxygen, pH, conductivity and temperature were determined in situ; nutrients and concentration of major solutes in the laboratory. Quantitative samples of phytoplankton (concentrated by sedimentation) and zooplankton (80 l; 48 µm) were obtained. Temporal variation of most physical, chemical and biological variables exceeded the spatial variation. Physicochemical variables were mainly associated in time with the hydrological regime. In the main channel water exchange originated by the flood of marshy environments caused dissolved oxygen, nitrate and sulphate to decrease and phosphate, conductivity, bicarbonate, calcium and magnesium to increase. The dilution-concentration action of flow was evidenced in chloride variation (conservative ion). The plankton variables would be affected directly and indirectly by water level, and also by seasonal climatic condition. The densities of phytoplankton and zooplankton increased during low flow. For some organisms, the variation in density exceeded the simple concentration effect of flow decrease. The lowest plankton densities coincided with a temperature minimum.

In aquatic ecosystems, the potential effects of toxic compounds are greatly determined by their physicochemical forms and bioavailability. The aim of this work was the assessment of metal toxicity to the green algae Pseudokirchneriella subcapitata in contaminated water samples of an urban river (Matanza-Riachuelo, Argentina). A total of 30 samples were analyzed. Physicochemical parameters and total and dissolved metal (Cd, Cu, Cr, Pb, Ni and Zn) concentrations were measured. Toxicity was related as a function of size fractionation and chemical speciation (using the program Mineql+) in the dissolved fraction (<1.2μm). Percentage inhibition of algal growth (%Iw) was estimated as the differences between cultures with and without EDTA additions in the filtered water samples. Concentrations of Cd (1-85μg.L -1 ), Cr (3-164μg.L -1 ), and Ni (10-99μg.L -1 ) in river waters were always lower than the respective EC50 for P. subcapitata, whereas concentrations of Cu (5-306μg.L -1 ), Pb (13-546μg.L -1 ), and Zn (4-380μg.L -1 ) in dissolved fractions resulted above the EC50 values in many samples. Only 10 samples showed algal growth inhibition (between 11.21% and 42.19%), which 8 of them also showed high concentrations of free ionic forms of Pd or Zn. Free ionic concentrations of Cu were always lower than the EC50 for the algae. Two samples resulted toxic to P. subcapitata in spite of the absence of toxic metal concentrations. Besides, 13 samples no inhibited the algal growth and high concentrations of Pd and Zn were obtained, producing expected %Iw values between 10.03% and 90.51%. The absence of toxicity in most samples with toxic metal concentrations could be related to the presence of organic ligands, colloidal organic matter, and antagonism effects among other metals, which reduce their bioavailability.