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Riparian areas of riverine plains develop extensive floodable areas named riverine wetlands, which are essential to the water cycle balance and ecosystem dynamics. In this study, we contrasted the hydrological and physicochemical variables of riverine wetlands of both peri-urban areas impacted by intensive farming and those of rural areas with the indicators of the biotic structure (taxonomic richness, Shannon diversity and total density) of benthic diatoms, phytoplankton, zooplankton, macroinvertebrates, chironomids, fishes, turtles, and birds. The study was performed on riverine waters of the Pampean plain, Argentina, with four seasonal samplings conducted in 2017–2018. Our results showed that the significant deepening of the groundwater level caused by aquifer overexploitation in peri-urban areas, as well as the declining surface water quality with higher phosphorus and total nitrogen concentrations, affected the taxonomic richness, diversity, and total density of the biotic assemblages of riverine wetlands. The taxonomic richness of birds, turtles, phytoplankton, chironomids, and fishes was the most sensitive to land use. Phytoplankton, chironomid, and fish diversity showed the greatest differences between rural and peri-urban riverine waters, while the total density of chironomids and birds showed the greatest differences according to land use. The results suggest that the socioeconomic development in those riverine wetlands that still maintain conditions close to the natural ones needs to be subject to guidelines derived from integrated basin management and sustainable urban planning.

Microplastics (MPs) are considered emerging pollutants and often enter aquatic ecosystems carried by currents and tides until they accumulate on the shorelines. In many cases they may be colonized by diverse microorganisms forming a community called plastisphere, which can even act as a reservoir for pathogenic microorganisms. This study carried out in the Río de la Plata estuary (southern coastal fringe, Argentina) focused on two main objectives, the analysis of the biofilm colonizing MPs under laboratory conditions, and the detection of bacteria indicating faecal contamination (Escherichia coli and Enterococci), in MPs from the intertidal sediment at coastal sites with different land uses, in the freshwater sector of the Río de la Plata estuary. The colonization experiment was carried out in the laboratory with water from the estuary for a period of 35 days (residence time of the water in the freshwater sector of the estuary). The results revealed a remarkable development and diversity of biofilm organisms from the second week of colonization on, covering the surface of the microplastic and thus masking this pollutant. On the other hand, the presence of faecal indicator bacteria in the MPs of the intertidal sediment was confirmed in all the studied sites, being proportionally higher on MPs found in areas influenced by sewage discharges. Los microplásticos (MPs) son considerados contaminantes emergentes y suelen ingresar en los ecosistemas acuáticos donde son transportados por las corrientes y mareas hasta acumularse en las costas. En muchos casos éstos pueden ser colonizados por diversos microorganismos conformando una comunidad denominada plastisfera y hasta pueden actuar como reservorio de microorganismos patógenos. En el estuario del Río de la Plata (Franja Costera Sur, Argentina), se realizó un estudio cuyo objetivo fue analizar el biofilm que se desarrolla sobre MPs en condiciones de laboratorio y analizar la presencia de bacterias indicadoras de contaminación fecal (Escherichia coli y Enterococos) en MPs hallados en el sedimento intermareal en sitios costeros con diferentes usos del suelo, en el sector de agua dulce del estuario del Río de la Plata. La experiencia de colonización se realizó en laboratorio con agua procedente del estuario durante un período de 35 días (tiempo de residencia del agua en el sector de agua dulce del estuario). Los resultados revelaron un notable desarrollo y diversidad de organismos del biofilm a partir de la segunda semana de colonización, recubriendo la superficie del microplástico y enmascarando así a este contaminante. Por otra parte, se confirmó la presencia de bacterias indicadoras fecales en los MPs del sedimento intermareal en todos los sitios estudiados, siendo proporcionalmente mayor en los MPs hallados en áreas influenciadas por descargas cloacales. Fil: Pazos, Rocío Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Suarez, Jimena Camila. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

Los bañados de desborde fluvial (BDF) desempeñan un papel importante en la ecología de las cuencas fluviales y en su capacidad de autodepuración. La demanda antrópica de espacio y agua pone en riesgo su biota y los beneficios ecosistémicos que brindan. Los objetivos del trabajo fueron analizar los ensambles de microorganismos que habitan en BDF de los partidos de La Plata y de Magdalena (Buenos Aires), relacionar sus variaciones con el impacto antrópico y evaluar cómo la microbiota puede ser indicadora de estos cambios ambientales. Se estudiaron los ensambles de diatomeas bentónicas, fitoplancton, zooplancton, ostrácodos y hongos de cuatro BDF: dos de ubicación periurbana con uso del suelo agrícola y hortícola (Del Gato y Carnaval) y dos de ubicación rural y uso ganadero (Cajaravillas y Chubichaminí). Las diferencias vinculadas con el uso del suelo se expresaron en la mayoría de los grupos, y los BDF se agruparon en rurales (con más especies sensibles a la contaminación y a la eutrofización) y periurbanos (donde fueron más importantes las especies muy tolerantes). En estos últimos, tanto los grandes grupos taxonómicos como los niveles tróficos estuvieron más relacionados con la vía detritívora que con la fotosintética, y en esos ambientes también fueron más relevantes las estrategias de los organismos para afrontar el estrés hídrico. Los resultados evidencian los efectos de la presión humana, la importancia que adquieren los microorganismos para evaluarla y la necesidad de una planificación adecuada del uso del territorio para evitar la pérdida de funciones y beneficios en estos ambientes., Riverine wetlands (RW) play a key role in the ecology of river basins and their selfpurification capacity. The anthropic demand for space and water puts their biota and the ecosystem benefits they provide at risk. The objectives of this work were to analyze the assemblages of microorganisms that inhabit the RW of La Plata and Magdalena (Buenos Aires), relate their variations to anthropogenic impact and evaluate how the microbiota can be an indicator of these environmental changes. Benthic diatoms, phytoplankton, zooplankton, ostracods, and fungi assemblages of four RW were studied: two of peri-urban location with agricultural and horticultural land use (Del Gato and Carnaval), and two of rural location and livestock land use (Cajaravillas and Chubichaminí). Differences linked to land uses were clearly expressed in most groups, with RWs grouped into rural (with more species sensitive to pollution and eutrophication) and peri-urban (with more species highly tolerant). In the latter, both the main taxonomic groups and the trophic levels were more restricted and more closely related to the detritivorous pathway than to the photosynthetic one, and the strategies to face water stress were emphasized. The results show the effects of human pressure, the importance that microorganisms acquire in its evaluation and the need of an adequate use of the territory to avoid the loss of functions and benefits of these environments., Instituto de Limnología "Dr. Raúl A. Ringuelet"

