In December 2019, China reported the emergence of a novel respiratory disease, Coronavirus Disease 19 (COVID-19), caused by the SARS-CoV-2. This highly infectious pathogen primarily targets the human respiratory tract, inducing symptoms similar to those of common influenza ( Zhou et al., 2020 ). The rapid global spread of SARS-Cov2 led to over 170,000 reported cases and approximately 6,500 deaths. The World Health Organization (WHO) declared a Public Health Emergency of International Concern on January 30, 2020, and elevated it to a pandemic on March 11, 2020 ( WHO, 2020 ).

Cameroon confirmed its first case on March 6, 2020, and the disease’s progression in country can be divided into three distinct periods. The initial exponential increase in cases from March 25 to May 26, 2020, was followed by a significant decline from June 25, 2020, to January 25, 2021. However, a resurgence was observed in February 2021 with over 3,000 new cases recorded within a week (from March 3 to 7, 2021). By October 13, 2021, 100,289 cases were reported ( Anonymous, 2021 ). Yaoundé, due to its high population density and mobility ( Nsegbe et al., 2023 ), has been a significant urban center of COVID-19 spread in Cameroon. The peak of confirmed cases was recorded in May 2020 ( Placard, 2021 ).

The measures deployed to fight against the pandemic, including individual and collective protection, have a significant impact on water quality. These measures include “barrier” gestures, enhanced hygiene practices leading to the widespread use of disinfectants and antiseptics, the use of protective tools like face masks, and the administration of various medications ( Poschet et al., 2020 ). These materials could potentially enter access natural waters and exacerbate existing pollution ( Mimouni, 2020 ; Moussima et al., 2020 ), particularly given the inadequacies in urban waste management system ( Ajeagah et al., 2018 ). Contamination of water resources with chemical components poses severe environmental and health concerns. For instance, water with a pH outside the normal range can lead to nutritional imbalances and hinder the growth and development of aquatic life ( Kouame, 2021 ). The integrity of water thus depends on various physicochemical parameters and their concentrations. To evaluate the effect of the COVID-19 response on the physico-chemical parameters of surface waters in Yaoundé, we monitored various parameters over 8 months period.

To assess the impact of the widespread use of disinfectants and various medication in response to COVID-19 on the surface waters of Yaoundé, various physicochemical parameters were measured in four rivers during an 8 months period.

Temperature plays a crucial role in the distribution and development of aquatic fauna and flora ( Chauhan, 2013 ). According to WHO (2002) , the required temperatures range for a healthy aquatic ecosystem is between 15˚C to 25˚C. In this study, they ranged from 23˚C to 30˚C; however, they were comparable to those reported by Ajeagah et al. (2018) , Awawou et al. (2021) (26.2˚C), and Zogning (2017) (27˚C) in their studies on the Mingoa and Mfoundi, respectively. The temperature of a river is primarily determined by the thermal regime and solar radiation. The P10 and P2 points showed the highest average temperatures. This could could be justified by the sampling hours, which were fixed between 6 am ± 1 hour and 11 am ± 1 hour, with the last sampling at 10:30 am - 11 am. However, temperatures above 25°C could pose a threat to the life of many aquatic organisms, particularly aerobic ones, due to the reduction in dissolved oxygen levels ( Walaa, 2018 ).

The levels of dissolved oxygen saturation varied during the study. Its values ranged from 33 to 81.25% in Mfoundi, 30 to 81.92% in Tongolo, and 43 to 88% in Mingoa. Compared to the values obtained by Ajeagah et al. (2018) (25%) in the Mingoa and by Lami (2002) (22.83% ± 17.15%) in the Tongolo, the values reported in our study for the same courses have slightly improved. This could indicate a decrease in mineralization activity by microorganisms in these environments due to a lesser input of organic pollutants. Indeed, in order to limit the spread of SARS-CoV-2, several restrictive measures were enacted during the Covid-19 health crisis by the Cameroonian government. These have led to the cessation (closure of public and private training institutions) or the slowdown (temporary closure of markets, bars, restaurants...) of numerous economic activities. Which would have favored the reduction of their pollution in these courses. This result has been found by Edward et al. (2021) in their study in the gulf Mannar. The difference observed between P3 and P2 of Jamot Hospital (Tongolo) could be justified by a drainage issue. Indeed, unlike P3, the water flow at P2 is heavily obstructed by the solid waste present there. Baok (2007) emphasizes in this regard that the oxygen content of the environment can only be enriched by the circulation of water in contact with air on one hand, and by the photosynthetic activity of its flora on the other. The difference between P1 (highest point) and P10 (lowest point) of Mfoundi was not statistically significant (P > 0.05). Which is contrary to the findings of Tanawa et al. (2003) who report that the state of the water in the Mfoundi river gradually deteriorates from upstream to downstream and worsens particularly at the P10 level (known as interchange no. 1 on the heavy axis Yaoundé-Douala). The similarity between the two points in this study would indicate a greater involvement of households in the pollution of this stream during the crisis.

