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Agricultural intensification is a process that is still undergoing in many emergent economies. In the Pampas (central Argentina), the former low-external input farming was replaced by a model based on genetically modified crops and the intensive use of pesticides (mainly glyphosate) in the last decades. Here, we analyzed changes in water chemistry (pH, conductivity, dissolved oxygen, nutrients, and carbonates) in 41 streams of Buenos Aires province between 2003/04 and 2015/16, and the impact of geology, soil type and land use change on water chemistry. We also used the SPARROW model to further analyze possible drivers of stream phosphorus loads. We observed modifications at reach scale in several streams, including changes in channel morphology, riparian vegetation and land use, and a moderate expansion of agriculture in most basins. Mean nitrate concentration did not significantly change between 2003/04 and 2015/16, and it was related to cropland cover in the basins. Dissolved phosphorus concentration increased in the streams, especially in the southern fluvial regions, but contrary to our expectations, phosphorus levels were not associated to land use but to pH. The SPARROW model also supported the link between water acidification and phosphorus concentration, and indicated that the whole basin acts as a phosphorus source. Possible explanations of fluvial acidification may be related to current agricultural practices, including higher inputs of labile carbon from croplands, soil acidification by nitrogenous fertilizers, and the generalized use of glyphosate. This highlights the necessity of adopting new agricultural paradigms to reduce the use of agrochemicals in Pampean basins.

Eutrophication is a key human-induced stressor in streams and often a consequence of intensified land use. We characterised how population density influences dissolved nutrients and its ratio in Pa...

In-stream nutrient retention is an important ecosystem function because it can regulate nutrient fate and export to downstream ecosystems. Temporal variation in nutrient retention in stream...

The aim of our study was to analyze dissolved organic carbon processing at a reach of a productive Pampean stream and to determine DOM retention efficiencies under a wide range of hydrological conditions. This study was carried out in Las Flores stream, a third-order stream located in the Lujan River basin in the northeast of the Buenos Aires province, Argentina. We selected a 2.2 km reach of Las Flores stream. On each sampling occasion, we collected water samples at the end of the reach, at two upstream tributaries, and at the potential hydrological contributors (end members) to stream flow. We determined DOC concentration and DOM optical properties in the stream and end members by combining absorbance–fluorescence spectroscopy techniques. The reach acts as a reactor for DOC concentration at baseflow condition, when in situ production processes prevailed. However, DOC is passively transported downstream at high flow condition. We found that humic and more degraded compounds predominate in DOM at the end of the reach. According to this, we proposed that both protein-like and humic-like compounds are produced at the study reach through in situ production processes, such as biological production from periphyton and macrophytes decomposition.

We characterized how land use influenced dissolved nutrients and periphytic algal biomass in an Andean basin from Northwest Patagonia. Nutrient export, especially dissolved inorganic nitrogen increased with human population density. However, no correlation between nutrient concentration and algal biomass was found, which could instead be limited by light availability. Our results suggest that local N-limited ecosystems are liable to eutrophication by increased demographic pressure and that alternative wastewater treatment strategies are necessary for sustainable growth.

Indicators of ecosystem health are effective tools to assess freshwater ecosystem impairment. However, they are scarcely used as a monitoring tool by local environmental agencies in Argentina. Here, we review the literature to analyze the use of ecosystem health indicators in freshwaters from Argentina. We found 91 scientific articles relating to the use of ecological indices to assess the impact of different environmental stressors in aquatic environments published between 1996 and 2019. We generated Google Earth map where we deployed the sampling sites and type of indices reported by each article. As biological indices were the most used, we also surveyed bioindication experts to gather information on their application. We found that most studies were concentrated mainly in Pampas (34%), Dry Chaco (20%), Espinal (12%), and Patagonian Steppe (10%) ecoregions. Biological indices (mainly with invertebrates) were more used than geomorphological or physico-chemical indices. Indices resulted useful to evaluate the impact of stressors in 63% of cases, being land use the most studied stressor. However, sampling design varied greatly among studies, making their comparison difficult. The information compiled here could help to the design of monitoring protocols, the adoption of regional indices, and the creation of a national inventory of ecosystem health status, which are mandatory to propose well-grounded conservation and management policies for freshwaters in Argentina.