The goal of this study was to analyse the presence, abundance, types, size and colours of microplastics (MPs) in the soft tissue of the golden mussel Limnoperna fortunei in the freshwater-mixohaline tidal zone of Río de la Plata estuary. 240 mussels were analysed and the MPs were observed in the 96% of samples, being more abundant in the mussels that settled down close to the main sewage discharges, and in the maximum turbidity front. The average number of MPs/g and MPs/ind was 2.08 and 0.43, respectively. The 90% of MPs, were fibres and the most frequent sizes were >500 ≤1000 µm. The high density of L. fortunei on the coast of the Río de la Plata, its potential predators, and the presence of MPs in the soft tissue, warn the important role this mussel plays in the transfer of this pollutant through the trophic web of the estuary. Fil: Pazos, Rocío Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Spaccesi, Fernando Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

Fil: Cochero, Joaquin. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Limnologia "Dr. Raul A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnologia; Argentina

Fil: Paz, Laura Estefania. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Limnologia "Dr. Raul A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnologia; Argentina

The picoplankton is an important component of aquatic food webs and plays a significant biogeochemical and ecological role in the environment. Little is known about this fraction of the plankton in temperate estuaries and especially in South America. In this article, we study the absolute and relative importance of the picoplankton along an annual cycle, and their relationship with physical and chemical variables in the Rio de la Plata estuary. We also review the existing research in estuaries around the world concerning this community and present our results in a global context. The seasonal variation in the abundance of the different groups analyzed was very noticeable. Phycocyanin-rich picocyanobacteria (Pcy) were the main component (in abundance and in biovolume) of the picophytoplankton (PPP) almost during the whole year, with a maximum abundance of 7.3 × 105 cell mL−1 in summer, three orders of magnitude higher compared to autumn-winter. Picoeukaryotes, larger phytoplankton and heterotrophic bacteria showed the same seasonal trend, although with a lower range of variation than that of the Pcy. Considering all the phototrophic planktonic fractions, in terms of biomass, the PPP reached a maximum of 43% of the total biomass in spring. The dynamics of PPP found in this area are consistent with the observed in other temperate estuaries, where temperature is the main variable that influences its development, and with a high seasonal variation. Additionally, the absolute and relative importance of Pcy showed a consistently increasing trend towards lower latitude estuaries. The review also showed us that there is scarce information related to the picoplankton fraction in the Southern Hemisphere, its sanitary implications due to their potential of toxicity or their ecological role in coastal zones. The results presented here show the importance of this fraction, not only in Rio de la Plata, but in many estuaries of the world, with a clear increase of relative abundance as we approach the equator.

The colonisation patterns of diatom assemblages were studied on artificial substrates in lowland streams with different urban pollution levels. Density, diversity, colonisation rates, ecological guilds and cell size classes of the assemblage at each of two sites were followed for 1 week. In addition, assemblages growing on glass, in natural epipelic biofilm and in water were compared at each site. At both sites, diatom colonisation on glass substrates had already started after 30-min exposure. At the less polluted site, total diatom density was higher, along with the proportion of low-profile and motile species, and there was a higher amount of diatoms in size class 5 (>1500μm3). At the more polluted site, species from the high-profile guild and size class 3 (300–599μm3) dominated. Ecological traits (ecological guilds and size class) were not sensitive enough descriptors of early colonisation in nutrient-rich lowland, but the largest cells constituted the first settlers. The results show that the assemblage developed on artificial substrata was a good proxy of the composition in the sediment.

Agricultural activities can affect the delivery of nutrients to streams, riparian canopy cover, and the capacity of aquatic systems to process nutrients and sediments. There are few measures of nutrient uptake and metabolism from tropical or subtropical streams in general, and even fewer from tropical regions of South America. We examined ammonium (NH₄ ⁺) and soluble reactive phosphorus (SRP) retention in streams in Brazil and Argentina. We selected 12 streams with relatively little or extensive agricultural activity and conducted whole-stream nutrient additions and measurements of gross primary production and ecosystem respiration. We used multiple linear regression to determine potential drivers of nutrient uptake metrics across the streams. Nutrient concentrations and retention differed significantly between land use categories. Both NH₄ ⁺ and SRP concentrations were higher in the agricultural sites (means of 161 and 495 μg l⁻¹, respectively), whereas metabolic rates were slower and transient storage was smaller. Our analysis indicated that agriculture increased ambient uptake lengths and decreased uptake velocities. The regression models revealed that ambient SRP had a positive effect on NH₄ ⁺ uptake and vice versa, suggesting strong stoichiometric controls. Drivers for nutrient uptake in streams with low-intensity agriculture also included canopy cover, temperature, and ecosystem respiration rates. Nutrient assimilation in agricultural sites was influenced by a higher number of variables (gross primary production for SRP, discharge, and transient storage for both nutrients). Our results indicate agricultural activity changes both the magnitude of in-stream nutrient uptake and the mechanisms that control its variation, with important implications for South American streams under agricultural intensification., Instituto de Limnología "Dr. Raúl A. Ringuelet"

In this study we explored the use of less conventional diatom metrics (motility, nuclear abnormalities and ecological guilds) to measure changes in diatom assemblages related to the transfer of epipelic communities from urban impacted streams to less impacted sites. Three lowland streams in the Argentine Pampean plain were selected, and two sites were established per stream for a total of six sites. Three sites were referenced as low urban impact (LI), as they run through peri-urban areas, and three downstream sites were referenced as high impact sites (HI), since they run through a dense urban area. Six germination trays filled with stream sediment were placed at each HI site, and three at each LI site, and left to be colonized for 30 days before transferring three trays from the HI sites to their respective LI site upstream. Samples were collected at days 0, 15, 30 and 45 days from each tray, and diatom species were identified and classified into ecological guilds. Motility variables were measure per individual of the genera Nitzschia (path length, velocity and maximum velocity) through video recording. Also, the frequency of abnormal nuclear locations and nuclear membrane breakage were determined. Results showed that there were no significant differences in any motility metrics, while the nuclei variables exhibited higher proportion of abnormally positioned nuclei and nuclear membrane breakage at the HI sites. The proportion of nuclear membrane breakage decreased over time in the translocated assemblage, indicating that it could be a sensitive indicator of water quality improvement, while the ecological guilds showed significant differences on the proportion of high profile and motile diatoms. The improvement in water quality produced a significative increase in the proportion of the high profile guild in the translocated biofilm. Despite the motile forms being the dominant group, the water quality improvement allowed the high profile forms, to colonize the substrate. These results emphasize the possible use of the ecological guilds as a suitable indicator of water quality improvement in Pampean streams, especially in nutrient rich environment. These tools can provide a rapid assessment of diatom condition and could be considered supplementary to biomonitoring protocols in lowland streams. Fil: Nicolosi Gelis, María Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Cochero, Joaquin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Donadelli, Jorge Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

