GEOTECHNOLOGIES IN THE ASSESSMENT OF EUTROPHICATION IN THE BARRA BONITA RESERVOIR, SÃO PAULO, BRAZIL

Objective: The objective of this study was to identify locations with higher eutrophication and, consequently, susceptible to the emergence of macrophytes in the Barra Bonita reservoir, SP. Theoretical background: The presence of macrophytes, such as Eichhornia crassipes, is influenced by ecological succession driven by human activities like introducing exotic species and discharging untreated nutrients. Effective management, including buffer zones, is suggested to reduce eutrophication, though some studies question their effectiveness on a large scale. Method: The analysis of the Normalized Difference Vegetation Index (NDVI) was conducted using remote sensing images obtained from Landsat 8 and 9 satellites. As a criterion for data acquisition, corresponding data for the study region were obtained over a one-year temporal scale with a maximum cloud cover of 0%. Results and conclusion: Analyzing the images, it is observable that the month of June 2023 exhibited areas with a higher vegetation index and, consequently, eutrophication processes. This trend gradually decreased in the months of August, September, and November. Research implications: The analysis of satellite images revealed specific patterns of eutrophication, suggesting a


INTRODUCTION
Reservoirs play various essential roles in human activities, such as storing large water volumes for human supply, agricultural and industrial activities, recreation, fishing, and primarily, power generation (Henry, 1999;Nogueira, 2005;Henry, 2014).In recent years, reservoirs have been facing eutrophication processes due to human activities, i.e., excessive accumulation of nutrients such as nitrogen and phosphorus, leading to uncontrolled algae growth and issues like decreased oxygen levels in water and aquatic organism mortality (Buzelli;Cunha-Santino, 2013;Trindade;Mendonça, 2014).Assessing vulnerability and methods to measure reservoir eutrophication are crucial tools for understanding the process (Figueirêdo, 2007;Freire;Souza Filho, 2022).Additionally, eutrophication, coupled with siltation and increased shoreline zones, favors the emergence of macrophytes or aquatic plants that can hinder power generation by impeding water flow through turbines, among other reported problems (Thomaz, 2002;Carvalho et al., 2005).Furthermore, geotechnologies can assess water quality (Tavares Junior et al., 2018;Pompêo;Moschini-Carlos, 2022).Satellite images provide a holistic and continuous view, enabling the detection of spatial and temporal changes, facilitating mitigating measures (Prado;Novo, 2007).The use of these tools can enhance effectiveness in monitoring eutrophication, allowing a quicker and more efficient response for the sustainable management of reservoirs like Barra Bonita in São Paulo.
The Barra Bonita reservoir, located in the middle Tietê basin, State of São Paulo, was built in 1963 for electricity production and is also used for fishing, recreation, tourism, navigation, and irrigation.Its event cycle is influenced by the hydrological characteristics of the basin (Tundisi et al., 2008).According to the authors, the reservoir is polymictic, with short periods of vertical stability and 114 tributaries.
The aim of this study was to identify locations with higher eutrophication, susceptible to the emergence of macrophytes in the Barra Bonita reservoir.It also aimed to assist with obtained data in implementing mitigating measures that could reduce the eutrophication process and consequently the proliferation of macrophytes.

THEORETICAL FRAMEWORK
In addressing the challenges posed by eutrophication processes and macrophyte proliferation in reservoirs, it is significant to draw upon a robust theoretical foundation encompassing various disciplines, including limnology, hydrology, ecology, and geotechnology.

Limnology and Eutrophication Theory
Limnology provides fundamental insights into the processes that control eutrophication.Central to this concept is the idea of nutrient loading, where excessive inputs of nitrogen and phosphorus from anthropogenic sources drive algae growth, leading to ecosystem degradation (Nwankwegu et al., 2019).Additionally, limnological theories clarify the dynamics of water column stability and mixing regimes, influencing nutrient cycling and oxygen availability (Zhang et al., 2019).

Hydrological Dynamics
Understanding the hydrological characteristics of reservoirs is pivotal for assessing vulnerability to eutrophication and macrophyte proliferation.Concepts such as residence time, flow regime, and hydrological connectivity play critical roles in modulating nutrient transport and sedimentation processes (Hoffmann, 2015).Moreover, the interplay between hydrological variability and ecological responses underscores the need for integrated approaches in reservoir management (Olden;Naiman, 2009).

