Freshwater, which represents 3% of the planet’s water ( Gleick, 1993 ), is an indispensable resource for humanity and the subsistence of an enormous biodiversity of flora and fauna on all continents ( Reid et al., 2019 ). Despite their importance, today, freshwaters are subject to environmental pressures from a set of anthropic factors along their shores (industrial, agricultural, water supply, recreation, etc.), which can deteriorate water quality ( Huang & Zhou, 2021 ) and trigger eutrophication ( Ali & Khairy, 2016 ), highlighting that associated with cases of eutrophication, the proliferation of algal blooms cause diseases in human health and compromise ecosystem services such as the reduction in food of aquatic origin, local tourism and availability of water for human consumption, among others ( Lee et al., 2015 ; Huisman et al., 2018 ). According to results of studies in aquatic ecology, phosphorus is an essential element for all forms of life ( Bunting et al., 2007 ); in high concentrations, together with nitrogen ( Yao et al., 2018 ), it triggers the phenomenon of eutrophication ( Vincon-Leite & Casenave, 2019 ), a condition of high production of algae and cyanobacteria that limit the availability of oxygen and cause mortality in aquatic fauna, highlighting that its scientific importance favored the study of the proliferation of harmful algal blooms ( Paerl & Huisman, 2008 ; Burford et al., 2020 ).

The Water Quality Index (WQI) is a fundamental tool for assessing water quality, applied globally to surface waters ( Uddin et al., 2021 ). The development of WQI models begins with the one developed in the 1960s by R. Horton, based on the 10 physical, chemical and biological parameters ( Horton, 1965 ). Years later, Brown, with the support of the National Sanitation Foundation, developed the NSF-WQI model, together with a group of 142 experts on the current topic ( Abbasi & Abbasi, 2012 ). Currently, there are a considerable number of indices have been developed for marine and continental environments ( Uddin et al., 2021 ), and given the need for tools to evaluate waterbodies with particular characteristics, new models continue to be implemented ( Chidiac et al., 2023 ).

The present research focuses on Lake Titicaca, a transboundary waterbody between Peru and Bolivia in South America, known for being the highest navigable lake in the world ( Chura-Cruz et al., 2013 ), but also because it is the center of attention of researchers dedicated to the environmental problems that have been considered, including studies on eutrophication ( Heredia et al., 2022 ), water quality assessment ( Farfán et al., 2015 ; Delgado et al., 2022 ), remote sensing ( Baltodano et al., 2022 ), antibiotic pollution ( Archundia et al., 2017 ), heavy metals ( Chui et al., 2021 ; Quiroga-Flores et al., 2021 ; Biamont-Rojas et al., 2023 ), algal blooms ( Duquesne et al., 2021 ) and microplastics ( Loayza et al., 2022 ), among others. Given the lack of tools to assess the evolution of the water condition in Lake Titicaca ( Mancilla, 2016 ), the objectives of this research were first to calibrate a water quality index for Lake Titicaca (WQI_T) based on physical, chemical, and microbiological parameters; second, assess the water quality and trophic state of the highly anthropic sector of Lake Titicaca.

Our study focused on developing a water quality assessment of Lake Titicaca, a waterbody of high ecological, economic and social importance in southern Peru ( Zilov, 2013 ). The deterioration of the lake’s health is caused by complex pollution related to urban demographic growth, inadequate management of solid waste and wastewater, and inadequate sanitary practices ( Quispe, 2024 ).

Given the need to integrate technical information from the different water quality parameters carried out by Peruvian state agencies, in order to efficiently interpret the water quality condition of Lake Titicaca, we propose the use of the water quality index model, which has the ability to convert multiple variables into a single value that describes water quality ( Banda & Kumarasamy, 2020 ). To this end, we calibrated a Water Quality Index for Lake Titicaca, the WQI_T, based on the original index proposed by Brown et al. (1970) for the US National Sanitation Foundation, the NSF-WQI. Comparing the efficiency of these two indices, the WQI_T revealed important differences in relation to the NSF-WQI, presenting water quality weights of less importance for the parameters T, DO and TU, and weights of greater importance for the parameters pH, NO₃, PO₄-P, TC, BOD₅ and TDS, highly related to eutrophication processes.

In fact, the water quality of Lake Titicaca established by applying the WQI_T showed a variation from moderate polluted bays to bad quality bays, such as Desaguadero and Yunguyo, and no bay reached good or very good quality. On the contrary, applying the NSF-WQI all bays were classified as waters of moderate quality. The water quality assessment of the Yunguyo and Desaguadero bays applying the WQI_T showed a significant difference with the others bays (p ≤ 0.05), being characterized as having bad water quality.