Se estudió el proceso de retención de fosfato en ríos de regiones subtropicales y templadas. Nuestra hipótesis fue que la retención difiere en las dos regiones. Esperamos más retención en la región subtropical que en la templada. Confirmado esto; las diferencias podrían deberse a una interacción entre la concentración de fosfato y la biomasa de algas, asociada a las diferencias de temperatura. La retención de fosfato se estudió realizando adiciones en continuo de P. La tasa de asimilación (U) y la velocidad de asimilación (Vf: 2.67 E-04 y 9.27 E-05 m/s para los ríos subtropicales y templados respectivamente) fueron asociadas con la concentración basal de fósforo, la biomasa de macrófitas y la zona de almacenamiento transitorio de acuerdo con un análisis de PCA. Así, los ríos de las regiones templadas responden más a la biomasa de las macrófitas que los ríos de las regiones subtropicales que se asociaron principalmente a las algas bentónicas. Estos resultados son novedosos para América del Sur y reflejan la importancia de la biomasa y producción de algas bentónicas o macrófitas en relación con la tasa de captación de P en diferentes ambientes. Phosphate retention processes in streams of a subtropical and a temperate region were studied. Our hypothesis was that retention differs in the two regions. We expect more retention in the subtropical region than in the temperate one. If that were confirmed the differences in stream phosphate uptake may be driven by an interaction between phosphate concentration and algal biomass, associated with differences in temperature. Phosphate retention was studied by carrying out constant rate additions. Nutrient uptake rate (U) and mass transfer coefficient (Vf: 2.67 E-04 y 9.27 E-05 m/s to subtropical streams and temperate one respective-ly) were associated with background phosphate, macrophyte biomass and transient zone storage according to PCA analysis. Thus, streams of temperate regions respond more to macrophyte biomass than streams of subtropical regions which were associated principally to benthic algae. These results are novel for South America and they reflect the relative importance of benthic algal or macrophyte biomass and production in relation to the P uptake rate at the different environments. Fil: Gultemirian, Maria de Lourdes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Biodiversidad Neotropical. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Biodiversidad Neotropical. Instituto de Biodiversidad Neotropical; Argentina Fil: Giorgi, Adonis David Nazareno. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Rodríguez Castro, María Carolina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Fernandez, Hugo Rafael. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Biodiversidad Neotropical. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Biodiversidad Neotropical. Instituto de Biodiversidad Neotropical; Argentina

The aim of our study was; (i) to characterize the composition of DOM in stream water and their potential sources (groundwater, overland flow, subsurface flow and rain water) and (ii) to analyze changes in DOM concentration and composition under different hydrological conditions (baseflow and high flow) in a third-order Pampean stream (Argentina). Pampean streams are mainly fed by the shallow aquifer under baseflow conditions and they lack of riparian forest. In addition, water velocity is low due to the gentle slope of the region and nutrient levels are high, favoring the development of rich macrophyte communities. DOM optical properties in the stream and end members were determined by combining absorbance-fluorescence spectroscopy techniques. Our results indicated that DOM chemical characteristics in the stream were mainly modulated by a differential contribution of end members to stream water depending on hydrological conditions. We observed that DOM in groundwater showed a microbial origin while DOM in runoff was terrestrially-derived. DOC concentration and inputs of humic substances from the riparian zone increased with discharge at high flow conditions. Due to the strong link between DOC properties and the riparian environment, structural alterations in the stream channel and changes in riparian vegetation (forestation) may result in changes in DOM composition and dynamics.

