Estructura del fitoplancton de seis sistemas lóticos de la provincia de Buenos Aires (Argentina)
Phytoplankton structure of six lotic systems from the province of Buenos Aires (Argentina). The phytoplankton structure of six lotic systems: Rodríguez, El Gato, El Pescado, Buñirrigo, Juan Blanco streams and Samborombón River was analyzed. Surveys were carried out seasonally from May 1997 to June 1998. Sample sites were established at 2 or 3 points depending on the length of the systems, from the headwaters to the mouths (E1 to E3). Physical and chemical variables (pH, temperature, conductivity, dissolved oxygen, transparency) were measured and alkalinity, suspended matter and nutrients were determined at each sampling site. Species composition, species richness, species dominance, diversity and evenness were assessed. Pennate diatoms and Chlorophyceae (Chloroccocales) were the best represented groups. The class Cyanophyceae was dominant in 60 % of the samples and the class Bacillariophyceae was subdominant in 42 % of the samples. The Samborombón River showed the minimum and maximum phytoplankton densities of the six systems during the period studied. Differences in specific composition were observed in E2 and E3 of El Gato and Rodríguez streams and in E3 of Buñirrigo stream, related to human impact. Mesosaprobic indicator species, such as Nitzschia palea, N. umbonata, Gomphonema parvulum, Navicula cryptocephala, Euglena acus, Strombomonas scabra, Crucigeniella rectangularis and Dictyosphaerium pulchellum, were found. In sampling sites E3 of El Pescado and Buñirrigo streams and Samborombón River, centric diatom species such as Aulacoseira granulata, Actinocyclus normanii, Cyclotella meneghiniana and other algae (Merismopedia tenuissima, Ulothrix aequalis, U. subconstricta) increased their densities. Diversity and richness indexes showed a non-defined pattern.
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