We analyzed the diatom assemblages inhabiting the epipelic biofilm of a Pampean stream, characterized by their high basal nutrient levels, when exposed to a continuous surplus of inorganic nutrients. An in situ experience was conducted, increasing concentrations of N and P in water 3-fold from the basal concentration. Nutrient enrichment was achieved by the use of fertilizer bags distributed along the reach. The period of exposure was of 14 months. The effects of nutrient enrichment were analyzed following a BACIPS ANOVA design. The changes in nutrient concentration were associated with a significant increase in diatom density and a decrease in species richness and diversity. The additional nutrient load also caused the change in the diatom taxa proportion, favoring motile forms, Nitzschia species mainly. The fertilization in La Choza, caused a mild to moderate effect, indeed not immediate, on the diatom assemblage. These delayed responses of moderate intensity could be related with intrinsic characteristics of diatom assemblages pre-adapted to nutrient-rich environments. The rising urbanization and agricultural activity in the Pampean plain, may seriously impair the biodiversity of its rivers if the entrance of nutrients to these ecosystems is not mitigated., Instituto de Limnología "Dr. Raúl A. Ringuelet"

The aim of this study was to explore the responses of the epipelic biofilm of a Pampean stream with little impact from human activity to two environmental conditions, with joint modifications in nutrients, temperature, water velocity and turbidity. The experiment was conducted using artificial channels and lasted five weeks. The biological variables measured included chlorophyll-a content, bacterial biomass, ash-free dry weight, total carbohydrate concentration, total respiratory activity, and biofilm composition. Results show that the species´ composition of the biofilm was affected, although no other structural or metabolic variables measured were. These results highlight the importance of including structural parameters to measure rapid changes in water quality, even when analyzing the effects of co-occurring variables., Instituto de Limnología "Dr. Raúl A. Ringuelet", Consejo Nacional de Investigaciones Científicas y Técnicas

The objective of the present article is to propose an index that allows the assessment of the fluvial habitat quality in lowland streams that run through urbanized areas, by the use of metrics related to the quality of the watercourse, the river banks, the riparian zone and the fluvial geomorphology. The metrics retained in the index (USHI, Urban Stream Habitat Index) include the cover percentage and quality of the aquatic vegetation; the main features of the river banks; the presence of exotic trees, of litter, of permanent structures (such as buildings) in the riparian zone and other major geomorphological alterations, such as dredging or channelization. The index is related to physical-chemical parameters that are linked to water quality, the imperviousness of the watershed and to other biotic descriptors, particularly the macroinvertebrate and diatom assemblages. The values of the index for the sites in the studied area revealed that 41.2% have a bad or very bad habitat quality, 27.8% a moderate habitat quality, while 31% have a good or very good habitat quality. The main issue detected in the studied sites involved the dredging or partial channelization of the reaches. Unlike other indices that evaluate the quality of the physical habitat through the use of the diatoms or macroinvertebrate communities, the USHI can be interpreted as a measure of the overall quality of the habitat, and uses indicators that do not require the identification of taxa, making it more accessible to non-specialists. Therefore it provides with a tool to evaluate the fluvial habitat quality of lowland streams that can be easily applied, particularly by professionals that take part in the management and decision making process regarding urbanized watersheds., Instituto de Limnología "Dr. Raúl A. Ringuelet"

The aim of this study was to propose an index [Index of Biotic Integrity for the Rio de la Plata (IBIRP)] formulated from a minimum of information-rich metrics related to biological integrity for evaluation of the coastal freshwater tidal zone of the Rio de la Plata estuary. A total of 23 sites were selected along 170 km of shoreline (34°27′10″ S, 58°30′21″ W and 35°23′28″ S, 57°08′50″ W), being influenced by different land uses. We explored 36 biotic descriptors (designated candidate metrics), each of which is considered as an environmental indicator, and selected four metrics: two related to phytoplankton assemblage (planktonic cyanobacterial density and phytoplanktonic total density) and two linked to benthic assemblages, the diatoms (percent most tolerant taxa), and the invertebrates (percent Tanaidacea). The metrics that were chosen to integrate the IBIRP provide valuable information about the biotic integrity both in terms of time and space. The multimetric index can assume values between 0 (very bad status) and 10 (very good status). The IBIRP correlated significantly with the second (p < 0.001) and fourth (p < 0.01) factors of the factor analysis—those being related to variables indicative of eutrophication and organic pollution. This index constitutes an approach that is capable of reflecting the anthropogenic impacts produced along the coast of the Rio de la Plata, with the inherent flexibility of being subject to modification and ultimate validation for estuaries of similar characteristics.

International audience; Since biofilms integrate the environmental effects of water chemistry, along with the physical and geomorphological characteristics of rivers and lakes, they have been widely applied in biomonitoring. In particular, diatoms are extensively used as reliable environmental indicators. Diatoms are microscopic, unicellular brown algae, which often dominate the algal biomass of biofilms. The shape and morphology of the siliceous skeleton, the frustules, unique to each taxon are used for taxonomical identification. The floras are diverse, in relation to their geographical location (climate, geology, relief) and to the quality of the aquatic environments they inhabit. Indeed, species are sensitive to the water physicochemical parameters and their presence/abundance is therefore correlated to water quality. Diatom sensitivity or tolerance towards different environmental parameters has long been studied and used to implement bioassessment methods. Such methods evolved from indices of saprobity designed first for European streams, to developments of various diatom indicators worldwide, able to highlight different types of pollution (pH, salinity, nutrients, toxicants). The objective of this chapter is to provide scientists and water managers with a broad overview of diatom tools helpful to monitor the ecological status of freshwater environments. We describe the applicability range and the limitations of the main existing methods, metrics (indices, traits) and types of surveys used, as well as the challenges faced by scientists to improve routine biomonitoring.

The structural features and O2 consumption of the epipelic biofilm in streams of the Pampean plain were explored. The study was conducted in three lowland streams subjected to different anthropic disturbances. Three sampling sites were selected in different sectors of these streams considering land use intensity (high, moderate, and low). Samples of the water and of the epipelic biofilm were taken seasonally. El Pescado stream is subjected to a low level of human impact and showed lower organic matter and nutrient contents than the Rodríguez and Don Carlos streams which are subjected to moderate and high levels of human impact. The biofilm composition of the three streams was represented by cyanophytes and diatoms but with different species composition and dominance; protozoans and nematodes were the characteristic heterotrophic groups in the three streams. The Rodríguez and Don Carlos streams showed the highest abundance of organisms. Multiple regression showed that O2 consumption, chlorophyll a and trophic index were significantly correlated with the oxygen demands. On the other hand, the Rodríguez and Don Carlos streams exhibited significant differences with the El Pescado stream in O2 consumption, trophic index, and chlorophyll a content. Our results demonstrated that the different biological descriptors responded to environmental variables that are influenced by the different land use intensities, being chlorophyll a, abundance of organisms, and O2 consumption the most sensitive variables to the changes water quality. Fil: Sierra, Maria Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina

