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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.

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.

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.

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.

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.

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"

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

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.