The pH allows for the evaluation of the acid-base balance of water ( Nkeng, et al., 2020 ). On average, it varied from 7.3 upstream to 7.7 downstream of the Mfoundi, it was 7.23 in Tongolo and 7.44 in Mingoa. No statistically significant difference (P > 0.05) was detected between the values recorded at points along the same watercourse during the study. The results obtained in this work at the Mfoundi and Mingoa sites differ from those obtained by INS-BGR (2013) , Menye et al. (2012) , Tanawa et al. (2003) at the first site and by Ajeagah et al. (2018) , Kouam (2004) at the second. Indeed, regarding the Mfoundi, these authors reported slightly acidic (6) to basic (9) pH levels from upstream to downstream of the course. At the Mingoa level, the reported values for this parameter were slightly acidic (<7). Which was not the case in our study. This difference reveals, compared to previous studies, a change in environmental conditions. This change could be related to the increase in households’ adherence to hygiene rules, particularly due to the increased use of soaps and detergents recommended during the crisis. Covid-19. These products, due to the failure of the waste sanitation system in most households, ended up in natural waters and caused an increase in their pH. The INS-BGR (2013) emphasizes that the pH of surface waters increases in accordance with the input of wastewater rich in alkaline detergents (soaps) and toilet wastewater. Moreover, this modification could also be related to the presence of azithromycin in the waters, a basic nature antibiotic ( Parnham et al., 2014 ) widely used during the health crisis to combat the coronavirus. Indeed, in their study on the water bodies of the city of Yaoundé, Scaccia et al. (2022) highlighted the presence of this antibiotic in trace amounts. The standard values required for water use would be between 6.5 and 9.2 ( WHO, 2011 ). The values recorded in P4 (6.3), P5 (6.2), and P10 (9.5) would indicate intense microbial degradation activities of organic matter ( Kouame, 2021 ) resulting from point source pollution. It is noted that an increase in the pH of the waters, as observed in this study, could promote the scaling of the channels and a poor effectiveness of chlorine during treatment, exposing users to risks of ocular and skin irritations during swimming and also being harmful to aquatic fauna. Zakaria (2020) , in his work on the coast of Mostaganem, emphasizes that basic pH levels would increase ammonia concentrations, which would be toxic to fish.

The incidence of Covid-19 cases was positively (rs = 0.812) and significantly (P = 0.014) correlated with oxidability in the Tongolo course. This result could be an indirect consequence of the psychosis generated by the outbreaks of Covid-19 cases in the city of Yaoundé. Indeed, during the outbreaks, this psychosis would have led the residents along this course to become more aware of the meticulous application of preventive measures, such as cleaning their environment, particularly the Nkoleton market located about 10 meters upstream from P3. The wastewater enriched with organic substances from these cleanings could thus have contributed to the pollution of this watercourse. The observed inversion of oxidability levels between P1 (10.26 mg/l of O₂) and P10 (7.11 mg/l of O₂) of the Mfoundi course during this study is contrary to the results reported by Tanawa et al. (2003) , Menye et al. (2012) in the same course. This change could be related to the increase in domestic wastewater discharges due to the implementation of hygiene measures. Mimi Coultas et al. (2020) , in their work titled Compendium on Handwashing in Resource-Poor Contexts, indicate that the chemical substances contained in soaps and alcohol-based hand gels could potentially harm the environment.

Electrical conductivity measures the ability of water to conduct electricity. It depends on the concentration of ions in the water ( Hajjar, 1997 ). The results obtained in this study for this parameter remained consistent across all courses when compared to those published in earlier studies for the same courses. The difference observed between P1 and P10 of the Mfoundi corroborated the results of Lami (2002) , Tanawa et al. (2003) , and Menye et al. (2012) . This could be justified by the scattered application of preventive measures such as handwashing during the COVID-19 response ( Sop et al., 2022 ), it would have limited the risks of water pollution in the city.

The widespread use of detergents, disinfectants, and antibiotics in response to COVID-19 in Yaoundé has had a profound impact on the water quality of city’s rivers. Specifically, the oxidability and pH levels at certain points of the rivers selected for the study have deteriorated compared to previous studies. Furthermore, a positive correlation (r_s = 0.812, P = 0.014) was observed between the incidence of COVID-19 cases with oxidability levels in Tongolo river, suggesting that the crisis has had a direct impact on the water quality. However, it is worth noting that the disproportionate application of hygiene measures among the populations of Yaoundé during the crisis has mitigated the effects of this deterioration. Despite these efforts, the lack of water flow observed in the beds of some rivers poses a significant threat to the integrity of the aquatic ecosystems in Yaoundé. Therefore, it is crucial that public authorities and the populations living along these rivers take action to address this issue and ensure the long-term sustainability of these ecosystems.