Phytoplankton assemblages in a Brazilian subtropical cascading reservoir system

Variations in the structure and dynamics of a phytoplankton community were determined during two periods in 2001, along five cascading reservoirs of the Iguacu River, Parana State (Brazil). The vertical dynamics of the phytoplankton was related to underwater light availability and the water column-mixing regime. Correlations between physical and chemical variables and phytoplankton abundance were evaluated by Canonical Correspondence Analysis (CCA). No significant cascading effect throughout the serial reservoirs was obtained. Thus, the main factor affecting the structure of the phytoplankton assemblages of each reservoir was the hydrodynamic pattern. The instability of the water column during the winter favored the development of R-strategist species such as Aulacoseira granulata, Urosolenia longiseta and U. eriensis in most reservoirs. In the period of the highest hydraulic stability and temperature, the greatest contribution to biomass came from Cyanophyceae S-strategists such as Aphanocapsa sp.l, Anabaena planctonica and A. crassa. The phytoplanktonic associations recorded were important indicators of the environmental conditions of these reservoirs.