Ecological Interactions
Ecosystem-based approaches emphasize the integrated interactions between biotic and abiotic components in shaping reservoir dynamics.Ecological theories elucidate the cascading effects of eutrophication on trophic structure, biodiversity, and ecosystem services (Saint-Béat et al., 2005).

Geospatial Analysis and Remote Sensing
Geotechnologies, including remote sensing and geographic information systems (GIS), provide fundamental tools for assessing and monitoring reservoirs.Remote sensing data offer information on water quality parameters, spatial and temporal distribution of potential macrophyte presence, and changes in land cover (Murray et al., 2017).Integrating geospatial analysis with ecological models facilitates predictive mapping and scenario planning, enhancing decision-making in reservoir management (He, 2003).
By integrating these theoretical frameworks, this study aims to contribute to understanding the consequences of eutrophication in the Barra Bonita reservoir, employing a multidisciplinary approach to infer sustainable management strategies.Through the synthesis of limnological, hydrological, ecological, and geotechnological perspectives, this research seeks to advance our understanding of reservoir dynamics and contribute to its conservation.

Data processing and analysis
The analysis of the Normalized Difference Vegetation Index (NDVI) was conducted using remote sensing images obtained from Landsat 8 and 9 satellites (Table 1).As a criterion for data acquisition, corresponding data for the study region were obtained over a one-year temporal scale with a maximum cloud cover of 0%, from the official website of the United States Geological Survey (USGS).For the calculation itself, the employed method (NDVI) was executed using QGIS software (version 3.28.14),where the difference between Near-Infrared (NIR) and Red bands is obtained and then normalized by the consequent sum, resulting in data that can vary from -1 to 1.The process is represented below: After completion, the data were organized in the software and represented to highlight potentially eutrophied areas (values greater than 0).

