Phytoplankton of the Paraguay and Bermejo rivers

Abstract

This article deals with the phytoplankton of the Paraguay and Bermejo rivers, with emphasis on the spatial and temporal patterns of its composition, density, biomass, diversity and community size structure. The phytoplankton of the Paraguay River is characterized by the dominance of small Chlorophyceae and Cryptophyceae, with a gradual increase in Bacillariophyceae towards its confl uence with the Paraná River. In the northern stretch, where the Pantanal drains, Bacillariophyceae exceptionally share the dominance with Chlorophyceae and Euglenophyceae. Small Chlorophyceae prevail in transparent waters, whereas Aulacoseira (A. granulata and A. herzogii) predominates throughout the main channel, in relation with a higher turbulence and lower water transparency. Density, biomass, diversity and species richness present an inverse relationship with the hydrometric level. Such variables are positively correlated with water temperature, and present a decrease from north to south in relation to increased suspended solids and conductivity and decreased water transparency. The Bermejo River, which is a main tributary of the Paraguay River in its lower section, is the main determinant of the sharp decrease in the main variables of the phytoplankton in the low Paraguay.