Lía C. Solari

Vertical distribution of phytoplankton in a pampean shallow lake

The San Miguel del Monte Pond is a shallow eutrophic regulated lake. During the sampling period, this pond experienced two phases of equilibrium, as described by PHILLIPS et al. ( 1978) and ScHEFFER et al. (1993) in shallow lakes. The study began during a turbid phase and continued until the growth of the submerged macrophyte Potamogeton pectinatus, which resulted in a clear-water period. The presence of this macrophyte probably produced shading, a reduction of turbulence and less availability of nutrients in the water column (MEI)ER et al. 1990). Few studies have analysed the vertical distribution of phytoplankton in ponds with successively different conditions of turbid and clear water. The aim of this study was to analyse changes in the vertical and temporal distribution, abundance, dominant species and succession processes of phytoplankton related to the alternative phases of equilibrium detected in the San Miguel del Monte Pond.

Phytoplankton chlorophyte structure as related to ENSO events in a saline lowland river (Salado River, Buenos Aires, Argentina)

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 Reynoldss 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.

Lotic and lentic environments of the Pampean Plain

The Pampean Plain landscape is characterized by a great number of shallow lakes (fl ushing, backwater, and seepage lakes) with different degrees of interconnection with rivers and wetlands. The Salado River is the major autochthonous water course of the Buenos Aires Province and is a typical lowland river. The fi rst phytoplankton studies in these shallow lakes were published during the middle of the last century, whereas investigations in lotic environments did not start until 1980. Cyanobacteria and Chlorophyta are usually dominant in all water bodies of the region, while Bacillariophyceae are abundant in certain environments. Chlorophyta and Cyanobacteria are mainly represented by euplanktonic species, whereas a high proportion of the diatoms are brought in from the periphytic and benthic communities. Shallow lakes of this region are characterized according to the presence or absence of submerged and emergent macrophytes and by the relative predominance of inorganic and organic suspended solids. These conditions infl uence the temporal and spatial variability of the phytoplankton structure because of the changes in water transparency, nutrient balance, and the zooplankton structure related to macrophyte development. The shallow lakes connected to the Salado River act as an alluvial valley, exchanging organisms between the river and the lakes, though this infl uence depends on the direction of the water fl ow, which can reverse during fl ooding events. The differences in the structure and dynamics of the phytoplankton of lotic and lentic environments of the Pampean Plain are related to the hydrological regime and geomorphologic-hydrologic features, to the incorporation of inocula from connected systems, and to the colonization of submerged macrophytes. In recent years, several hydraulic constructions have been made, which have modifi ed the natural dynamic system and thus, infl uenced the normal exchange of phytoplankton assemblages in this complex system.

The effects of hydraulic works and wetlands function in the Salado-River basin (Buenos Aires, Argentina)

Man-made activities exert great influences on fluvial ecosystems, with lowland rivers being substantially modified through agricultural land use and populations. The recent construction of drainage canals in the upper stretch of the Salado-River basin caused the mobilization of huge amounts of salts formerly stored in the groundwater. The main aim of this work was to analyze the effect of the discharges of those canals into the Salado-River water, under different hydrologic conditions, and the role of the wetlands and shallow lakes placed along the canals’ system. Physicochemical variables were measured and water samples were taken during times of high water, mean flows, drought, and extreme drought. The environmental variables and the plankton development were related to the hydrologic regime and reached minimum values during floods because of low temperatures and dilution. Local effects on the water’s ionic composition became pronounced during droughts because of groundwater input. Nutrient concentrations were mainly associated with point wastewater discharges. Conductivity, ion concentrations, total plankton biomass, and species richness increased in the Salado-River downstream site, after the canals’ discharges. The artificial-drainage system definitely promotes the incorporation of salts into the Salado-River basin. In this scenario, a careful hydraulic management is needed to take into account this issue of secondary salinization that threatens the economic exploitation of the region. The wetlands present in this study acted as service environments not only helping to reduce salt, nutrient, and suspended-solid concentrations downstream but also contributing a plethora of species and plankton biomass into the Salado-River main course.

Succession of microconsumers in waterlogged pampean soils (Buenos aires, Argentina) and its significance for nearby wetlands

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.