Phytoplankton of a small lowland stream related to water quality and hydraulic discontinuities

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The structure and dynamics of the phytoplankton related to water quality and hydraulic characteristics were studied in a 12 km reach of the Rodríguez Stream, a tributary of the Río de la Plata, in Argentina. This stream includes a small artificial pond. Samples were taken seasonally along a year at three sites at the stream and once at the pond. Water quality according to physical and chemical parameters was better at site 1 than at downstream sites. Changes in phytoplankton assemblages descriptors were observed with increasing nutrients and water residence time. The spatial water quality changes are coincident with the hydraulic discontinuity. This situation suggests a synergic effect among the phytoplanktonic descriptors that were analyzed. There was a high percentage of species belonging to euglenoids followed by green algae and diatoms. Loricated euglenoids were dominant at site 1, clorococcalean greens dominated at the pond and decreased progressively downstream, at sites 2 and 3. Gomphonema parvulum, Nitzchia palea and N. umbonata were abundant in the latter sites, where current velocity increased. Our results also show the integrating character of the phytoplankton in this small stream where it can reach considerable development due to the low gradient ($latex <10^{-3} $), and specially to the presence of the pond.

Palabras clave
phytoplanktonic descriptors
Rodríguez Stream

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