Inés O'Farrell

Algal assemblages across a wetland, from a shallow lake to relictual oxbow lakes (Lower Parana River, South America)

This study deals with the variation in the algal assemblages across the transversal dimension of a wetland of the Lower Paraná River from October 1998 to September 1999. The relationship between the algal composition and the physico-chemical variables is analyzed by means of Canonical Correspondence Analysis.The abundant floating macrophytes generated a severe reduction in light penetration in the relictual oxbow lakes (ROLs), almost anoxic conditions and high values of P, N and K. Such characteristics accounted for their very particular algal flora dominated by Cyanobacteria and several diatoms, species probably mixotrophic regarding their capacity to grow in darkness and tolerate very low oxygen contents. In the shallow lake, the phytoplankton comprised many small autotrophic green algae, accompanied by many flagellates of the classes Cryptophyceae, Euglenophyceae and Dinophyceae. Our results indicate that the macrophyte cover was probably the stirring factor in the selection of algal species along the transitional zone comprising a floodplain shallow lake and several ROLs.

The assessment of water quality in the Lower Luján River (Buenos Aires, Argentina): phytoplankton and algal bioassays

The monitoring of river phytoplankton and several hydrological, physical and chemical variables, in combination with bioassays using Selenastrum capricornutum Printz, allowed the characterisation of three distinct reaches of the Lower River Luján. The upstream stretch, characterised by the lowest depth and discharge, registered the highest nutrient, dissolved heavy metal and chlorophyll a concentrations in accordance with low phytoplankton diversity and the occurrence of several species typical of organically polluted lowland rivers. A downstream improvement, concomitant to increasing river discharge, is revealed by a progressive decrease of organic pollution parameters even though algal toxicity is registered through bioassays. The water input from the Parana River through the G. Arias Channel plays an important role in the regulation of the limnology of the Lower Luján River. As a result of marked increasing discharge, depth and width, there is a decrease in nutrient concentration and phytoplankton density and an increase in dissolved oxygen concentration. Likewise, algal growth rates in the bioassays showed less toxic effect.

Do steady state assemblages occur in shallow lentic environments from wetlands

This study was conducted in a wetland on the Lower Paraná River (Argentina). Algal assemblages from a permanent shallow lake, three relict oxbow lakes (ROL) and an isolated small pond were sampled fortnightly during the spring-summer time of 1998–99. The algal assemblages strongly differed among the permanently vegetated ROLs and the shallow lake and small pond, both temporarily covered by floating macrophytes. Anoxic conditions and poor light penetration have been pointed out in previous studies to be the leading factors in the selection of algal species in this wetland. Two or three Cyanobacteria species and some large pennate diatoms dominated the algal biomass in the ROLs, whereas only the former taxa contributed significantly to density. Assemblages were characterised by their low diversity and species richness; the highest SIMI values between consecutive samplings were registered in the ROLs. Even though some characteristics of the Z and S2 Reynolds associations describe the ROLs assemblages, they do not strictly match with them. Changes in algal density and biomass were more pronounced in the shallow lake. An alternative dominance of Cryptomonas spp., Peridinium spp., Euglena spp., Monoraphidium spp., Oscillatoriales and Chroococcales occurred, with a consequent replacement of the Y, Lo, W1X1,S2, K, and F Reynolds associations. Variation in the macrophytic cover of the small pond accounted for the fluctuations in its algal composition. When the cover was complete, the assemblages resembled those of the ROLs, whereas when its surface was free of plants, they were more similar to those of the shallow lake. Steady state assemblages related to the persistence of a dense macrophyte cover may be found in wetlands in environments with constant and extreme ecological conditions such as those recorded in the ROLs. In contrast, “unpredictable” assemblages can be expected in shallow lakes with fluctuating conditions.

Morphological variability of Aulacoseira granulata (Ehr.) Simonsen (Bacillariophyceae) in the Lower Paraná River (Argentina)

Abstract In this paper we consider the morphological variation within a natural population of Aulacoseira granulata (Ehr.) Simonsen in relation to environmental factors. This species was dominant in the phytoplankton of the Lower Paraná River (Argentina) and exhibited seasonal fluctuations of cell dimensions. The mean cell diameter was directly correlated with the river water level and inversely with pH and nitrate concentrations, whereas cell length was directly correlated with transparency and nitrate concentration and inversely with suspended solids. This pattern was similar to that observed for filament length. The cell length : diameter ratio was inversely related to water level and discharge and directly related to pH, transparency, and nitrate concentration. Maximum diameters did not coincide with maximum lengths. A tendency to maintain cell volume throughout the annual cycle was observed, which probably relates to both buoyancy and photosynthetic capacity. These results associate the water ascendancy and the size recovery phases to discharge. Cells become smaller on the ebbing of the flood phase, and the decreasing depth increases the probability that the alga will be disentrained from the turbulent field. The loss during low water would act as a stimulus for auxosporulation, contributing to the production of large cells to start off the next population.

Comparative analysis of the phytoplankton of fifteen lowland fluvial systems of the River Plate Basin (Argentina)

The phytoplankton of fifteen lowland courses of the River Plate Basin is compared and characterized by means of multivariate analyses.The cluster analysis performed with the more abundant species of each fluvial system reveals three main groups. Rivers with a high discharge and a large floodplain are grouped on the basis of the dominance of several Aulacoseira species. An eutrophic flora typified by Cyclotella meneghiniana, Synedra ulna and several green algae occurs in the smaller rivers with high conductivity, low transparency and important discharge variations. A third group comprises the Uruguay River and its tributaries, characterized by the presence of several pennatae diatoms and flagellates: Amphipleura pellucida, Surirella tenera, Terpsinoe musica, Navicula cuspidata, Eudorina elegans, Pandorina morum and Peridinium gatunense.The Principal Component Analysis based on a data matrix of physical, chemical and hydrological parameters revealed similar results. The ordination according to the first two components reflects the geographic location of these systems in the River Plate Basin, responding to a decreasing gradient of conductivity, pH and solids and an increasing minimum temperature. The rivers were ordinated by means of a Correspondence Analysis based on the main algal groups.

Phytoplankton ecology and limnology of the Salado River (Buenos Aires, Argentina)

The phytoplankton community structure and dynamics of the Salado River were studied during 18 months (1987–1989). The abundance ranged from 2722 ind ml−1 at site I (5/1988) to 498094 ind ml−1 at site 5 (2/1989). Many species were indicative of brackish and alkaline waters and mesosaprobic systems.

The sudestadas : a hydro-meteorological phenomenon that affects river pollution (River Luján, South America)

Abstract The “sudestadas” are short-term hydro-meteorological phenomena that produce a “hydraulic plug”, preventing the normal drainage of the water courses in the Río de la Plata Estuary. The purpose of this study was to analyse the influence of the sudestadas on the water quality of the Lower River Luján, which discharges into the Río de la Plata Estuary (59°37′W; 34°43′S). Samplings were conducted from November 1998 to July 2001 at six sites on the lower stretch of river. Physical and chemical variables were measured and a nonparametric test was applied to each variable, grouping samples affected and non-affected by sudestadas, which further diminish the water quality of the river. The frequency and intensity of sudestadas are predicted to increase as a consequence of the Earths warming; thus, this research suggests that the environmental risk produced by the effects of global change, will be greater.