RESULTS AND DISCUSSION
The Normalized Difference Vegetation Index (NDVI) is a measure that utilizes spectral data to assess the quantity and condition of vegetation in a specific area.It is often used in remote sensing and satellite image analysis to monitor plant health and vegetation cover (Minhonhi et al., 2017).Generally, high indices are associated with healthy vegetation, while lower values may indicate a lack of vegetation or less favorable conditions for plant growth.The NDVI was calculated from March 2023 to November, excluding July and October 2023 where no images were available.
In figures 2, 3, and 4, it can be observed that the eutrophication process is concentrated in the Tietê River, São Paulo, Brazil.Analyzing the remaining images, it is noticeable that June 2023 presented locations with a higher vegetation index, and consequently, eutrophication processes (Figure 5), which decreased in August (Figure 6), September (Figure 7), and November (Figure 8).Analyzing the images, it is noticeable that the month of June 2023 exhibited areas with a higher vegetation index and, consequently, eutrophication processes (Figure 5).This trend gradually decreased in the months of August (Figure 6), September (Figure 7), and November (Figure 8).
A study based on samples collected at 30 stations between 1990 and 2002 had already highlighted a significant increase in the water trophic level in the Barra Bonita reservoir (Prado;Novo, 2007).According to the authors, water quality degradation was related to land use changes and population growth detectable through geotechnologies.Additionally, the pollutant potential in the basin is not uniform, emphasizing the need for more localized control measures in the most vulnerable areas.
The most occurring macrophytes in the Barra Bonita reservoir were Brachiaria mutica, Brachiaria subquadripara, and Eichhornia crassipes (Carvalho et al., 2003).Aquatic vegetation colonization occurs during ecological succession, primarily driven by human activities such as the introduction of exotic species, discharge of untreated phosphorus and nitrogen, and fertilizers from extensive sugarcane plantations in the watershed (Thomaz, 2002;Tundisi et al., 2008;Santos;Medeiros, 2023).According to the authors, developing effective methods with low environmental impacts remains a challenge, and management may be necessary to reduce populations in situations like excessive proliferation in reservoirs.
Reducing the eutrophication process in reservoirs involves implementing strategies and practices that reduce nutrient entry from the soil to the tributaries forming the reservoirs.Therefore, one of the most important measures for reducing eutrophication is reducing the nutrient source.Several studies on agricultural ecosystem management have pointed to the potential benefits of buffer zones in improving tributary quality in regions with small agricultural areas (Ribaudo et al., 2001;Turner and Rabalais, 2003;Schröder et al., 2004).Buffer zones can be defined as areas of vegetation on riverbanks that play a crucial role as an ecological filter between agroecosystems and water bodies.Thus, it is possible to mitigate the negative impacts of conventional agricultural practices on aquatic ecosystems.However, despite positive evidence of the environmental benefits provided by these buffer zones, some studies question their effectiveness as an ecological filter on a large scale, especially in areas with intensive agriculture or at the watershed level (Correll, 2005;Lovell and Sullivan, 2006;Stutter;Richards, 2012).
The satellite image results of this study provide a holistic view of eutrophication, indicating areas susceptible to macrophyte presence.However, upon examining the images and indices, it is evident that the primary sources of nutrients for the tributaries forming the Jurumirim reservoir were notably more eutrophied in the municipalities of Botucatu, SP, and Anhembi, SP in June 2023.The images allowed the identification of specific eutrophication patterns, suggesting a direct correlation between the geographical distribution of nutrients and the potential proliferation of macrophytes (Diniz et al., 2005).Valente et al. (1997) compared nutrient concentrations in Lavapés stream, Botucatu, SP, with those of the Capivara River, both flowing into the Barra Bonita reservoir, SP.According to the authors, both had similar characteristics, indicating similar rural pollution, but Lavapés was more affected by urban pollution in Botucatu.In any case, the authors concluded that urban pollution from Botucatu not only harms water quality in the municipality but also contributes to eutrophication in the Barra Bonita reservoir.
In this context, it is noteworthy that effective management of water residence time can be considered a crucial strategy to reduce short-term phosphorus concentration in reservoirs.Wiegand et al. (2016) conducted a comparison of the eutrophication state and its possible causes in two tropical reservoirs, one in a humid climate and another in a semi-arid climate.According to the authors, by estimating trophic state during the wet and dry seasons of the municipalities, the study concluded that the high water residence time in the semi-arid basin, due to the reduced runoff coefficient and excessive evaporation, indicates that semi-arid lakes are more vulnerable to eutrophication during dry periods, requiring more restrictive management to control nutrient production.
Based on the presented discussion, it can be concluded that the Barra Bonita reservoir faces significant challenges related to eutrophication, influenced by changes in land use, population growth, and agricultural practices.The increase in the water trophic level, highlighted in studies from 1990 to 2002, points to water quality degradation primarily attributed to the introduction of exotic species, discharge of untreated nutrients, and fertilizers from sugarcane plantations in the watershed.The presence of macrophytes, such as Brachiaria mutica, Brachiaria subquadripara, and Eichhornia crassipes, reflects ecological succession driven by human activities.Managing these populations is challenging, and more efficient methods with low environmental impacts are needed to address excessive proliferations.The implementation of buffer zones emerges as a possible strategy in reducing nutrient entry into tributaries feeding the reservoirs, requiring further in-depth studies.Despite success on another continent, some studies question the effectiveness of these zones, especially in areas with intensive agriculture.However, effective management of water residence time is considered a crucial strategy to reduce short-term phosphorus concentration in reservoirs, as highlighted in research emphasizing the vulnerability of semi-arid lakes to eutrophication during dry periods.
The analysis of satellite images revealed specific patterns of eutrophication, suggesting a direct correlation between the geographical distribution of nutrients and the potential proliferation of macrophytes.Additionally, comparative studies of nutrient concentrations in tributaries indicate that urban pollution, besides affecting local water quality, contributed to eutrophication in the Barra Bonita reservoir.

CONCLUSION
In conclusion, integrated management, including localized control measures, nutrient source reduction strategies, effective macrophyte management, and buffer zone implementation, can be significant strategies to address challenges associated with eutrophication in the Barra Bonita reservoir.These actions should be adapted to the specific characteristics of the watershed, taking into account climatic factors, agricultural practices, and urban impacts.

Figure 1 :
Figure 1: Study area located in Barra Bonita and further municipalities, São Paulo.Source: Prepared by the authors (2024).

Figure 2 :
Figure 2: NDVI of the study area in March 2023.Source: Prepared by the authors (2024).

Figure 3 :
Figure 3: NDVI of the study area in April 2023.Source: Prepared by the authors (2024).

Figure 4 :
Figure 4: NDVI of the study area in May 2023.Source: Prepared by the authors (2024).

Figure 5 :
Figure 5: NDVI of the study area in June 2023.Source: Prepared by the authors (2024).

Figure 6 :
Figure 6: NDVI of the study area in August 2023.Source: Prepared by the authors (2024).

Figure 7 :
Figure 7: NDVI of the study area in September 2023.Source: Prepared by the authors (2024).

Figure 8 :
Figure 8: NDVI of the study area in November 2023.Source: Prepared by the authors (2024).

Table 1 .
Technical information of Landsat 8 and 9 satellites used in the analysis.