Fluvial systems are particularly sensitive to changes in the terrestrial ecosystems where they are embedded, receiving simultaneously the impact of multiple stressors. The design of adequate management policies requires analyzing fluvial systems as social-ecological systems, because the decoupling of natural and social systems can lead to a severe mismatch between maintaining ecological integrity and the pursuit of human well-being. Pampean streams are especially prone to the impact of human activities because they are located in a region that provides almost half of the agricultural production of Argentina and concentrates 66% of the whole population of the country. In the present work, we conceived a general social-ecological framework that links the occurrence of multiple stressors and their impacts on ecosystem services, with changes in environmental perception of streams, which in turn feedback over institutional actions at the watershed's governance. We identified four current key drivers of the dynamics in Pampean streams: a dominant agro-industrial model for the region, a command-and-control governance regime mainly based on an engineering hydraulic perspective, the real estate market speculation of surrounding lands, and the persistence of structural poverty in urban areas. The resulting dynamics resembles the occurrence of different kinds of social-ecological traps, i.e., a highly stable but undesirable state of the system that is difficult to escape. Based on this analysis, we provide a leverage point perspective to avoid this trap. Together, this approach could be applied to other fluvial systems of the world to link the ecological and social domains to multiple stressors analysis, and to improve institutional fit for the sustainability of fluvial social-ecological systems.

En este estudio se describen el ciclo de vida y la ecología poblacional de la planta acuática Egeria densa Planch. (Hydrocharitaceae) en un arroyo de la llanura pampeana. El trabajo se efectuó en tres etapas. En la primera, se realizó un muestreo de 21 meses en dos arroyos pertenecientes a la cuenca del río Luján (provincia de Buenos Aires) que presentaban sendos manchones de E. densa. Este muestreo tuvo como objetivo determinar la influencia de distintos factores (características físicas y químicas del agua, tipo de sedimentos, presencia de perifiton, herbívoros y otras especies de macrófitas) sobre la biomasa y contenido de nutrientes de E. densa, y analizar su ciclo fenológico. La segunda etapa del trabajo consistió en la realización de experiencias en invernáculo a fin de examinar la absorción de nutrientes por esta macrófita. En la tercera etapa se elaboró un modelo matemático descriptivo de la dinámica poblacional de E. densa en un arroyo de llanura. Los resultados del trabajo de campo sugieren que los factores que influyen sobre la biomasa y el contenido de nutrientes en E. densa, actúan con distinta intensidad de acuerdo al grado de desarrollo de la población de macrófítas. Las grandes crecientes actúan como perturbaciones que pueden llegar a arrastrar a las macrófitas aguas abajo, creando un claro que posteriormente es recolonizado por E. densa. En las primeras etapas, el desarrollo de esta planta acuática estaría regulado principalmente por factores abióticos, tales como la concentración de nutrientes en el medio, mientras que en etapas posteriores tendrían mayor incidencia los factores bióticos, como la presencia de otras especies de macrófitas (principalmente Lemnaceas), el ficoperifiton y los herbívoros.

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

The Pampean region covers a large surface in central Argentina, but despite the extensive agricultural activities and the high nutrient levels recorded in streams of the region, few authors have analysed the influence of land use on water quality. Here, we evaluated the relationships among catchment attributes (size, morphometry and land cover) and water chemistry in 23 Pampean streams in different seasons (autumn, spring and summer) and at three spatial scales: whole catchment and two scales of riparian buffers (200 and 500 m adjacent to both stream margins). Chloride concentration was positively related to catchment area and negatively related to drainage density. Nitrate level was strongly associated to cropland, but soluble phosphorus concentration showed no relationships with any type of land cover. Land cover at the buffer scale seemed to be more influential than land cover at the whole catchment for nitrogen concentration. The main impact of cropland was the increase of nitrate concentration, while cattle breeding was negatively associated to photosynthetically active radiation (PAR) in autumn and summer and to dissolved oxygen concentration in spring. Our results highlighted the importance of local land use and riparian conservation on streamwater quality.

Nutrient retention has been studied intensively in streams, but some mechanisms and processes are not yet entirely understood, especially in open-canopy streams. We evaluated PO43−, as soluble reactive P (SRP), and NH4+ uptake in 2 chronically enriched Pampean streams with different macrophytic abundance. We performed short-term nutrient additions to quantify SRP and NH4+ uptake metrics throughout the year, and we investigated which hydrological and biological factors influenced nutrient uptake. The results showed that SRP and NH4+ uptakes were high in relation to pristine and impaired streams elsewhere, and they did not saturate despite elevated background nutrient levels. NH4+ areal uptake rate fit in a 1st-order uptake model, but an exponential model described the relationship between SRP areal uptake rate and SRP concentration. Consistent with this finding, SRP uptake velocity tended to increase linearly, and SRP uptake length decreased linearly with SRP concentration. The analysis of factors influencing uptake metrics suggested that SRP uptake mainly depended on heterotrophic demand, whereas NH4+ uptake was enhanced under more autotrophic conditions. Our results showed that nutrient uptake metrics of enriched Pampean streams were similar to those observed in streams with low nutrient levels. Fil: García, Victoria Julieta. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina Fil: Gantes, Hilda Patricia. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Giménez, Loreta. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina Fil: Hegoburu, Cecilia. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Ferreiro, Nicolas Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina Fil: Sabater, Francesc. Universidad de Barcelona. Facultad de Biología. Departamento de Ecología; España Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina

The effect of phosphorous enrichment on decomposition rate, exoenzymatic activities (β-glucosidase, cellobiohydrolase and alkaline phosphatase), and macroinvertebrate abundance in Typha latifolia leaves were assessed in a 2nd order Pampean stream (Central Argentina). Phosphorous was added to a downstream reach while another reach located upstream was kept intact and, once significant differences in phosphorus concentration in water were attained, leaf bags were attached to each reach bottom. T. latifolia leaves lost 77% of their initial weight along 154 days and decomposition rates were not significantly different between reaches. Besides, neither exoenzymatic activities nor macroinvertebrate abundances differed between reaches. However, an increment in leaf phosphorus content, attributed to immobilization by decomposer microorganisms, was detected in the enriched reach.

Environmental heterogeneity in natural ecosystems influences several parameters at the population and community levels. In freshwater ecosystems, habitat heterogeneity can be provided by macrophyte species with different structural shapes. Previous studies suggest that aquatic plants with more complex architectures will support higher number, biomass, and taxon richness of macroinvertebrates than plants with simpler shape. We investigated the influence of macrophyte structural heterogeneity (quantified by fractal dimension) and food availability (represented by epiphytic biomass) on several parameters (number of individuals, biomass, body size distribution, taxon richness, and diversity) of the macroinvertebrate community in a Pampean stream. Four submerged macrophyte species (Egeria densa, Elodea ernstae, Ceratophyllum demersum, and Stuckenia striata) and associated macroinvertebrates were sampled in late spring, summer, and autumn. Plants were photographed and fractal dimension was estimated from the images by the box-counting method. Fractal dimension was independent of plant surface area per unit of macrophyte biomass and differed significantly among species. Mean fractal dimension varied between 1.29 and 1.62, and increased following the sequence E. densa → S. striata → E. ernstae → C. demersum. Macrophyte species with higher fractal dimension supported a greater abundance of macroinvertebrates, especially those of small body size (500–1,000 μm); but fractal dimension was unrelated to macroinvertebrate biomass, richness, and diversity. However, overall animal biomass was significantly associated to the epiphytic abundance. Consequently, macrophyte heterogeneity influences macroinvertebrate density and body size distribution, while animal biomass depends on epiphytic food resources provided by plants.

SUMMARY 1. Wind is considered the dominant factor controlling phytoplankton distribution in lentic environments. In canyon-shaped reservoirs, wind tends to blow along the main axis generating internal seiches and advective water movements that jointly with biological features of algae can produce a heterogeneous phytoplankton distribution. Turbulence generated by wind stress and convection will also affect the vertical distribution of algae, depending on their sinking properties. 2. We investigated the vertical and horizontal distribution of phytoplankton during the stratification period in Sau Reservoir (NE Spain). Sites along the main reservoir axis were sampled every 4 h for 3 days, and profiles of chlorophyll-a and temperature were made using a fluorescent FluoroProbe, which can discriminate among the main algal groups. Convective and wind shear velocity scales, and energy dissipation were calculated from meteorological data, and simulation experiments were performed to describe nonmeasured processes, like vertical advection and sinking velocity of phytoplankton. 3. Wind direction changed from day to night, producing a diel thermocline oscillation and an internal seiche. Energy dissipation was moderate during the night, and mainly attributed to convective cooling. During the day the energy dissipation was entirely attributed to wind shear, but values indicated low turbulence intensity. 4. The epilimnetic algal community was mainly composed of diatoms and chlorophytes. Chlorophytes showed a homogeneous distribution on the horizontal and vertical planes. Diatom horizontal pattern was also homogeneous, because the horizontal advective velocities generated by wind forcing were not high enough to develop phytoplankton gradients along the reservoir. 5. Diatom vertical distribution was heterogeneous in space and time. Different processes dominated in different regions of the reservoir, due to the interaction between seiching and the daily cycle of convective-mediated turbulence. As the meteorological forcing followed a clear daily pattern, we found very different diatom sedimentation dynamics between day and night. Remarkably, these dynamics were asynchronous in the extremes of the seiche, implying that under the same meteorological forcing a diatom population can show contrasting sedimentation dynamics at small spatial scales (approximately 10 3 m). This finding should be taken into account when interpreting paleolimnological records from different locations in a lake.