The aim of this study was to analyze the density, diversity, biomass and assemblage composition of the phytoplankton in relation to environmental conditions (physical, chemical, hydrological and meteorological variables), measured under the different scenarios caused by the ENSO phenomenon in the period between 2005 and 2012, in six sampling sites in the tidal freshwater zone of the Río de la Plata estuary, covering almost 100 km of coastline. The results revealed changes in the structure of the phytoplankton, such as a significant reduction of diversity, and decreases in biomass and phytoplankton density, particularly during El Niño phases. Cyanobacteria were more abundant in the neutral periods, Chlorophyceae dominated La Niña phase while Bacyllariophyceae dominated El Niño. However, no complete replacement of species between cycles was observed. The results obtained were highly variable due to the inherent natural variability of the Río de la Plata, emphasized by the anthropogenic impact in this area., Instituto de Limnología "Dr. Raúl A. Ringuelet"

Benthic diatoms are often used for assessing environmental conditions, such as water quality and habitat conditions in stream and river systems. Although laboratory experiments have shown that each diatom species have different levels of tolerance to different stressors, few studies have been conducted in laboratory settings that analyze the responses of the diatom assemblage to the effects of multiple simultaneous variables. The aim of this study was to evaluate some structural responses (such as species composition and diversity) of the diatom assemblage on a short time scale to the effects of the simultaneous increase in four variables that are directly linked to the environmental changes affecting the Pampean streams: turbidity, nutrients (phosphorous and nitrogen), water velocity and temperature. To this end we conducted a five-week laboratory experiment using artificial channels where we simulated two environmental conditions (LOW and HIGH) employing epipelic biofilm from a mesotrophic stream. The results obtained in the experiment show that the structure of the diatom assemblage in the epipelic biofilm is affected by the simultaneous modification of temperature, water velocity, nutrient concentration and turbidity. These modifications in the assemblage included moderate decreases in diversity, small decreases in the proportion of species sensitive to eutrophication and saprobity, moderate increases in the IDP (Pampean Diatom Index) values and moderate changes in the percentages of the stalked growth-forms. The relative abundance of species such as Luticola mutica, Navicula cryptocephala and Navicula lanceolata were negatively affected by both treatments; other species such as Planothidium lanceolatum, Caloneis bacillum, Encyonema minutum, Humidophila contenta, Luticola kotschyi, Nitzschia amphibia, Navicula veneta, Pinnularia subcapitata var. subcapitata were positively affected by the HIGH treatment; and Nitzschia fonticola was positively affected by both treatments. The results suggest that, in the very short term of the bioassay conducted, the diatom assemblage can modify its structure to respond in a sensitive manner to the abrupt changes in multiple physical-chemical variables. Fil: Cochero, Joaquin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - la Plata. Instituto de Limnología "dr. Raul A. Ringuelet". Universidad Nacional de la Plata. Facultad de Cs.naturales y Museo. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - la Plata. Instituto de Limnología "dr. Raul A. Ringuelet". Universidad Nacional de la Plata. Facultad de Cs.naturales y Museo. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - la Plata. Instituto de Limnología "dr. Raul A. Ringuelet". Universidad Nacional de la Plata. Facultad de Cs.naturales y Museo. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina

The Rio de la Plata is located on the East coast of South America and is a shallow, large-scale, turbid coastal plain estuary that covers an approximate area of 35,000 km 2 . Despite the socio-economic importance of the Rio de la Plata, studies related to the biological aspects of this ecosystem are scarce, particularly in the freshwater tidal zone. The objective of this study was to explore the diatom composition and distribution of density, biomass and empty frustules along the fluvial–mixohaline axis. Furthermore it was to analyze the spatial succession of diatom assemblages and the environmental variables during an extensive sampling carried out during spring 2001. Two replicate sub-surface water samples were collected at 29 sites for quantitative analysis of phytoplankton. The greatest number of species, mainly pennate taxa, were observed in the freshwater tidal zone. The average density of the diatoms was 59 cells ml − 1 . Chains or filaments of centric diatoms were frequent and dominant in the samples along the fluvial–mixohaline axis. The carbon content of the diatoms increased downstream, with an average value of 2.5 μg C l − 1 in the freshwater tidal zone and 4.4 μg C l − 1 in the mixohaline zone. Diatoms supplied 65% of the total phytoplankton carbon content in the freshwater tidal zone and reached 17% in the mixohaline zone. Canonical Correspondence Analysis (CCA) allowed us to identify two species assemblages in the Rio de la Plata differentiated mainly by salinity, pH and silicate gradients. In our study species exclusive to brackish waters were not identified, but some freshwater and marine taxa with wide ranges of salinity tolerance were observed.

Phytoplankton community composition, structure and biomass, spatial distribution patterns in relation to abiotic factors and life-form strategies were assessed in spring 2001 in the Río de la Plata estuary. 224 taxa were identified, with a mean total density of 110 cell ml-1. Although cell numbers were of the same order of magnitude in the upper freshwater tidal zone and in the mouth of the estuary, the maximum carbon contents of phytoplankton were observed in the latter, due to the presence of large dinoflagellate cells. Diversity values ranged between 0.3 and 3.2 bits.ind.-1 Chlorophytes and cyanophytes were dominant upstream; diatoms were the most important downstream (in the maximum turbidity front). In the outer, mixohaline zone, diatoms and pyrrophytes were dominant. The phytoplankton of 48% of the Río de la Plata estuary is dominated by riverine specimens. The dominant phytoplankton morphologies in the Río de la Plata were filaments (Planctonema lauterbornii, Ulothrix cf. subconstricta) or chains (Aulacoseira spp., Skeletonema costatum), which provide extensive light absorbing surfaces. Canonical correspondence analysis allowed identification of two species assemblages, one containing freshwater taxa and one with brackish-marine species. In the first group it was recognised that there was a secondary grouping due to the light gradient. Along the fluvial-mixohaline axis it was recognised that the dominant R-strategy species were replaced by S-strategists in the outer sector of the estuary. Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Hualde, Paula R.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Bauer, Delia Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

The objective of this study is to determine which are the characteristic algal communities coating the stems of Scirpus califomicus (MEY) STEUD in the Rio de la Plata. Taxonomic composition, disposition of the algae in the biofilm, spatial patterns of abundance and species diversity and life-form strategies were analysed. Samples of periphyton were taken in winter and spring 1999 and summer and autumn 2000, at five sampling sites. Physical-chemical characteristics were measured in summer and autumn 2000. Ten stems of the bulrush were cut randomly and the bottom 15 cm kept and put into a flask with distilled water. The biofilm was removed by brushing, integrating the material collected in each sampling site. The algae that comprised the biofilm analysed showed a simple architecture consisting of a layer dominated mainly by biraphid diatoms, with planktonic diatoms and/or filamentous chlorophytes entrapped in the surface film. The matrix analysed in this study corresponds to early stages of succession characterized by R-selected taxa, considered to be pioneer species and good colonizers in a disturbed system. During spring, winter and autumn, diatoms were dominant, but with different species compositions, while in summer green algae were abundant. The epiphyton of the bulrushes studied was characterized by a low proportion of epiphytic specimens sensu strictu (adnate, stalked and erect forms) exhibiting a high proportion of immigrants from the benthos and plankton.

