Las diferencias en la topografía determinan diferencias en la frecuencia, intensidad y duración de las fases de inundación y sequía así como la oferta de hábitat para micro y macroorganismos, mientras que distintos usos del suelo afectan la concentración de nutrientes, y la actividad biológica influye en la dinámica de nutrientes del ambiente. El conocimiento de los efectos de la inundación sobre la fertilidad de los suelos y sobre el río permitirá conocer las respuestas de ambos ecosistemas frente a los disturbios que provoca el uso actual de la tierra y los que ocurrirán con las acciones planificadas de manejo y regulación del cauce principal en un futuro cercano. El objetivo general del trabajo fue identificar las especies de microorganismos (productores y consumidores) del agua de inundación que se suceden desde el anegamiento a la desecación en suelos con distinto uso y pendiente así como también conocer los principales cambios físico-químicos, contenido de nutrientes y materia orgánica del agua en los diferentes estadios de inundación del suelo, y finalmente construir modelos conceptuales de funcionamiento de estos sistemas naturales y sus efectos sobre las características del agua de escorrentía superficial.

In floodplains, the passive dispersal by drift occurs when water bodies become connected, and is a key feature offering pathways for the recolonization of periodically flooded habitats. Mesocosms with experimental flooding were used to document the succession of small invertebrates under differing hydrologic regimes with the intent of identifying which animals were capable of moving among habitats. The mesocosm experiments were performed in soils in sandy areas of the Salado-River basin including a mixed-use plot, a plot for breeding livestock, and a plot currently without any use located in different topographic positions (upper, middle, lower). Forty-seven taxa were found including ciliates, amoebae, rotifers, and microcrustaceans. The maximum total specific richness was recorded in middle soil and the minimum in upper soil because of a shorter colonization time. Higher mean densities occurred in middle and lower topographies, while the upper exhibited lower values. The ciliates and rotifers were the dominant. During the intermediate stage of flooding, the high density and biomass of the primary producers resulted in high concentration of dissolved oxygen (photosynthesis exceeded respiration). In contrast, in the final stage, low densities of microalgae led to low concentrations of dissolved oxygen and a high density of consumers (preponderance of respiration over photosynthesis). These flooded areas are significant as sources of microorganism inocula into the river. An abundance of microbiota arising from temporary wetlands and floodplains is fundamental for a successful recruitment of native-fish species.

We analyzed the phytoplankton present in the lower sector of the Salado River (Buenos Aires, Argentina) for 10 years (1995–2005) and detected significant changes occurring in chlorophyte abundance and species richness during La Niña event (1998–1999), which period was analyzed throughout the entire basin (main stream and tributaries). We compared the physicochemical and biologic variables between two El Niño–La Niña–Southern Oscillation (ENSO) periods – El Niño (March 1997–January 1998) and La Niña (May 1998–May 1999) – to identify possible indicators of a relationship between climatic anomalies and chlorophyte performance. Chlorophyte density increased during the La Niña. Under normal or extreme hydrologic conditions, mobile (Chlamydomonas spp.) and nonmobile (Monoraphidium spp.) chlorophytes codominated. These species belonged to Reynolds's functional groups X1 and X2, those typical of nutrient-enriched environments. Comparative analyses between El Niño and La Niña periods indicated significant differences in physicochemical (K+, dissolved polyphenols, particulate reactive phosphorus, alkalinity, pH) and biologic (species diversity and richness, phytoplankton and chlorophyte total densities) variables between the two periods at all basin sites. During the La Niña condition, species richness was greater owing to interconnected shallow lakes and drainage-channel inputs, while the Shannon diversity index was lower because of the high abundance values of Monoraphidium minutum. A detailed analysis of the chlorophytes in the entire basin, indicated that changes in density and species dominance occurred on a regional scale although diverse chlorophyte assemblages were identified in the different sectors of the Salado River basin. After La Niña event, the entire basin had the potential to revert to the previous density values, showing the resilience to global environmental changes and the ability to reestablish the general conditions of stability.

Phytoplankton and environmental variables were studied during a year in a shallow eutrophic lake and its inflow in the 'depressed Pampa' of Argentina. The taxa were classified according to the competitors, stress tolerators, ruderals and intermediate categories (CSR) model and were allocated to morphofunctional groups (MFG). As was expected according to the trophic state, scarce transparency and mixing condition, the R-strategists (several cyanobacteria and pennate diatoms) dominated almost all of the study period. C-strategists were more scarce and favoured by short periods of transparency, whereas only three species were attributed to the S-strategist category. Each MFG was represented by a single species at a time, resulting in a high functional miscellany in this shallow lake. Thus, we can assume that the CSR approach is a useful tool to explain the success of the different species of phytoplankton in this shallow lake.

No abstract