María dos Santos Afonso

Algal assemblages across a wetland, from a shallow lake to relictual oxbow lakes (Lower Parana River, South America)

This study deals with the variation in the algal assemblages across the transversal dimension of a wetland of the Lower Paraná River from October 1998 to September 1999. The relationship between the algal composition and the physico-chemical variables is analyzed by means of Canonical Correspondence Analysis.The abundant floating macrophytes generated a severe reduction in light penetration in the relictual oxbow lakes (ROLs), almost anoxic conditions and high values of P, N and K. Such characteristics accounted for their very particular algal flora dominated by Cyanobacteria and several diatoms, species probably mixotrophic regarding their capacity to grow in darkness and tolerate very low oxygen contents. In the shallow lake, the phytoplankton comprised many small autotrophic green algae, accompanied by many flagellates of the classes Cryptophyceae, Euglenophyceae and Dinophyceae. Our results indicate that the macrophyte cover was probably the stirring factor in the selection of algal species along the transitional zone comprising a floodplain shallow lake and several ROLs.

Cadmium toxicity in tadpoles of Rhinella arenarum in relation to calcium and humic acids

Bioassays were carried out to study the differences in cadmium (Cd) toxicity to premetamorphic tadpoles of Rhinella arenarum, in aqueous solutions with variable contents of calcium in the presence and absence of humic acids, and to analyze the relationship between the free Cd2+ ion concentrations calculated by chemical modeling and the biological results. The correlation analysis of the free Cd2+ concentration calculated by chemical speciation and the analytical Cd yielded a direct relationship between the degree of toxicity and the slope value. The lowest slope was obtained from the treatments with lowest free Cd2+ ion concentration and lowest toxicity, whereas the highest slope was obtained from the most toxic treatment. At comparable concentrations of free Cd2+, intralarval Cd increased as the Ca in the solution decreased. At equal contents of Ca, in the presence of humic acids, the content of Cd in larvae was higher and the toxicity values lower. The results obtained in this study show that waterborne Ca could offer some protection from metal uptake and accumulation by competitive inhibition in the uptake mechanism that involves active transport via cell membrane. In the systems with humic materials, a certain proportion of the Cd present in the solution was associated to them and thus became less bioavailable.

The crater lake of Copahue volcano (Argentina): geochemical and thermal changes between 1995 and 2015

Abstract The crater lake and associated hydrothermal features of Copahue volcano have been studied intensively over the last 20 years (1995–2015). The geochemical and isotopic compositions of the waters provide insights into the processes occurring in the volcanic–hydrothermal system, the crater lake and the thermal springs. Variations in the temperature and chemical composition of the waters reveal fundamental changes in the system that precede and accompany the magmatic and phreatic eruptive events at Copahue. A conceptual model of the summit system was developed involving the intrusion of slivers of magma in the hot acidic hydrothermal cell, the saturation of waters with secondary minerals leading to reduced permeability, the blocking of fluid pathways by liquid sulphur during heating events and the transport of gas from the magma through the ductile–brittle transition into the hydrothermal system. Geophysical data were integrated with the chemical data to provide new insights into the behaviour of the deep magmatic system that feeds the volcano edifice. Multidisciplinary studies were used to identify precursory signals of eruptive activity to give an early warning of pending volcanic hazards. Several geochemical ratios in river water were identified as potential indicators of upcoming volcanic activity, which could be used in co-operation with the community and local authorities to deal with these volcanic hazards.