The diatom community growing on cobbles and sand substrata along the Cuarto River (Cordoba, Argentina) was studied during 2000 and 2001. Multivariate analyses of the data (PCA and CCA) showed distinct differences in water chemistry and substrata types between the upstream sites (sites 1–10) and downstream sites (sites 11–19). Sites 1–10 supported an epilithic diatom community associated with low water conductivity and gravel substrata. This consisted of adnate Achnanthes (A. biasolettiana, A. minutissima), as well as stalked (Gomphonema aff. angustum, Reimeria uniseriata, Fragilaria capucina var. rumpens) or prostrate (Nitzschia lacuum) taxa. Downstream sites were associated with high conductivity, fast flowing waters and finer substrata (sand, silt), and were colonised by prostrate diatoms, including several species of Navicula (N. pupula, N. mutica, N. veneta, N. insociabilis) and Nitzschia (N. umbonata, N. palea). Variations in water flow caused significant changes in the diatom communities of the river. During periods of low flow (winter and autumn), chain forming (Diatoma vulgaris, D. moniliformis) or stalked (Synedra ulna) taxa partially replaced the former community of Navicula and Nitzschia in the downstream sites. High flow (in summer) led to diatoms of large size being replaced by smaller size diatoms, such as Achnanthes lanceolata, Navicula mutica, Hantzschia amphioxys and Amphora montana. The severe effect of floods in the lower part of the Cuarto led to these taxa having a much higher proportion in the lower stretch of the river, taking advantage of the subaerial conditions created by the floods. Local episodes of water pollution were associated with a transient shift towards the dominance of Navicula pupula, Synedra ulna, Nitzschia lacuum and Reimeria uniseriata during winter, when inputs were least diluted because of the low flow.

(2002). Planktonic and periphytic coastal algae of the Rio de la Plata, Argentina. SIL Proceedings, 1922-2010: Vol. 28, No. 1, pp. 250-253.

(2002). Ecological status of the Pampean plain streams and rivers (Argentina) SIL Proceedings, 1922-2010: Vol. 28, No. 1, pp. 259-262.

The theory of ecological stoichiometry holds that heterotrophs are mostly homeostatic and exhibit less variation in body stoichiometry than do autotrophs. Most studies of stream foodweb stoichiometry have been done in low-nutrient environments. Little is known about foodweb stoichiometry in nutrient-rich streams, in which a higher level of stoichiometric homeostasis should be expected, mainly because imbalances between resources and consumers are low and nutrient availability may meet biotic requirements. We analyzed elemental content (C, N, P) and stoichiometric ratios (C:N, C:P, N:P) of basal resources, macroinvertebrates, and fishes in a nutrient-rich Pampean stream and compared these values to those from other studies. We manipulated P and N in a 1-y fertilization experiment to analyze biotic stoichiometric responses to additional nutrient input to this naturally enriched system. Soluble reactive P concentration in the treatment reach was doubled relative to the background concentration. Consumers had lower C:P and N:P than those in other lotic systems, whereas P content and C:P and N:P of basal resources were within the ranges observed for other systems. Most components of the trophic web were not affected by fertilization, and only epiphyton, fine benthic organic matter, and macroinvertebrate species (Palaemonetes argentinus and Pomacea canaliculata) changed their nutrient content or stoichiometric ratios. Imbalances in C:N and C:P occurred between primary consumers and their resources, particularly among macroinvertebrate collectors and detritivorous fishes feeding on FBOM. Most basal resources and consumers were strictly homeostatic for P content and the stoichiometric ratios, but a lower degree of homeostasis occurred in the epiphyton, P. canaliculata, and collectors feeding on epiphyton. A high degree of stoichiometric homeostasis exists across the various components of the food web in this nutrient rich stream, regardless of their trophic position. Fil: Feijoó, Claudia Silvina. Universidad Nacional de Lujan. Departamento de Cs.basicas. Laboratorio de Ecologia; Argentina Fil: Leggieri, Leonardo Ramón. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación En Biodiversidad y Medioambiente; Argentina. Universidad Nacional de Luján; Argentina Fil: Ocon, Carolina Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Muñoz, Isabel. Universidad de Barcelona. Facultad de Biologia. Departamento de Ecologia; España Fil: Rodrigues Capitulo, Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Giorgi, Adonis David Nazareno. Universidad Nacional de Lujan. Departamento de Cs.basicas. Programa Ecologia de Protistas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Colautti, Dario César. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús (San Martin); Argentina Fil: Ferreiro, Nicolas Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación En Biodiversidad y Medioambiente; Argentina. Universidad Nacional de Luján; Argentina Fil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Limnología "dr. Raul A. Ringuelet"; Argentina Fil: Sabater Cortés, Sergi. Universidad de Barcelona. Facultad de Biologia. Departamento de Ecologia; España. Institut Català de Recerca de l; España. Universidad de Girona; España

During the fall and winter of 1995, ninety-two samples were collected from twenty-three sampling stations in the Matanza-Riachuelo river basin. Epipelic diatoms and physico-chemical parameters were studied to establish water quality. Ecological methodologies (species richness, diversity and saprobity indices and multivariate analysis) were employed to assess the impact of the pollution on diatom assemblages. Suspended solids, biological and chemical oxygen demands increased downstream, and dissolved oxygen, pH and transparency increased upstream. Conductivity was variable. This fluvial system is affected by organic and inorganic pollutants, many of them toxic. The presence of heavy metals, phenols, pesticides, hydrocarbons among other chemicals, mostly account for the impoverishment of the diatom assemblages of the basin, thus leading to changes in assemblages structure. Changes in water quality were better reflected by species richness, diversity indices and by principal component analysis than pollution indices in this basin.

Epipelic diatom assemblages have been used to evaluate water quality in the Matanza-Riachuelo river basin. Twenty-three sites were sampled in April, May, June and August 1995, before the setup of restoration programs. The Matanza-Riachuelo river basin is located in N.E. Buenos Aires province. It runs across one of the most industrialized and populous zones of Argentina, for which it is subject to strong anthropogenic disturbances. A total number of 97 taxa were identified of which only 37 were abundant. Navicula cryptocephala, Nitzschia palea and N. umbonata occur in the whole basin though more frequent downstream; Stephanodiscus hantzschii and Cyclotella meneghiniana develop where the river becomes wider and deeper. The distribution of the diatomological flora in the Matanza-Riachuelo basin was affected by the increasing concentration of organic matter carried by the river and downstream for the input of chemical compounds, several of which are toxic to aquatic life (i.e. heavy metals, phenols, hydrocarbon, etc.). The assemblage was dominated by pollution-tolerant species, which implies strong to very strong contamination in a great portion of the basin.

