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The study shows changes on physical and chemical water parameters and of trophic state in a large reservoir in the Brazilian semiarid region following decreasing reservoir volume due to rainfall shortage during four consecutive years. The monitoring period, between November 2011 and May 2014, assessed approximately 50% water volume reduction and 10 meters' decrease of reservoir water level that degraded water quality. Decrease in reservoir volume, strong evaporation and the permanent influence of anthropogenic activities, favored the concentration of salts and accumulation of nutrients and of increasing pH. Thermal stratification of the water column occurred when volume was maximum and lead to a significant reduction in dissolved oxygen in the hypolimnion (0.07 to 2.62 mg L-1). Diminishing volume resulted in mixing of the hypolimnion nutrient-rich and oxygen-poor waters in the entre water column and changed the initial oligotrophic condition to eutrophic. However, the temporal scale of the response of the reservoir's trophic state differs in the different areas of the reservoir. Whereas deeper areas accumulating nutrients from aquaculture and agriculture progressively became mesotrophic and eventually eutrophic; shallower regions far from direct anthropogenic influences, changed their trophic sate much later, but rapidly turned into super-eutrophic conditions, probably due to more intense sediment resuspension and water mixing. Trophic State Index followed nutrient increase during most of the period. However, it also responded to an increase in chlorophyll a concentrations when the reservoir achieved its minimum volume, in particular in the shallower areas. The results suggest that this type of reservoir systems are vulnerable to eutrophication during extended drought periods and that a better assessment of the maximum support capacity for reservoir activities, particularly aquaculture, must be re-assessed taking into consideration worst case scenarios forecasted by global climate change. [ABSTRACT FROM AUTHOR]

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Land-use intensification impacts freshwater ecosystems in many agricultural landscapes, including the Rio de la Plata grasslands in Southeastern South America. We evaluated water chemistry of rivers, streams and lakes associated with a protected river-floodplain in the Queguay River basin in Uruguay. From 2019–2021, we measured phosphorus (P) and nitrogen (N) forms, dissolved oxygen (DO), turbidity and other chemical parameters; basin-scale land-use via Sentinel satellite imagery; and trophic status based on nutrient concentrations and Principal Components Analysis (PCA). Results highlighted low nutrient concentrations in the Queguay rivers, indicating this watershed as a national reference for evaluating eutrophication in Uruguay. Levels of total nitrogen (TN) in rivers averaged 430–510 µg/L and total phosphorus (TP) averaged 44–51 µg/L, classifying them as oligotrophic-mesotrophic systems. Streams ranged in TP from 51 to 97 µg/L and in TN from 545 to 701 µg/L, suggesting meso- to eutrophic states according to TP levels, and potential anthropogenic eutrophication in smaller basins with crop cover > 25%. Floodplain lakes are a conservation target within the protected area and had TP averaging 74–109 µg/L, suggesting eutrophic-hypereutrophic states based on TP and potential external nutrient inputs. The positive correlation between basin-scale crop cover and mean TP and soluble reactive phosphorus (SRP) concentrations suggested that land-cover plays a role in nutrient levels of fluvial and lotic systems. This study establishes a baseline for water chemistry in lakes and fluvial ecosystems in the Queguay River basin within an extensive agricultural region of the La Plata River Grasslands. By providing the first time series dataset of water chemistry for this area, we fill a major geographical gap regarding the limnological attributes of freshwater ecosystems in northwestern Uruguay. Results highlight the value of the Queguay basin for freshwater habitat conservation, and the vulnerability of these systems to land cover changes and nutrient loading from external sources. • La Plata grassland land-cover change impacts freshwater ecosystems. • Freshwater lagoons are critical but understudied ecosystems in river-floodplains. • The Queguay Rivers are potentially reference systems for low nutrient concentrations nation-wide. • Fluvial and lagoon systems differ in terms of oxygen, turbidity and phosphorus concentrations. • We provide a baseline for protected area management and habitat conservation of aquatic species. [ABSTRACT FROM AUTHOR]

