Relevant factors in the eutrophication of the Uruguay River and the Río Negro

doi:10.1016/j.scitotenv.2020.143299

Science of The Total Environment

Volume 761, 20 March 2021, 143299

https://doi.org/10.1016/j.scitotenv.2020.143299 Get rights and content

Highlights

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The algal bloom at Uruguay was promoted by the Climate Change.
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Chl-a levels were increased by temperature, electrical conductivity and pH of rivers.
•
Reduction in agricultural P emissions to rivers would not avoid high Chl-a levels.
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The replacement of agriculture by natural prairie do not avoid the algae bloom.

Abstract

In recent decades, there has been increasing eutrophication of rivers and lagoons in Uruguay and solutions leading to water purification are being sought. The growing pollution has been attributed to nitrogen and phosphorus compounds exported from the river basins with intensification of agricultural production and the absence of tertiary treatment for urban and industrial effluents. Although nitrogen and phosphorus are relevant to eutrophication, there are also other factors that can promote eutrophication and algal blooms. This paper reports a broad analysis of water quality variables recorded over 9 years (2009–2018) at 17 sampling stations on the Uruguay River and 16 sampling stations on the Río Negro, and explores their relationship with the changes of chlorophyll a (Chl-a) concentrations using a generalized linear model and a neural network simulation (NNS). The input variables were total phosphorus; total suspended solids; electrical conductivity of water (EC_w); alkalinity; water temperature (T); water pH (pH) and sampling month. The NNS explained 79% of Chl-a variations and showed the most relevant variables to be T, EC_w, and pH. Moreover, the NNS showed that replacement of current land uses by natural prairie would not significantly reduce Chl-a concentrations. The results showed that the main factors that drive Chl-a concentrations (i.e., algae) are not directly linked to agriculture land use.

Graphical abstract

Introduction

The contamination of freshwater bodies is one of the most important environmental impacts threatening sustainable development. This is agreed in the Millennium Development Goals (Sachs, 2012), the risk analysis of the World Economic Forum (Bakker, 2012; World Economic Forum Water, 2009), and by the International Organization for Standardization (e.g., ISO, 2014) through the Water-footprint Use in Life Cycle Assessment working group. All of these promote both the protection of freshwater bodies and the water quality in them, mainly against anthropic eutrophication processes.

Eutrophication is the enrichment with minerals and nutrients that increase the natural primary production (i.e., algae and macrophytes) of water bodies (lentic and lotic) (Wetzel, 2001). Therefore, concentrations of nitrogen, phosphorus, and chlorophyll a (Chl-a) are monitored to assess the degree of anthropic eutrophication being promoted in water bodies (Wetzel, 2001). The eutrophication could result from point and/or diffuse pollution sources such wastewater discharges and fertilizers washed from basins, respectively. High Chl-a concentrations generally correspond to high amounts of algae in a freshwater body. Thus, it is possible to infer that increasing Chl-a concentrations are positively correlated with increasing risk of harmful cyanobacteria (Watson et al., 1997).

Since 1982, Uruguay has been reporting anthropic eutrophication, and environmental management of these impacts has focused on control of emissions of nitrogenous and phosphorus compounds (Conley et al., 2009; Wetzel, 2001). However, Wetzel (2001) has pointed out that can be other factors that promote eutrophication and thus algal blooms. Variables such as water temperature changes (Konopka and Brock, 1978; Rose and Caron, 2007; Zhao et al., 2019); loss of freshwater herbivores (Delpla et al., 2009; Yoshimura and Endoh, 2005); increasing electrical conductivity of the water (EC_w) (Casanova et al., 2009) by ions in runoff from agricultural areas (Armstead et al., 2016; Bowling, 1994; Bowling and Baker, 1996; Delpla et al., 2009; Schuytema et al., 1997); and increases in pH may all promote cyanobacteria growth (Gao et al., 2015; Zhao et al., 2019). These key influences highlight the importance of land use changes in basins. Greater soil cover can reduce soil erosion; plants protect the soil, as reflected in the (universal soil loss equation, revised universal soil less equation) USLE/RUSLE models C-factor. Better soil cover can also reduce total phosphorus export to freshwater, as identified for the Río Negro (Beretta, 2019), and the EC_w according to Carrasco-Letelier et al. (2014) and Carrasco-Letelier and Beretta-Blanco (2017).

Neural network simulation (NNS) is a data analysis tool that has recently become available for most personal computers. This methodology allows a multidimensional analysis of datasets relating diverse problems (Karul et al., 2000). NNS can identify contributing relationships with a specific variable and simulate potential scenarios (Gao et al., 2015; Huo et al., 2013). Thus, this process has been used to determine the influence of changes in several freshwater variables; a diagnostic process with less bias than when variables are defined on the basis of expert experience (Gao et al., 2015; Huo et al., 2013; Karul et al., 2000).