Structural characteristics of spring phytoplankton from the Southern Coastal Fringe of the Rio de la Plata between 58°32′ and 57°41′ W, up to 10 km from the coast line are analysed. From 126 recorded taxa, 69 had a relative frequency above 5%. Main frequencies coincided with main quantitative abundances, corresponding to centric diatoms of the genus Aulacoseira – A. granulata var. augustissima especially – Stephanodiscus hantzschii and the coccal green Dictyosphaerium pulchellum and D. subsolitarium; the exception was the Cyanophyceae Microcystis aeruginosa, which showed the greatest quantitative abundance, but was not among the most frequent species. Centric diatoms accounted for most of the biovolume. Average phytoplankton density was 259 cells ml-1 (11–3700 cells ml-1). Diversity index (H′) varied from 0.51 to 2.78 bits ind-1. Phytoplankton size structure was: 43% picoplankton, 56% nanoplankton and 1% microplankton. Quantitative data on selected species were assessed for similarity among stations, 74% are joined in a group dominated by Aulacoseira sp. pl. and codominated by coccal green algae. The groups dominated by Stephanodiscus hantzschii and Microcystis aeruginosa included stations affected by pollution. Algal patchiness was greatest following low tide river currents. The assessed phytoplankton was eutrophic, oligohalophilous and mesosaprobic, a composition in agreement with some characteristics of a true potamoplankton.

El objetivo del presente estudio fue analizar las respuestas estructurales (densidad de organismos, biomasa total, biomasa autotrófica e índice autotrófico) y funcionales (producción primaria neta, respiración, producción primaria bruta y tasa de asimilación) del biofilm epipélico en tres sitios de un arroyo pampeano que presentan diferentes proporciones del uso del suelo expuesto a la urbanización. Los resultados mostraron que el aumento en el porcentaje del uso del suelo destinado a la urbanización fue evidenciado por los cambios en la calidad del agua y por los descriptores funcionales y estructurales del biofilm epipélico. En tal sentido, la calidad del agua disminuyó en los sitios más urbanizados como consecuencia principalmente del incremento de nutrientes. Esto repercutió en la producción primaria neta, la tasa de asimilación y el cociente producción primaria bruta/respiración, reflejando que el biofilm fue marcadamente autotrófico y más eficiente en términos fotosintéticos. Estos cambios en los descriptores metabólicos se condicen con el patrón seguido por la clorofila, que reflejó un cambio en el estado trófico de mesotrófico a eutrófico. Asimismo, la composición específica en aquellos sitios mostró cambios a favor del incremento de organismos tolerantes a la polución., The purpose of the present study was to analyze the structural (organisms density, total biomass, autotrophic biomass and autotrophic index) and functional (net primary production, respiration, gross primary production and assimilation rate) responses of the epipelic biofilm at three sites of a Pampean stream presenting different proportions of land use exposed to urbanization. The results showed that the increase in the percentage of land use destined to urbanization was evidenced by the changes in the water quality and by the functional and structural descriptors of the epipelic biofilm. In this regard, the quality of water diminished at the most urbanized sites mainly as a consequence of the nutrient increase. This affected the net primary production, the assimilation rate and the gross primary production/ respiration ratio, reflecting that the biofilm was markedly autotrophic and more efficient photosynthetically. These changes in the metabolic descriptors are coincident with the pattern followed by the chlorophyll which reflected a change in the trophic state from mesotrophic to eutrophic. Likewise, the specific composition in those sites showed changes favoring the increase of organisms tolerant to pollution., Instituto de Limnología "Dr. Raúl A. Ringuelet", Instituto de Botánica "Dr. Carlos Spegazzini"

We assessed the effects of nutrient enrichment on three stream ecosystems running through distinct biomes (Mediterranean, Pampean and Andean). We increased the concentrations of N and P in the stream water 1.6–4-fold following a before–after control–impact paired series (BACIPS) design in each stream, and evaluated changes in the biomass of bacteria, primary producers, invertebrates and fish in the enriched (E) versus control (C) reaches after nutrient addition through a predictive-BACIPS approach. The treatment produced variable biomass responses (2–77% of explained variance) among biological communities and streams. The greatest biomass response was observed for algae in the Andean stream (77% of the variance), although fish also showed important biomass responses (about 9–48%). The strongest biomass response to enrichment (77% in all biological compartments) was found in the Andean stream. The magnitude and seasonality of biomass responses to enrichment were highly site specific, often depending on the basal nutrient concentration and on windows of ecological opportunity (periods when environmental constraints other than nutrients do not limit biomass growth). The Pampean stream, with high basal nutrient concentrations, showed a weak response to enrichment (except for invertebrates), whereas the greater responses of Andean stream communities were presumably favored by wider windows of ecological opportunity in comparison to those from the Mediterranean stream. Despite variation among sites, enrichment globally stimulated the algal-based food webs (algae and invertebrate grazers) but not the detritus-based food webs (bacteria and invertebrate shredders). This study shows that nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but that its magnitude and extent within the food web are complex and are strongly determined by environmental factors and ecosystem structure., Instituto de Limnología "Dr. Raúl A. Ringuelet"

La valoración ambiental de las riberas es una tarea necesaria para conocer el estado ecológico de los cursos de agua, lo cual permite evaluar los méritos de eventuales experiencias de manejo, impactos antropogénicos o bien detectar problemas ambientales de forma temprana. En tal sentido, el objetivo de este estudio fue desarrollar una metodología para evaluar la calidad del hábitat de la Franja Costera Sur del Río de la Plata. La información obtenida de 21 sitios de muestreo, visitados entre los años 2005 y 2008, fue resumida y simplificada en un índice, denominado Índice del Hábitat para el Río de la Plata (IHRPlata). Para obtener el índice se requiere evaluar cuatro descriptores: 1) sucesión espacial de la vegetación costera, 2) modificaciones costeras por introducción de infraestructuras, 3) ocurrencia de residuos en la línea de costa, y 4) indicadores biológicos de déficit de oxígeno. Según los valores del IHRPlata obtenidos, 32% de los casos analizados revelaron una calidad del hábitat muy buena, 20% buena, 24% moderada, 18% mala y 6% muy mala. Los valores del índice alcanzaron correlaciones significativas con parámetros relacionados con la calidad del agua y con indicadores de la calidad biótica, lo que fortalece su utilidad para evaluar la calidad del hábitat costero., Evaluating the coastal environment is a necessary task to assess the ecological status of water bodies, which allows to determine the merits of management programs, anthropogenic impacts, or to detect early signs of environmental impacts. To this purpose, the objective of this research was to develop a methodology to assess the habitat quality in the Franja Costera Sur of the Río de la Plata. The information obtained from 21 sampling sites, visited from 2005 to 2008, is summarized and simplified in an index, named the Habitat Index for the Río de la Plata (IHRPlata). To obtain the index four separate descriptors are to be evaluated: 1) the spatial succession of the vegetation, 2) any man-introduced infrastructures in the coast, 3) the presence of litter and debris in the coastline, and 4) the visualization of biological indicators that imply oxygen deficit. According to the obtained values for the IHRPlata, 32% of the analyzed cases revealed a very good habitat quality, 20% good, 24% moderate, 18% bad and 6% very bad habitat quality. The obtained values were also significantly correlated with parameters related to water quality and to biotic indicators, which strengthens its usefulness to evaluate the coastal habitat., Instituto de Limnología "Dr. Raúl A. Ringuelet"

