Fitoplancton de ambientes inundables del rio Parana (Argentina) : estudio comparativo entre areas libres y vegetadas

Le phytoplancton a été étudié dans deux mares (Llanta et Perdida) et dans un marais (Choui) sur des îles du rio Parana (27° 30°S, 58° 55 O). Des échantillons ont été prélevés dans la végétation des rives et dans les eaux libres en période de crue et d'étiage entre décembre 1981 et février 1983. La densité et la diversité spécifique ont été analysées en fonction des variables de l'environnement. En période d'étiage, la diversité la plus faible et la densité la plus forte (7706 ind/ml) ont été trouvées dans le marais à #Panicum grumosum, avec dominance de l'Euglénophycée #Trachelomonas volvocina. Dans les mares, les plus fortes densités ont été relevées dans la mare Llanta dans les zones à #Eichhornia azurea et #Salvinia sp., et dans la mare Perdida, dans les herbiers à #Polygonum acuminatum

Spatio-temporal variability in underwater light climate in a turbid river-floodplain system. Driving factors and estimation using Secchi disc

The underwater light climate has important effects on primary producers. The aim of this research was to evaluate its variability in a turbid river-floodplain system. Photosynthetically active radiation (PAR) was measured in the Middle Paraná River during different hydrological phases to (a) analyse the photosynthetically active radiation attenuation coefficient (k) and euphotic depth (Zeu) as well as their associations with optically active components and (b) develop and evaluate indices and regression models based on Secchi disc (SD) measurements to estimate k and Zeu. Values of k were higher in the fluvial system than in the floodplain and during low-water stage than high-water stage. Particulate components controlled the light climate variability. Chromophoric dissolved organic matter and chlorophyll-a had significant effects during floods. The estimation of k and Zeu was sensitive to temporal but not to spatial variations. The highest prediction accuracy was observed when using specific non-linear regressions for each hydrological phase, especially for Zeu estimation (low stage: k = 1.76 × SD−0.80, Zeu = 2.62 × 1/SD−0.80; high stage: k = 2.04 × SD−0.53, Zeu = 2.26 × 1/SD−0.53). The indices k × SD and Zeu/SD were significantly different from those proposed for clear water environments. It is concluded that temporal variations should be considered when estimating k and Zeu in turbid river-floodplain systems because of the temporal heterogeneity in optically active components. Considering that ecological implication of the light climate depends on Zeu:depth ratio, we propose to estimate Zeu instead of k. Finally, indices proposed for clear water environments are not recommended to be applied to turbid environments.Fil: Mayora, Gisela Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Devercelli, Melina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin

Abundance and diversity of phytoplankton in the Paran\ue1 River (Argentina) 220 km downstream of the Yacyret\ue1 reservoir

Patterns in the temporal composition, abundance and diversity of the phytoplankton community of the Paraná river prior to and after the initial filling phase of the Yacyretá reservoir are analyzed. The study site is located 220 km downstream from the Yacyretá reservoir and 30 km downstream from the confluence with the Paraguay river. Because both rivers remain separate and unmixed at the study site, we compared the possible effects of the impoundment on both river banks (left and right banks) in hydrological periods with similar duration and magnitude of the low and high water phases. Physical and chemical conditions measured on the right bank (water from the Paraguay river) were similar at both periods (pre and post-impoundment) whereas conductivity, pH and orthophosphate concentration increased on the left bank (water from the High Paraná river and Yacyretá reservoir) after the impoundment. Changes in phytoplankton density and diversity were observed only in samples collected from water flowing from the reservoir (left bank). The density of Chlorophyceae (Chloromonas acidophila, Chlamydomonas leptobasis, Choricystis minor, Chlorella vulgaris, Scenedesmus ecornis, Monoraphidium minutum, M. contortum and M. pusillum) and Cryptophyceae (Rhodomonas minuta, Cryptomonas marssonii and C. ovata) increased while Cyanophyceae (Cylindrospermopsis raciborskii, Raphidiopsis mediterranea and Planktolyngbya subtilis) and Bacillariophyceae (Aulacoseira granulata and its bioforms) decreased compared to previous studies conducted on the left bank of the Paraná river. Phytoplankton collected from the right bank of the river did not differ in pre and post- impoundment samples because they originate from the Paraguay river, which remains relatively unaffected by human activities

Behavioural responses of freshwater zooplankton vary according to the different alarm signals of their invertebrate predators

Single behavioural interactions between two freshwater planktonic crustaceans and invertebrate predators from different zones of the common environment were studied. The planktonic prey organisms were the cladoceran Ceriodaphnia dubia and the copepod Notodiaptomus conifer. The three invertebrate predators were the shrimp Macrobrachium borellii, the crab Trichodactylus borellianus and the larva of the dipteran midge Chaoborus. Feeding experiments were first performed to document the effects of exposure time and prey density on predation. All the selected predators fed on C. dubia and N. conifer, and predation rates were dependent on prey density and exposure time. The ability of microcrustaceans to modify their swimming and avoidance behaviour when faced with cues from each predator was then analysed. The cladoceran was more vulnerable to decapod predation, whereas the prey microcrustaceans were taken equally by the dipteran larvae. The analysis showed that the microcrustaceans detected the presence of at least one invertebrate predator through the predator alarm signals. The cladoceran responded to chemical signals from the three predators but copepods modified their behaviour only in the presence of infochemicals of M. borellii. The different outcomes suggest that macrocrustaceans have different vulnerabilities when faced with the same predator and so have evolved specific and different strategies to reduce invertebrate predation.Fil: Gutierrez, Marìa Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Rojas Molina, Florencia Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: de Azevedo Carvalho, Debora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin