A reply to "Relevant factors in the eutrophication of the Uruguay River and the Río Negro".

A recent paper by Beretta-Blanco and Carrasco-Letelier (2021) claims that agricultural eutrophication is not one of the main causes for cyanobacterial blooms in rivers and artificial reservoirs. By combining rivers of markedly different hydrological characteristics e.g., presence/absence and number of dams, river discharge and geological setting, the study speculates about the role of nutrients for modulating phytoplankton chlorophyll-a. Here, we identified serious flaws, from erratic and inaccurate data manipulation. The study did not define how erroneous original dataset values were treated, how the variables below the detection/quantification limit were numerically introduced, lack of mandatory variables for river studies such as flow and rainfall, arbitrary removal of pH > 7.5 values (which were not outliers), and finally how extreme values of other environmental variables were included. In addition, we identified conceptual and procedural mistakes such as biased construction/evaluation of model prediction capability. The study trained the model using pooled data from a short restricted lotic section of the (large) Uruguay River and from both lotic and reservoir domains of the Negro River, but then tested predictability within the (small) Cuareim River. Besides these methodological considerations, the article shows misinterpretations of the statistical correlation of cause and effect neglecting basic limnological knowledge of the ecology of harmful algal blooms (HABs) and international research on land use effects on freshwater quality. The argument that pH is a predictor variable for HABs neglects overwhelming basic paradigms of carbon fluxes and change in pH because of primary productivity. As a result, the article introduces the notion that HABs formation are not related to agricultural land use and water residence time and generate a great risk for the management of surface waterbodies. This reply also emphasizes the need for good practices of open data management, especially for public databases in view of external reproducibility.

Plastics and microplastics on recreational beaches in Punta del Este (Uruguay): Unseen critical residents?

Beaches are social-ecological systems that provide several services improving human well-being. However, as one of the major coastal interfaces they are subject to plastic pollution, one of the most significant global environmental threats at present. For the first time for Uruguayan beaches, this study assessed and quantified the accumulation of plastic and microplastic debris on sandy beaches of the major touristic destination Punta del Este during the austral spring of 2013. Aiming to provide valuable information for decision-making, we performed a detailed analysis of plastic debris, their eventual transport pathways to the coast (from land and sea), and the associated persistent pollutants. The results indicated that the smallest size fractions (<20 mm) were the dominant size range, with fragments and resin pellets as types with the highest number of items. PAHs and PCBs were found in plastic debris, and their levels did not differ from baseline values reported for similar locations. The abundance of plastic debris was significantly and positively correlated with both the presence of possible land-based sources (e.g. storm-water drains, beach bars, beach access, car parking, and roads), and dissipative beach conditions. The analysis of coastal currents suggested some potential deposition areas along Punta del Este, and particularly for resin pellets, although modeling was not conclusive. From a local management point of view, the development and use of indices that allow predicting trends in the accumulation of plastic debris would be critically useful. The time dimension (e.g. seasonal) should also be considered for this threat, being crucial for locations such as Uruguay, where the use of beaches increases significantly during the summer. This first diagnosis aims to generate scientific baseline, necessary for improved management of plastic litter on beaches and their watersheds.

Fish composition and species richness in eastern South American coastal lagoons: additional support for the freshwater ecoregions of the world.

The relationships between fish composition, connectivity and morphometry of 103 lagoons in nine freshwater ecoregions (FEOW) between 2·83° S and 37·64° S were evaluated in order to detect possible congruence between the gradient of species richness and similarities of assemblage composition. Most lagoons included in the study were <2 km(2) , with a maximum of 3975 km(2) in surface area. Combined surface area of all lagoons included in the study was 5411 km(2) . Number of species varied locally from one to 76. A multiple regression revealed that latitude, attributes of morphometry and connectivity, and sampling effort explained a large amount of variability in species richness. Lagoon area was a good predictor of species richness except in low latitude ecoregions, where lagoons are typically small-sized and not affected by marine immigrants, and where non-native fish species accounted for a significant portion of species richness. Relationships between species and area in small-sized lagoons (<2 km(2) ) is highly similar to the expected number in each ecoregion, with systems located between 18·27° S and 30·15° S attaining higher levels of species richness. Similarities in species composition within the primary, secondary and peripheral or marine divisions revealed strong continental biogeographic patterns only for species less tolerant or intolerant to salinity. Further support for the FEOW scheme in the eastern border of South America is therefore provided, and now includes ecotonal systems inhabited simultaneously by freshwater and marine species of fishes.