No abstract

El Pescado (literally ‘the fish’) is a lowland stream in the pampean plain, entering the Rio de La Plata at 35˚55¢S,57˚45¢W, 70 km SW of the city of Buenos Aires. The stream develops as a potamon with variable flood frequency along its 36 km length. The water is slightly soft, with relatively high conductivity and dominance of HCO–3 , Na + and Cl – ions. Three fish families, Characidae (29%), Pimelodidae (14.6%) and Loricariidae (12.8%) are most abundant among 55 species. Differences in species composition among four stations with different environmental traits were observed. Flooding slightly affected the fish fauna. In open-water and rocky-bottom stations, both species number and number of individuals were correlated with conductivity, an indication of the decrease of fish density with floods. Species richness was well over worldwide means. Diversity values, higher than in small tropical streams in Venezuela and southern Brazil, ranged from 0.32 to 2.59, being higher during autumn and spring. The stream has a high persistence, with the composition of the fish community maintained or recovering as soon as flooding ends. One Synbranchus marmoratus specimen was found among rocks, the first record of this behaviour for the species.

No abstract

An investigation was performed in a pond (San Miguel) directly associated with the Salado River (Pampean grasslands). The aim of the study was to analyze river interactions. Physical and chemical conditions and the plankton community were studied in order to determine the effects of changes in hydrological conditions. The main factors that influenced the physical–chemical characteristics of the pond have been identified by means of cluster analysis. Conductivity was the main factor that determined groups in sampling periods. Changes in conductivity were clearly associated with the water‐level of the pond and depended on the flow of the Salado River and fluctuations of its conductivity, which is itself a function of evaporation and the inflow of underground water of high conductivity. Other factors that affected the physical–chemical conditions included concentrations of phosphorus and polyphenols, the main allocthonous sources. The concentration of these compounds was decreased in low water conditions. Principal Components Analysis suggested that there were four major regulatory factors in the pond, as follows. First, a dry season, with a prolonged isolation phase and a considerable increase in conductivity, turbidity and suspended solids, during which the abundance of plankton was greatly reduced, as were incorporated benthic species. A high and sudden increase of river flow determines the second regulatory factor, the flood season, when dissolved allocthonous material enters, conductivity decreases and there is a conspicuous dominance of cyanophytes, protozoans and crustaceans. The different degrees of hydrological connection with the Salado River produce changes in dissolved phosphorus forms and the composition of the plankton. The third regulatory factor is an increase in particulate material. This is associated with the highest total phosphorus values as a consequence of the phosphorus– particle relationship. The opposite situation, the fourth regulatory factor (a decrease in particulate material), produces clear water conditions with a dominance of chlorophytes, planktonic diatoms and rotifers.

In Buenos Aires province, the river Samborombon, which is typical of the plain, was sampled monthly during 1986-1987 in research carried out both on its physico-chemical characteristics and the communities present -plankton, phytobenthos, zoobenthos -. In this paper the structure and function of phytoplankton and benthic algal communities in the different sections of the river are compared. Both communities have definite limits as their components are not shared and show different strategies. Likewise, the environmental conditions along the river are variable, leading to differences in the planktonic and benthic algae at the locations studied. Algues planctortiques et benthiques d'une rivière de plaine (Argentine) : étude comparativeMots clés : phytoplancton, phytobenthos, structure, dynamique, rivière, Argentine Dans la province de Buenos Aires, la rivière Samborombon, rivière typique de plaine, a été étudiée de mars 1986 à février 1987 dans le cadre d'une recherche relative aux caractéristiques physico-chimiques et aux communautés du plancton, du phytobenthos et du zoobenthos. Dans ce travail, la structure et la fonction du phytoplancton et du phytobenthos sont comparées dans diverses sections de la rivière. Les communautés ont des limites définies parce que leurs composantes sont différentes et montrent des stratégies contraires. Les caractéristiques du milieu sont variables tout au long de la rivière et déterminent des variations locales

Annual changes in the algal density and concentrations of chlorophyll a, total phosphorus, and organic matter were analyzed in water and sediments at four sites characterized by the presence or absence of submerged and emergent macrophytes, during turbid-and clear-water conditions to determine the contribution of the algal components of the plankton and the epipelon and to identify the most typical species in each community. Three states were recognized: one turbid and two clear, with different submerged macrophyte cover. The peaks of phytoplankton and epipelon occurred in the turbid phase, whereas the highest proportion of true epipelic algae in sediments was reached in the second clear phase. The Oscillatoriaceae dominated during the turbid phase in the water and throughout the entire year within the sediments. IntroductionIn shallow lakes where alternative equilibrium states can occur, the importance of the relationships between epipelic and pelagic algae has recently been highlighted (VADEBON-COEUR et al., 2002). In such lakes the structure and biomass of epipelon varies in relation to spatial heterogeneity created by the presence or absence of emergent and submerged macrophytes. According to CYR (1998), the biomass of benthic algae varies among sites and according to depth in small oligotrophic and mesotrophic lakes.The benthic assemblages are made up of autochthonous species plus others incorporated from the plankton and periphyton. Thus, changes in environmental conditions can promote an increase in the density of those algae that can develop in both the sediment and the water column, giving them a competitive advantage over the algae that can live in only one of those lake habitats. The seasonal distribution of algal productivity observed by HANSSON (1996) in North American lakes, and by BARKO et al. (1977) andFLÖDER et al. (2006) in small ponds, suggests an exchange of algal organisms between the planktonic and epipelic populations.The phosphorus in shallow lakes tends to accumulate in the sediments and in the macrophytes (SØNDERGAARD et al., 2003;ROONEY and KALFF, 2003). Thus, the nonplanktonic algae 154 M. A. CASCO et al.

No abstract

Biomass assessments of algae in wetlands usually include only the phytoplankton community without considering the contribution of other algal associations to total algal biomass. This omission prevents an accurate evaluation of the phytoplankton community as an integral part of the total ecosystem. In the present work, the biomass contributions (expressed as chlorophyll-a content per m 2 of lake) of phytoplankton, epiphyton on both submerged and emergent macrophytes, and epipelon were measured in Lacombe Lake, Argentina, for the purpose of (1) establishing the relative importance of the phytoplankton and (2) evaluating the entire contribution of algal biomass within the context of the Goldsborough & Robinson conceptual model. Our sampling was carried out monthly for a year in sites representative of different conditions with respect to water depth and type of macrophytes. Physicochemical analyses of water were performed following standard methods. Plankton was collected in a five-level profile at deeper stations and in subsurface samples at the shallow one. Samples of sediment obtained with corers were collected for epipelon sampling and segments of plants were cut at different levels, so as to obtain the epiphytes by scraping. Pigment was extracted with aqueous acetone and calculations were made by means of the Lorenzen equation. According to the Goldsborough & Robinson model, a Lake State developed here during the winter (phytoplankton maxima: 150 mg chlorophyll-a per m 2 ). Then, through the subsequent growth of the submerged macrophytes, an Open State was observed, characterized by a maximum epiphyton biomass (at 3,502 mg chlorophyll-a per m 2 ) along with lower levels of phytoplankton biomass. The epiphytic algae on the emergent macrophytes were always present but attained only relatively low biomass values (maximum: 120 mg of chlorophyll-a per m 2 in February). The epipelon biomass varied between 50 and 252 mg chlorophyll-a per m 2 , registering a considerable contribution of settled algae from the water column (phytoplankton). This study contributes to our knowledge of wetland dynamics through its assessment of the rapid changes in the relative contributions of both planktonic and attached algae to the total algal biomass within the context of specific environmental factors.