Diatoms are an integral and often dominant component of the benthic microalgal assemblage in estuarine and shallow coastal environments. Different toxic substances discharged into these ecosystems persist in the water, sediments, and biota for long periods. Among these pernicious agents, the toxicity in diatoms by metal is linked to different steps in the transmembrane and internal movements of the toxicant, causing perturbations in the normal structural and functional cellular components. These changes constitute an early, nontaxonomic warning signal that could potentially serve as an indicator of this type of pollution. The aim of this work was to study the environment-reflecting short-term responses at different levels of organization of epipsammic diatoms from the Río de la Plata estuary, Argentina that had been exposed to hexavalent chromium within experimental microcosms. To this end we monitored: (i) changes in the proportion of the diatoms in relation to other algal groups at the biofilm community level; (ii) shifts in species composition at the diatom-assemblage level; (iii) projected changes in the densities of the most representative species at the population level through comparison of relative growth rates and generation times; and (iv) the cytological changes at the cellular and subcellular levels as indicated by the appearance of teratological effects on individuals and nuclear alterations. The epipsammic biofilms were exposed for 96 h to chromium at a concentration similar to that measured in highly impacted sites along the coast (80 μg L−1). Chromium pollution, at this concentration and short exposure time did not affect the algal biomass and density of these mature biofilms. The biofilm composition, however, did change, as reflected in a decline in cyanophytes and an increment in the proportions of diatoms and chlorophytes; with Hippodonta hungarica, Navicula novaesiberica, Nitzschia palea, and Sellaphora pupula being the most frequent and abundant species. The most notable shifts related to chromium exposure were a decrease in the relative abundance of H. hungarica and a significant increase in the proportion of N. palea. Moreover, the species analyzed in the treatment microcosms showed higher growth rates than in the controls – N. palea grew faster, while H. hungarica replicated more slowly. The total nuclear abnormalities – as recorded in Fallacia pygmaea and N. novaesiberica – were significantly higher in the treatment microcosms; whereas in N. palea, the dominant species in treatment microcosms, neither nuclear alterations nor abnormal frustules were observed. Fil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raul A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raul A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

Our objective was to assess the effect of the physical habitat degradation in three lowland streams of Argentina that are subject to different land uses. To address this matter, we looked into some physical habitat alterations, mainly the water quality and channel changes, the impact on macrophytes’ community, and the structural and functional descriptors of the epipelic biofilm and invertebrate assemblages. As a consequence of physical and chemical perturbations, we differentiated sampling sites with different degradation levels. The low degraded sites were affected mainly for the suburban land use, the moderately degraded sites for the rural land use, and the highly degraded sites for the urban land use. The data shows that the biotic descriptors that best reflected the environmental degradation were vegetation cover and macrophytes richness, the dominance of tolerant species (epipelic biofilm and invertebrates), algal biomass, O₂ consumption by the epipelic biofilm, and invertebrates’ richness and diversity. Furthermore, the results obtained highlight the importance of the macrophytes in the lowland streams, where there is a poor diversification of abiotic substrates and where the macrophytes not only provide shelter but also a food source for invertebrates and other trophic levels such as fish. We also noted that both in benthic communities, invertebrates and epipelic biofilm supplied different information: the habitat’s physical structure provided by the macrophytes influenced mainly the invertebrate descriptors; meanwhile, the water quality mainly influenced most of the epipelic biofilm descriptors., Facultad de Ciencias Naturales y Museo, Instituto de Limnología "Dr. Raúl A. Ringuelet"

The rivers and streams in the pampean plains are characterized by a low flow rate due to the low slope of the surrounding terrain, high levels of suspended solids, silty sediment in the benthos, and reduced rithron; the riparian forest of this region has been replaced by low-altitude grasslands. Many of these environments contain a wide coverage of aquatic reeds, both submerged and floating, making the pampas limologically extraordinary. These terrains have undergone a gradual transformation in response to the progress of urbanization and agricultural activity in recent years with a resulting loss of biodiversity, leaving only few sites that continue to reflect the original characteristics of the region. Because of human activities in combination with the global climate change, variations have occurred in biological communities that are reflected in the structure and function of populations and assemblages of algae, macrophytes, and invertebrate fauna or in the eutrophication of affected ecosystems. The objective of this article is to describe the principal limnologic characteristics of the streams that traverse the Buenos Aires Province and relate these features with the predicted future global changes for the area under study. Considering the future climate-change scenarios proposed for the pampean region, the projected increment in rainfall will affect the biological communities. Higher rainfall may enhance the erosion and generate floodings; increasing the transport of sediments, nutrients, and contaminants to the ocean and affecting the degree of water mineralization. Changes in discharge and turbidity may affect light penetration in the water column as well as its residence time. The modifications in the use of the soil will probably favor the input of nutrients. This latter effect will favor autotrophy, particularly by those species capable of generating strategies for surviving in more turbid and enriched environments. An accelerated eutrophication will change the composition of the consumers in preference to herbivores and detritivores. The increase in global population projected for the next years will demand more food, and this situation coupled with the new scenarios of climate change will lead to profound socioeconomic changes in the pampean area, implying an increase in demand for water resources and land uses. Fil: Rodrigues Capitulo, Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina Fil: Giorgi, Adonis David Nazareno. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Luján. Departamento de Ciencias Básicas. Programa Ecología de Protistas; Argentina Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján; Argentina

A translocation experiment of epipelic biofilm was performed in order to explore the effects on this biological complex when exposed to different water qualities. To carry out such an experiment we employed artificial substrata placed at two sites within a stream that receives a textile effluent: at site 1, located upstream from this influx, and at site 2, downstream. After a 4-week colonization, the substrata at each site were switched in location between sites 1 and 2.The analysis of the epipelon was performed once a week between April and July 2008. In order to evaluate the disturbance on the biofilms we assessed structural (biofilm composition, chlorophyll “a” and ash-free dry weight) and metabolic (net and gross primary production, respiration, and assimilation rate) features. With the metabolic variables that showed significantdifferences, resistance was calculated. The taxonomic and metabolic variables analyzed responded differently in accordance with the type of environmental challenge presented. In this regard, the biofilm developing at the site upstream from the textile effluent that was later transferred to the downstream site proved to be more resistant to the environmental perturbations with respect to its composition, but not at the level of its metabolic descriptors. Indeed, in the translocated and nontranslocated biofilms growing downstream we observed diatom species with morphological deformations in their frustules, fact that clearly reflects the environmental stress at this site. On the other hand, the biofilm transferred in the opposite direction, in turn, rapidly exhibited tendencies to compensate for its lower biological integrity, but responded more slowly at the metabolic level. Finally, the observation of the changes occurred in the biofilms as a consequence of the worsening and improvement of the water quality could be efficiently evaluated through this experiment. Fil: Sierra, Maria Victoria. 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; Argentina Fil: Gomez, Nora. 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; Argentina

