Estrutura e dinâmica do fitoplâncton nos reservatórios de Segredo e Parigot de Souza (Estado do Paraná, Brasil).

The main aim of this study was to evaluate phytoplankton spatial and temporal distribution in the presence of abiotic (climatological and hydrological) and biotic (predation by zooplankton) factors in two subtropical reservoirs located in Paraná State, Brazil. Sampling was performed every three months during 2002 in Segredo and Parigot de Souza reservoirs, along longitudinal axis (upper, transition and lacustrine regions) of different depths. 118 taxa were identified, with Chlorophyceae as the most specious group. In general, the values of species richness, diversity and equitability were low, although high values were observed in Segredo reservoir. In general, for phytoplankton density, values observed were above 300 ind.mL-1 in both reservoirs, except in December, when high values were registered, with dominance of nanoplaktonic species (<20µm) belonging to Bacillariophyceae, Cyanobacteria and Cryptophyceae groups. Low biomass values (< 1mm3.L-1), in both reservoirs, were mostly verified during the study period. Biomass values above 1mm3.L-1 were observed in Parigot de Souza's upper region in April, and in Segredo's lacustrine region in December, with dominance of Microcystis aeruginosa Kütz and Anabaena circinalis Rab., respectively. Vertical gradients of the phytoplanktonic attributes analyzed were verified, and higher values were registered in the mixed layer. Considering the phytoplankton biomass values, both reservoirs were oligotrophic for the duration of the study. The results show that the biomass values were more similar in the different zones of each reservoir than between zones of the two reservoirs. The distinct characteristics in reservoir morphometry and hydrology, unique for these systems, and the extension and watershed uses probably constitute the determining factors on phytoplankton structure in these artificial ecosystems.