Regarding phosphate, one of the parameters most related to eutrophication, these two bays had an average concentration of 0.38 ± 0.4 mg∙L⁻¹, and can be classified as hypereutrophic, as this average exceeded the maximum limit concentration for this category, 0.211 mg∙L⁻¹, following the classification adopted by Barreto et al. (2013) , as shown in Table 4 . Therefore, the uses attributable to this condition are only irrigation and energy production ( Table 5 ). The results applying the NSF-WQI were unable to identify this significative difference, as all bays were classified as waters of moderate quality. This result indicates that the calibration of the WQI_T was adequate, as it allows inferring and estimating the water quality of Lake Titicaca with greater precision, where the water ecosystem health is deteriorated by wastewater pollution, a common occurrence in urbanized lakes ( Radwan et al., 2019 ), or aquaculture pollution, a triggering factor for the proliferation of cyanobacteria bloom according to studies carried out in lakes where this aquaculture activity takes place ( Buley et al., 2021 ; Xu et al., 2022 ). Thus, the WQI_T calibrated for Lake Titicaca can be used as an efficient tool to assess water quality in high Andean lentic waterbodies in South America, which do not exist ( Uddin et al., 2021 ).

According to Peru’s water quality standard for category 4, established for the conservation of the country’s lakes, the parameters that exceeded the standard values were PO₄-P (0.035 mg∙L⁻¹) and BOD₅ (5 mg∙L⁻¹) in all bays, and TC (1000 MPN mL⁻¹) in Yunguyo bay. These high values are directly related to eutrophication processes. A similar situation was found by Bajaña et al. (2022) , working in two reservoirs in the Paute River Hydrographic Basin, south inter‑Andean region of Ecuador, where the PCA results indicated that the eutrophication gradient was positively correlated with phosphate (r = 0.83) and BOD₅ (r = 0.83), where the average total phosphate concentration was 0.73 ± 1.51 mg∙L⁻¹, classifying the basin water as hypereutrophic. Eutrophication is also one of the main environmental problems in aquatic systems in southern Brazil (e.g., Lobo et al., 2010, 2015 ; Klamt et al., 2019 ), highlighting that the increase of nutrients in a waterbody, mainly phosphate, is one of the main sources of eutrophication, mostly by agricultural fertilizers, animal waste, domestic and industrial sewage. Eutrophication is a process that can render an aquatic ecosystem unusable for human supply, power generation, and leisure area ( Powley et al., 2016 ).

Eutrophication and algal bloom events are common in waterbodies with high population density, such as Lake Taihu in China ( Yan et al., 2022 ), Lake Victoria in Africa ( Simiyu et al., 2022 ), Lake Ontario in Canada ( Molot et al., 2022 ) or Lake Erie in the United States ( Francy et al., 2016 ). For South America, we show the context of water quality in Lake Titicaca, where algal blooms were reported in the inner bay of Puno ( Farfán et al., 2015 ) and Cohana ( Achá et al., 2018 ), places with high phosphorus loads from untreated waters in Puno (Peru) in the first case, and La Paz (Bolivia) in the second case, a worrying situation. However, in our study, we reported a eutrophication process based on high values of PO₄-P (0.09 to 0.46 mg∙L⁻¹) for all bays of Lake Titicaca, critical because the waters could develop algal blooms at these levels ( Walker & Havens, 1995 ; Gu et al., 2020 ; Song et al., 2024 ). Likewise, we draw attention to the high levels of coliforms (>1000 MPN ml∙L⁻¹) in the Yunguyo and Desaguadero bays, which is risky for human health ( Soller et al., 2010 ), since these places are frequented by tourists ( Gascón, 2022 ).

The high BOD₅ values in the bays in Lake Titicaca are mainly influenced by wastewaters from nearby cities ( Farfán et al., 2015 ; Heredia et al., 2022 ) and the aquaculture industry ( Tanjung et al., 2024 ), which focuses on rainbow trout aquaculture developed in the littoral zone of the Lake Titicaca ( Chura & Mollocondo, 2016 ). BOD₅ plays an important role in assessing water pollution ( Koda et al., 2017 ; Tanjung et al., 2019 ), highlighting that this parameter explains the biological oxidation processes, knowledge necessary to understand the water quality state ( Qi et al., 2021 ; Aguilar-Torrejón et al., 2023 ). Organic matter originates from various anthropic wastes and natural process ( Pivokonsky et al., 2006 ; Tanjung et al., 2022 ).

We concluded that the WQI_T calibration was adequate, as it allows inferring and estimating the water quality of Lake Titicaca (Peruvian sector), with greater precision, based on physical, chemical, and microbiological parameters, where the predominant condition was moderate quality and bad quality in the 10 bays analyzed.

The deterioration of the lake’s health is caused by complex pollution related to urban growth, inadequate solid waste and wastewater management, and inadequate sanitation practices, highlighting the need for water management to mitigate the eutrophication process. In the future, it is necessary to have information available that allows estimating lake quality indices, so it is extremely important that the competent authorities carry out periodic monitoring (seasonal frequency) in the evaluated sectors and expand new sampling locations that need to be evaluated to improve water quality management.

The WQI_T calibrated for Lake Titicaca can be used as an efficient tool to assess water quality in high Andean lentic waterbodies in South America.

We would like to thank the University of Santa Cruz do Sul, Brazil, for granting an institutional scholarship to the first author to pursue a postgraduate course in the Environmental Technology Program. We also thank the Lake Titicaca Special Project of the Ministry of Agriculture and Irrigation of Peru.