Low current velocities, high nutrient levels, the lack of riparian forest vegetation, and the development of dense and rich macrophyte communities characterize Pampean streams. The objective of this study was to describe the main physical, chemical, and biological characteristics of a headwater Pampean stream as well as to analyze the role of macrophytes and phytobenthos. The study was conducted in a stream considered to be not much disturbed by human activities. Samples of water and organisms (macrophytes, benthic algae and invertebrates) were taken monthly for 14 months in two sampling stations, in fast flow and slow flow sites. Macrophyte biomass and diversity increased in spring and summer, and they decreased in autumn, when the plant community was greatly affected by an important flood. Phytobenthos biomass was lower in late summer, possibly due to the establishment of a dense cover of the floating macrophyte Lemna gibba L. Density of amphipods and gastropods greatly increases in spring and summer, jointly with the macrophyte development. Analysis of correlation showed that current velocity is the most important factor influencing macrophyte biomass and phytobenthos structure, while depth, nutrients, and herbivores are linked factors. Pampean streams could be considered systems dynamically fragile, because habitat heterogeneity is generated by aquatic vegetation, a substratum that varies along time.

Macrophyte complexity has been associated with high abundance and richness of macroinvertebrates. While the effect on richness has been attributed to an increase in the number of niches, the effect on abundance has been explained by a higher availability of space for small individuals, refuge and/or food. For studying effects of complexity on macroinvertebrates we used complementary approaches of laboratory choice and field colonization experiments, with macrophytes (Egeria densa and Elodea ernstae) and plastic imitations of contrasting fractal dimension. We investigated whether macroinvertebrates may actively select complex habitats by Hyalella sp. choice experiments. Then, we tested effects of complexity on macroinvertebrate density, biomass, richness, diversity and body size using colonization experiments. Finally, a caging experiment was performed to study interacting effects of complexity and predation. The active choice of complex substrates by Hyalella sp., and the significant positive relationship between macrophyte fractal dimension and macroinvertebrate density supports the existence of a positive effect of complexity on abundance. As macroinvertebrate length was not associated with fractal dimension, such differences could not be attributed to a higher space available for smaller invertebrates in complex plants. Finally, neither macroinvertebrate density nor size were reduced by fish predation in the Las Flores stream. 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 Lujan. Instituto de Ecologia y Desarrollo Sustentable; Argentina. Universidad Nacional del Comahue; Argentina Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján; Argentina Fil: Giorgi, Adonis David Nazareno. Universidad Nacional de Lujan; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rosso, Juan Jose. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Mar del Plata; Argentina

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

The Argentine pampean streams exist in a traditionally agricultural region, but they also receive a supply of contaminants in the more urban zones. The objectives of this study are: 1. To analyze the physical, chemical and microbiological characteristics of the streams in the Lujan river basin; and 2. To describe the annual variation of some chemical characteristics (primarily nutrients) in one of the streams. Two samplings (winter and summer) were made in 20 streams to determine some physical, chemical and microbiological variables. Topographical variables were also estimated in each stream. Additionally, in one of the streams (Las Flores), monthly measurements of current velocity, flow, and dissolved nitrogen, phosphorus and oxygen were taken during 4 years. The streams are characterized by high nutrient contents. There is no strong heterogeneity in stream chemical characteristics within the same period, however seasonal variations are important . The principal components analysis suggests the existence of two axes of variation within the basin. One reflects a pollution gradient that increases downstream; the other axis shows that waters change from high to low alkalinity and from less to more eutrophic. In Las Flores stream, nutrient concentration varied greatly between years. The concentration of soluble reactive phosphorus was significantly correlated with stream flow (P < 0.05). Though there was no correlation between nitrate levels and flow, some estimates indicated that these ions are in high concentrations in the groundwater. The physiognomical characteristics of pampean streams differ greatly from those described in temperate streams of the northern hemisphere, and it may be assumed that they also exhibit distinct functional characteristics.