The relationship between microbenthic communities, the habitat characteristics, and physical and chemical gradients was studied in the Rio de la Plata estuary. Five replicates of the surface layer were collected seasonally, in 10 sampling sites influenced by different land uses. The distribution of microbenthic communities was governed by two gradients, the first one determined by anthropic factors, related to pollution, and the second one to conductivity and turbidity. The higher densities of producers were observed in sites characterized by fine sediments. During winter, spring, and summer cyanophytes were abundant, while in autumn the diatoms, particularly birraphideans, dominated. The ciliates were the most abundant group among consumers, particularly in winter, and their spatial distribution was influenced by the turbidity. The whole study area has an eutrophic condition. Turbidity and the enrichment with nutrients and organic matter explained 50% of the variability in the species’ distribution. Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Licursi, Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Cochero, Joaquin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

This paper deals with the species composition and the population dynamics of the diatoms that were found in the limnetic plankton of the Embalse Rio III over IhoII and a half years. 32 taxa were identified. The larger populations were : Aulacoseira liraia var. alpigena, Aulacoseira granulata, Aulacoseira granulata var. angustissima, Aulacoseira gra­ nulata var. angustissima f. spiralis, Actinocyclus normanii f. subsalsa and Cymbella affinis. The diatom flora found in Embalse Rio III were alkalophilous, otigohalobous eutrophic indicators. This algal group was often very dominant among the phytoplankton. This phenomenon usually took place during certain periods in autumn and winter. The iso­ thermal mixing is one of the principal factors for the increase of the diatom populations of the reservoir. However A. granulata was the dominant diatom in number, biomass and permanence ; this alga shows massive development during the two periods of thermal stratification.

We explored the effects of a textile industry effluent on water quality, habitat quality and structural and functional responses of benthic communities in a lowland stream. Two sampling sites were selected: site 1 was located 300 m upstream of the outflow from the textile factory and site 2 was 500 m downstream from the discharge point. Samples of water, microbenthos, invertebrates and aquatic plants were taken seasonally. The effluent from the textile industry modified the structure of the microbenthic assemblages downstream, increased the density of organisms and the biomass of primary producers, but diminished the species richness. The oxygen consumption of the microbenthic community was 3 x higher downstream of the effluent and abnormal frustules of diatoms were noticed. The richness and abundance of invertebrate taxa were lower at the impacted site. The invertebrate modes of existence and the functional feeding groups were also significantly affected. This study is an important baseline for assessment of lowland streams with high water residence time and a notable development of hydrophytes. It will also provide a baseline for the monitoring and restoration, or remediation, programs using the metrics of biotic integrity, particularly in South American countries where such metrics are rarely employed.

Community composition, structure, biomass and tolerance to pollution of biofilms growing on Schoenoplectus californicus were assessed, together with physico-chemical parameters of the water, to evaluate their usefulness as water quality indicators along the Argentinian shore of the freshwater zone of the Río de la Plata Estuary. Monthly sampling was carried out at three sites, one of which, located in a nature reserve, was considered as the reference site while the others were affected by sewage outlets and port activity. The reference site had higher dissolved oxygen and turbidity values, the polluted sites greater conductivity, oxygen demands, phosphate, ammonium and nitrite values. Diatoms dominated the biofilms at the reference site; ciliates reached higher densities at the polluted sites. Significant differences were found in species numbers, species diversity index, biomass, tolerance to pollution of the taxa, and the Pampean Diatom Index, between the reference and the impacted sites. This study shows that the biofilms growing on Schoenoplectus californicus are good indicators of the changes in the water quality of their environment. Fil: Bauer, Delia Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina Fil: Hualde, Paula R.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

The structure and dynamics of the phytoplankton related to water quality and hydraulic characteristics were studied in a 12 km reach of the Rodríguez Stream, a tributary of the Río de la Plata, in Argentina. This stream includes a small artificial pond. Samples were taken seasonally along a year at three sites at the stream and once at the pond. Water quality according to physical and chemical parameters was better at site 1 than at downstream sites. Changes in phytoplankton assemblages descriptors were observed with increasing nutrients and water residence time. The spatial water quality changes are coincident with the hydraulic discontinuity. This situation suggests a synergic effect among the phytoplanktonic descriptors that were analyzed. There was a high percentage of species belonging to euglenoids followed by green algae and diatoms. Loricated euglenoids were dominant at site 1, clorococcalean greens dominated at the pond and decreased progressively downstream, at sites 2 and 3. Gomphonema parvulum, Nitzchia palea and N. umbonata were abundant in the latter sites, where current velocity increased. Our results also show the integrating character of the phytoplankton in this small stream where it can reach considerable development due to the low gradient ($latex

Two invertebrate species (Hyalella curvispina and Palaemonetes argentinus) and one macrophyte (Egeria densa) from a naturally high nutrient content system (Pampean rivers of La Plata, Argentina) were evaluated for their potential use in situ assays aiming to assess changes in water quality. Invertebrates were individually placed in cylindrical chambers in polluted sections of rivers and in reference upstream sites. Mortality after 48 h was high in polluted and reduced in control sites. Mortality was also higher in situ assays than in laboratory static tests. Standard sections of the macrophyte were also deployed at the reference and control sites. Growth (7 days) in terms of mass increment (but not in length) was consistently reduced in polluted sites. Results of benthic invertebrate and periphitic algae surveys were consistent with the in situ tests: pollution resulted in a decrease in the number of taxa, taxa replacement and in changes in the value of the biotic indices Indice Biotico PAMPeano and Indice de Diatomeas Pampeano, indicating deterioration of water quality. In situ assays have a high potential as environmental tools in integrated approaches of bioassessment programs. Fil: Graça, Manuel A.S.. Universidad de Coimbra; Portugal Fil: Rodrigues Capitulo, Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina Fil: Ocon, Carolina Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina Fil: Gomez, Nora. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología ; Argentina

The ecology of Aulacoseira (formerly Melosira) granulata and its different forms in ~30 reservoirs scattered throughout Spain has been analysed. Nutrient concentration explained a significant proportion of the variation in cell characteristics. In general, there is a tendency to find large cells with less densely areolated valves in nutrient-rich waters. Ecological differences between the type and its variety angustissima mainly concern calcium concentrations Aulacoseira granulata showed a unimodal response to the concentration of calcium, and preferred lower concentrations of calcium than the variety angustissima. The ecological differences between the a and -y forms (statuses) of A.granulata were also the object of our investigation. The y status occurred in all the reservoirs studied in summer, but in only 3 of 28 reservoirs studies in the overturn period. Light and turbulence explained the ecological differences observed between the two forms. The y status showed a certain preference for less turbulent waters compared with the a status, which is a more cosmopolitan form in its requirements.