In the current situation, Uruguay does not have any management tool to predict the increase in Chl-a concentrations, and it was assumed that the phosphorus load is the main factor that promotes eutrophication. However, Wetzel (2001) and Allan and Castillo (2007) have pointed out that eutrophication is a multi-factor process. Therefore, to achieve sound environmental management, a tool that reduces bias and the influence of the researcher's beliefs on the diagnosis is required. In this direction, the simulation of the NNS could help both in the identification of the main variables that promote eutrophication; as in the simulation of the change in water quality due to the change in land use of the basin, if this information is used by NNS. For current freshwater problems, an NNS would offer an unbiased diagnostic tool and simulation capability to assist environmental management of the basin's land use.

If we recognize that the eutrophication process is a multidimensional event, a reductionist and unidimensional way of analysis would not identify all relevant variables linked to increasing Chl-a. Any resulting model would fail to adequately simulate the potential risk of algal blooms when key factors are modified. The hypothesis of this work is, therefore, that an NNS can identify the variables that drive increases in Chl-a concentrations, and consequently the factors that must be managed to avoid algal blooms. To explore this hypothesis, a database of freshwater variables from the Uruguay River and the Río Negro were analyzed using an NNS to identify which freshwater and land use variables are most closely linked to Chl-a changes and how the rivers would be affected if the use of the land would be restored to its natural (i.e., natural prairies).

Section snippets

Database of freshwater variables

A database of freshwater variables (Table 1) was acquired from the open database service of the National Environmental Agency (MVOTMA, 2019) for the period 2009-05-01 to 2018-11-30 for the Río Negro and for the period 2014-06-01 to 2018-11-30 for the Uruguay River (Fig. 1). The freshwater variables included: Chl-a expressed in μg L⁻¹; alkalinity (ALC), expressed in mg L⁻¹CaCO₃; electrical conductivity of water (EC_w), expressed in μS cm⁻¹; total phosphorus (TP), expressed in μg L⁻¹; total

Annual change analysis

The annual values in the period 2009–2018 showed decreasing trends in pH, ALC, EC_w and TSS and an increasing trend for T in the Río Negro (Table 3). The TP and Chl-a concentrations did not show any temporal trend. In the Uruguay River, ALC, TP and T values decreased over time; pH, EC_w and TSS increased, and Chl-a did not change significantly.

Freshwater changes linked to Chl-a concentrations

The recorded Chl-a concentrations (Fig. 3) in the studied rivers had positive, but low, correlations with: T (R² = 0.25; p < 0.01), EC_w (R² = 0.14;

Discussion

The differences in the annual trends of freshwater variables between the Uruguay River and the Río Negro could be the result of differences in water sampling before July 2014; subsequently, a common sampling procedure (i.e., subsurface sampling) was applied to both rivers.

While the Uruguay River had lower values than the Río Negro for ALC, TSS, and EC_w, both rivers had an increasing trend in TSS, and EC_w. The mean annual temperature showed a downward trend in the Uruguay River, but increased in

Conclusion

NNS was found to effectively determine the variables linked to changes in Chl-a concentrations. However, to improve the Chl-a estimation, it is necessary to increase the spatial and temporal frequency of sampling.

The average values of Chl-a in the channel waters of the Uruguay River and the Río Negro depend mainly on T, EC_w, and pH. However, when using all eight variables analyzed here, it is possible to estimate Chl-a values, with acceptable accuracy, in the range from 5 to 50 μg L⁻¹. Current

CRediT authorship contribution statement

Andrés Beretta-Blanco: Conceptualization, Investigation, Methodology, Validation, Formal analysis, Writing - original draft. Leonidas Carrasco-Letelier: Conceptualization, Investigation, Writing - review & editing, Visualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Relevant factors in the eutrophication of the Uruguay River and the Río Negro

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Database of freshwater variables

Annual change analysis

Freshwater changes linked to Chl-a concentrations

Discussion

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Limnologica

Eur. J. Agron.

Environ. Int.

Procedia Environ. Sci.

Ecol. Model.

Lancet

Harmful Algae

Curr. Biol.

Sci. Total Environ.

Mapa solar del Uruguay: versión 1.0: memoria técnica

Trends in water temperature of large European rivers and lakes

Stream Ecology - Structure and Function of Running Waters

The effects of elevated specific conductivity on the chronic toxicity of mining influenced streams using Ceriodaphnia dubia

PLoS One

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Aquat. Microb. Ecol.

Water security: research challenges and opportunities

Science

Impacto de la erosión en el contenido de fósforo total en agua del Río Negro

Soil quality decrease over 13 years of agricultural production

Nutr. Cycl. Agroecosyst.

Cianobacterias y cianotoxinas en ecosistemas límnicos de Uruguay

Innotec

Occurrence and possible causes of a severe cyanobacterial bloom in Lake Cargelligo, New South Wales

Mar. Freshwater Res

Major cyanobacterial bloom in the Barwon-Darling River, Australia, in 1991, and underlying limnological conditions

Mar. Freshw. Res.

Fluctuations of the population of Daphnia laevis Birge 1878: a six-year study in a tropical lake

Braz. J. Biol.

Soil erosion by water estimated for 99 Uruguayan basins

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