(1998). Annual variation of periphyton biomass in two plain streams with different macrophyte abundance. SIL Proceedings, 1922-2010: Vol. 26, No. 4, pp. 1698-1701.

Habitat heterogeneity is one of main factors determining distribution of organisms, and vegetation is of primary importance in shaping structural environment in aquatic systems. The effect of macrophyte complexity on macroinvertebrates has been well researched, however, much remains to be revealed about influence of complexity on epiphytic algae. Here we used fractal dimension to study effect of complexity at two scales, macrophyte architecture and leaf shape, on several parameters of epiphytic algal community (number of individuals, biomass, taxon richness, and diversity) in a Pampean stream. Four submerged macrophyte species with different complexities and associated algae were sampled in late spring, summer, and autumn. Important differences were found in fractal dimension of whole plant and leaves among macrophyte species. Particulate organic matter and chlorophyll-a associated positively to leaf fractal dimension, but not to plant fractal dimension, partially supporting the hypothesis of a positive effect of macrophyte complexity on periphyton biomass. No association was found of fractal dimension with algal abundance, taxon richness or diversity. Complementary, a mesocosms experiment was performed with plastic imitations of different plant fractal dimension. After four weeks, there were differences in chlorophyll-a and autotrophy index between treatments which suggested a positive effect of complexity on autotrophic periphyton biomass. Differences in plant fractal dimension effect between sampling and experiment were attributed to differences in environment and algal community between stream and experimental trays. Results indicate that well-known positive effect of macrophyte complexity on macroinvertebrates might be partially explained by a positive effect of complexity on periphyton biomass. 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 Lujan; 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: Feijoó, Claudia Silvina. Universidad Nacional de Lujan; Argentina

Weather variations change stream hydrological conditions, affecting the stream function. A seasonal study in three well-conserved first-order Pampean streams was carried out to test the hypothesis that rainfalls are the main drivers of whole-stream metabolism, through their effects on hydrology. We estimated the stream metabolism and metabolic contribution of six relevant communities (angiosperms, macroalgae, seston, epiphyton, epipelon,and hyporheos) during late spring, summer, and winter and examined the relation between gross primary production (GPP) and photosynthetic active radiation (PAR). Our results showed that the decrease in available streambed light due to the dissolved organic carbon after rainfalls was the main factor related to the decrease in the ecosystem and community metabolisms. For instance, GPP oscillated from *10 gO2 m-2 d-1 in early spring (low flows) to *3 gO2 m-2 d-1 in summer (high flows). Ecosystem respiration (ER) was less sensitive than GPP to rainfalls due to the increase of hyporheic respiration. Rainfalls also caused a significant loss of downstream macroalgal biomass. At a day scale, the high PAR of late spring and summer saturated GPP during the afternoon, and the low temperature of winter mornings constrained GPP. Hence, the knowledge of weather changes is key to understanding the main hydrological drivers of stream function. Fil: Leggieri, Leonardo Ramón. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina. Ministerio de Ciencia, Tecnología e Innovación Productiva. Agencia Nacional de Promoción Cientifíca y Tecnológica; Argentina Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina Fil: Giorgi, Adonis David Nazareno. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Ferreiro, Nicolas Andres. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina Fil: Acuña, Vicenç. Universidad de Girona. Institut Catala de Recerca de L; España

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"

We identified factors influencing biomass and nutrient content in E. densa in an enriched pampean stream of Argentina. Physical (current velocity, temperature), chemical (pH, conductivity, dissolved oxygen, nutrient content in water and sediments), and biological variables (biomass and nutrient content of E. densa, biomass of periphyton and other macrophytes) were estimated at each sampling occasion, and mean monthly values estimated. Biomass and nutrient content in E. densa were correlated with these physical-chemical and biological variables. Biomass was positively correlated with ammonium in stream water (P