Abstract: The input of nutrients into rivers and other aquatic ecosystems is one of the most common causes of damage to the integrity of the ecosystem. The use of biological communities for water quality assessment is a common and effective practice. Among these biological indicators, the fish community is one of the most used in monitoring programs and ecosystems studies and so there is much knowledge about their response to the process of eutrophication. In this study we analyzed the fish composition, diversity and evenness in fourteen reaches, of low stream orders within a basin with differing land use intensity. The structure of the fish community was related with physiochemical water composition by canonical correspondence analysis, which enabled us to group the fish species according to their specific tolerance to eutrophication. Oligotrophic habitats were characterized by a higher evenness, larger individuals, a fish composition of at least 40% of sensitive species (Crenicichla scotti, Gymnogeophagus gymnogenis, Gymnogeophagus sp., Heptapterus mustelinus, Hoplias malabaricus and Rhineloricaria sp.) and less than 20% of very tolerant ones (Astyanax fasciatus, Cnesterodon decenmaculatus, Corydoras paleatus, Cyphocharax voga, Hisonotus sp., Pimelodella australis). Eutrophic reaches showed the opposite community features and a fish composition with more than 40% of very tolerant species and less than 20% of sensitive ones. Among the fourteen study reaches, four were classified as oligotrophic, six as mesotrophic and four as eutrophic. [Copyright &y& Elsevier]

Abstract: Human activities are radically changing natural land cover and increasing the delivery of soil, organic compounds, nutrients, toxic agrochemicals and other contaminants to aquatic ecosystems. The eutrophication of streams, rivers, lakes, reservoirs and costal zones is one of the most important consequences of human activities. In this study we assessed the trophic status of 28 wadeable stream reaches of the Santa Lucía basin, an important economic region of Uruguay. We developed a Trophic State Index of Benthic Invertebrates (TSI-BI), the first of its kind for South American lotic systems. The methodological approach consisted of determining the ambient trophic gradient via canonical correspondence analysis based on the benthic invertebrate abundance matrix and an environmental variable matrix. The rescaled site scores served as environmental variables in the weighted averaging model (WA), to weight the benthic abundances and then find the optimum and tolerance of each of the sampled genus. These data were used to estimate the TSI-BI scores. These scores, in conjunction with the total phosphorus concentrations (TP), were used to group the study reaches when running a cluster analysis. The basic statistical parameters of the defined groups serve as an input to identify the threshold values of TP and TSI-BI corresponding with the different trophic states. The boundaries of TSI-BI and TP demarcating mesotrophic and eutrophic states were 8 and 71μg/l, respectively, and can be considered the limits between impaired and less altered reaches. The results also indicated that the trophic status of the reaches is related to land use intensity. A change in land use management seems to be critical for the preservation of one of the most important water supply systems in Uruguay. [ABSTRACT FROM AUTHOR]

Abstract: Many biotic and abiotic processes contribute to variability in phytoplankton diversity in aquatic ecosystems. Depending on their intensity and on their frequency, these may drive non-equilibrium dynamics and enhance the species diversity. Different studies propose that biodiversity buffers ecosystem functioning against environmental fluctuations leading to more predictable aggregate community or ecosystem properties. Salto Grande reservoir is polymictic and eutrophic with recurrent summer cyanobacterial blooms. The aim of this study was to determine the key variable(s) related with phytoplankton diversity in order to predict the possible occurrence of an algal bloom at the ecosystem. A preliminary analysis of the data matrix suggested non-linear relationships between diversity and the selected variables: phytoplankton abundance and the vertical attenuation coefficient (Kd). The best function fitting of the scatter plot of phytoplankton diversity versus phytoplankton abundance was a rational function. The inflection point of phytoplankton diversity estimated by the second derivate of this function was 2.7bitcell−1 which corresponded to an abundance of 3000cellsml−1. The relationship of phytoplankton diversity and vertical attenuation coefficient values also show a humped distribution pattern (Gauss function). The inflection point of this function corresponded to a diversity value of 1.9bitcell−1 and 2.5m−1 for Kd. These inflection points were, respectively, related with the resource competition among the present species and the light limitation conditions. The showed patterns of diversity and the estimated threshold values could be integrated to construct a predictive model for the reservoir based on phytoplankton diversity and the probable ambient conditions of the reservoir. [Copyright &y& Elsevier]