Variation in phytoplankton composition and limnological features in a water-water ecotone of the Lower Paraná Basin (Argentina)

The study site is located on the left margin of the Paraná River (33° 15' S; 60° 30' W). In this work, we provide information of a spatial gradient. between the main channel and a shallow lake (Monnel), both permanendy connected. Changes in the co~positio? and dynamics of phytoplankton along this transitional system are described. Six sampling sites were established from the River Paraná to a floodplain lake. Phytoplankton samples were collected monthly between May 1995 and May 1996. Several physical and chemical features were measured. Some variables presented a clear trend along the transect in agreement with observations of different authors for other floodplain river systems (HAMILTON & LEWIS 1987, ENGLE & MELACK 1993). Suspended solids and nitrates decreased from the river to the shallow lake. The drop in suspended matter is due to its deposition on the floodplain and its retention by aquatic plant root systems. Macrophytes, phytoplankton and bacteria are probably responsible for nitrogen reduction from the river to the floodplain lakes. Transparency was very low throughout the system, increasing with high waters, while the opposite occurred under low water levels. Temporal fluctuations in suspended matter approximately follow the river regime, increasing during rising waters and dropping when levels decrease. Nevertheless, in January when the shallow lake reached its minimum depth, an important resuspension of sediments produced an increase in suspended solids, which direcdy affected the transparency. Unlike nitrates, SRP seems to be less influenced by water-level fluctuations. Chlorophyll a showed typical values of mesoeutrophic systems (mean 6.06 pg L-'), with maximum concentrations in November. lt was inversely correlated to water level. In December, the decrease in transparency seems to be the main cause of the decline of algal biomass. A clear dominance of diatoms followed by green algae was observed as in other reaches of the Lower Paraná River (O'FARRELL et al. 1996). The only algal class that revealed a clear decrease along the ecotone was the Bacillariophyceae. The dominant taxa were: Aulacoseira ~~nu­ lata var. granulata, A. granulata var. angusttsstma, Cyclotella meneghiniana, Chlamydomonas spp. and Synura uvella. For the first three species, a clear decrease towards the lake was observed. The reduction of the vertical extent of the turbulence towards the shallow lake may explain the diatom decrease. Nevertheless, diatoms dominated al! along the transitional system, probably due to the persistence of the turbulent conditions. Since Montiel Lake presents a permanent connection with the main channel, this condition would not allow the development of significant populations of typicallentic species. Rstrategies species, such as Chlamydomonas spp. , tended to increase in the lake during high water phases. The same was observed by GARCIA DE EMILIANI (1993) for a shallow Middle Parana lake. The ordination of the species obtained by principal component analysis gave an accurate description of th~ir distribution along the ecotone. The laek of a spanal differentiation was clearly shown together with the variation of the most relevant centric diatoms of this system. [The full paper will be published elsewhere. ]