An Index of Riparian Quality was applied to a Pampean stream with moderate human impact (Las Flores stream, Luján river basin) to evaluate its degree of conservation. Aquatic macrophyte cover and diversity were also registered in 12 different reaches along the stream to analyze its relationship with the riparian index. In addition, the richness and diversity of the fish community were registered in two reaches with contrasting use of riparian zones and channel morphology. The results of the survey indicate that conservation of the riparian zones is difficult in the stream due to the increase of farming activities, the development of cropland near the margins, the invasion of an alien tree Gleditsia triacanthos (acacio negro) and the development of urban zones. Alteration of the riparian zone also modified the structure of the macrophyte communities and could be associated with a reduction in the richness and diversity of the fish community. We consider that the needs for conservation and recuperation of the riparian zones must be transmitted to the land owners. Activities should be coordinated to maintain the water quality and habitat diversity in order to enhance the basin health., Instituto de Limnología "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

In order to determine the environmental problems as perceived by the community of Lujan (Province of Buenos Aires, Argentina), a survey was carried out. The relationship between environmental perception and socio-economic level was also studied. The most important problems reported by the community were the floods, which occur when the Lujan river overflows its banks, and the contamination of this river as it flows through Lujan city. Deficiencies in the tourist infrastructure were also considered to be serious. The least perceived problem was that of industrial air pollution. Positive correlations between environmental perception, socio-economic standing and levels of instruction were observed.

Streams of the pampas region (central Argentina) are characterized by the lack of riparian forests and by low current velocities, and receive a large amount of industrial and domestic effluents. Intensive agriculture in the region has greatly increased during the last years; nevertheless, regional studies to determine baseline conditions are not available. Here, we characterized pampean streams considering physico-chemical water features and the composition of macrophyte communities, and we established if the regions proposed by an early hydrological regionalization show differentiated water chemistry and macrophyte assemblages. Forty-one streams were sampled in autumn, spring and summer. Streams were characterized by alkaline waters, with high conductivities and dissolved oxygen concentrations, and they can be classified as eutrophic. A good discrimination among regions was achieved based on streamwater features in all seasons; variables that most contribute to the discrimination were related to pH, eutrophication, and conservative ions. Regional differences may be due to variations in water availability, parent material, and land use practices. The aquatic plant communities reflected the eutrophic state of waters, but macrophyte composition was not useful to differentiate among regions because it is not affected by differences in water chemistry. Macrophyte species grouped according to their growth form and assemblage distribution were related to chemical variables (as conductivity and nitrate) and possibly to flow conditions.

We estimated phosphate uptake in an eutrophic and macrophyte-rich Pampean stream, in a basin where dominant land use was agricultural activity. Our hypothesis was that phosphate retention may be linked to macrophyte abundance, especially that of submerged plants. Four short-term constant-rate phosphate addition experiments were carried out in late spring, summer, autumn and winter under different discharge, background phosphate concentration and macrophyte abundance. Changes in added soluble reactive phosphorus (SRP) concentration were monitored along the studied reach and corrected for dilution to estimate phosphate uptake length (Sw) and other spiraling metrics (Vf or mass transfer coefficient, and U or nutrient uptake rate). All spiraling metrics indicated a higher SRP retention in spring than in autumn, despite macrophyte cover was similar in both sampling dates (39% and 51%, respectively). Phosphate retention was not detected neither in summer (when macrophyte biomass and especially that of submerged plants was the highest one) nor in winter. Ambient SRP concentrations in the stream were an order of magnitude higher than concentrations observed in pristine streams; nevertheless, under the varying conditions of discharge and phosphate levels observed in our study, we found no retention in summer and winter, a phosphate uptake comparable with those measured in impaired streams in autumn, and with those measured in pristine streams in late spring. © 2010 Elsevier GmbH. Fil: Feijoó, Claudia Silvina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina Fil: Giorgi, Adonis David Nazareno. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina Fil: Ferreiro, Nicolas Andres. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina