Pesticide Residues in Ethiopia’s Highland Lakes Pose High Risk to Tiny Aquatic Life

In the highlands of northwestern Ethiopia, two of the country’s most important freshwater bodies are quietly accumulating a chemical legacy of modern agriculture. A new study published in the open-access journal Heliyon has measured organophosphate pesticide residues in the water and fish of Lake Tana, Ethiopia’s largest lake and the source of the Blue Nile, and the smaller, deeper Lake Hayqe. The findings reveal a striking asymmetry: while the concentrations detected are too low to pose a meaningful non-cancer health risk to people eating the fish, they are high enough to represent a serious acute threat to Daphnia, the microscopic crustaceans that form a critical foundation of the aquatic food web.

Organophosphate pesticides are among the most heavily used chemical pest controls on Earth, accounting for roughly 40 percent of global pesticide consumption. They were widely adopted as replacements for the notoriously persistent organochlorine pesticides of the mid-twentieth century because they break down more quickly in the environment. Yet that relative impermanence has not kept them out of aquatic ecosystems. Spray drift, agricultural runoff, groundwater leaching, careless disposal of empty containers, and the washing of application equipment all provide routes by which these compounds reach lakes and rivers, where they can persist long enough to be taken up by organisms. In Ethiopia, chlorpyrifos-methyl is commonly applied in crop production, while diazinon finds its way into the environment partly through flea collars used on sheep, dogs, and cats in rural areas.

The research team, led by Tarekegn Fentie Yimer of the University of Gondar, targeted Lake Tana and Lake Hayqe because neither had ever been assessed for the ecological and human health risks of these compounds, despite intensive pesticide use by smallholder farmers and horticulture companies in their catchments. Lake Tana covers 3,150 square kilometers at an altitude of 1,786 meters, is recognized by UNESCO as a Biosphere Reserve, and supports endemic fish, bird, and plant species alongside a fishery with an estimated potential of 10,000 tons per year. Lake Hayqe, by contrast, is a closed-basin lake with a watershed of only about 77 square kilometers but an average depth of 37 meters and a maximum depth of 81 meters. Both lakes supply Nile tilapia, a staple food for surrounding communities, making them ideal natural laboratories for tracing pesticides from farm fields to dinner plates.

Between February and May 2023, the researchers collected water from 20 sites on Lake Tana and 5 sites on Lake Hayqe, along with 30 Nile tilapia from three Lake Tana fishing districts and 10 fish, five tilapia and five common carp, from Lake Hayqe. Fish muscle tissue was freeze-dried, homogenized, and extracted using the QuEChERS method, a technique whose acronym promises a process that is quick, easy, cheap, effective, rugged, and safe. All samples were then analyzed by gas chromatography coupled with triple quadrupole mass spectrometry, one of the most sensitive analytical approaches available for trace organic contaminants. The method performed well: recoveries ranged from 96.7 to 106.0 percent, well within the accepted 80 to 120 percent window, and detection limits were low enough to quantify every residue found.

Of the eighteen organophosphate pesticides screened, only a handful appeared. In water, chlorpyrifos-methyl, ethion, and malathion were detected at every site sampled, with chlorpyrifos-methyl ranging from 1.41 to 1.91 micrograms per liter. That figure is dramatic when set against the United Kingdom Environment Agency’s non-statutory maximum acceptable concentration of just 0.01 micrograms per liter for the protection of freshwater life, meaning the lakes carry concentrations more than a hundred times that benchmark. Malathion, by contrast, remained below the World Health Organization’s drinking-water guideline of 2 micrograms per liter. In fish muscle, four compounds were detected: chlorpyrifos-methyl, diazinon, ethion, and malathion, at concentrations generally between about 11 and 25 micrograms per kilogram. Notably, malathion levels in fish exceeded the WHO maximum residue limit of 2 micrograms per kilogram, while diazinon stayed comfortably within its 200 to 500 microgram limit.

The spatial patterns told a story about sources. Statistical analysis revealed significant differences among sampling sites for every detected compound, with the highest chlorpyrifos-methyl and malathion values clustering near the mouths of tributary rivers such as the Gumara, Rib, and Megech, which drain intensive agricultural areas like Fogera and Gorgora, and near Bahir Dar, the region’s largest city. Principal component analysis, a multivariate technique that condenses many correlated measurements into a few underlying axes of variation, confirmed that ethion and chlorpyrifos-methyl dominated the first component explaining nearly 60 percent of the variance in water data, while malathion distinguished the Megech site. Tilapia consistently accumulated more pesticide than common carp in Lake Hayqe, suggesting species-specific differences in uptake and metabolism.

The ecological risk assessment produced the study’s most alarming numbers. The team calculated risk quotients by dividing measured environmental concentrations by predicted no-effect concentrations derived from acute toxicity data for three representative organisms: fish, Daphnia, and algae, chosen to span primary producers, primary consumers, and higher trophic levels. For Daphnia, the risk quotients for chlorpyrifos-methyl reached 275 in Lake Tana and 268 in Lake Hayqe, values far above the threshold of 1 that signals high risk. Ethion also posed high risk to Daphnia, with quotients between roughly 19 and 22, while malathion registered a medium risk. Fish and algae fared considerably better, with low to medium risk quotients across the board. The pattern identifies Daphnia as by far the most sensitive receptor, a finding consistent with studies in Ethiopia’s Lake Hawassa and in surface waters of northwestern Bangladesh and Greece.

The human health picture was far more reassuring. Using standard exposure assumptions, a fish consumption rate of 30 grams per day and an adult body weight of 60 kilograms, the researchers estimated daily intakes and hazard quotients for each pesticide. Every hazard quotient came in well below 1, and the combined hazard index was approximately 0.06 for both lakes, indicating a negligible non-carcinogenic risk from eating fish under the conditions assessed. Diazinon contributed the largest share of the hazard index, yet even at maximum detected concentrations the total remained an order of magnitude below the level of concern. The authors caution, however, that these estimates rest on generalized consumption and body weight assumptions that may not reflect every local community, and that the assessment addressed only non-cancer endpoints.

The study’s authors are candid about its limitations. Composite sampling, while analytically practical, blurred spatial resolution, particularly in Lake Hayqe where all water samples were merged into a single composite. Sampling occurred during a single season, so rainfall-driven pulses of pesticide runoff may have gone unmeasured, and the smaller sample size from Lake Hayqe limits direct comparison between the lakes. Still, the baseline established here carries a clear message for policymakers: pesticide governance in Ethiopia remains underdeveloped, many generic pesticides circulate without full registration, and farmers often handle chemicals without precautions. The researchers call for multi-season monitoring, larger and non-composited samples, and site-specific exposure data, alongside improved management practices to stem the flow of organophosphates into lakes that sustain fisheries, tourism, and hydropower. Protecting the plankton at the base of these ecosystems, the data suggest, may be the most urgent task of all.

Subject of Research: Ecological and human health risk assessment of organophosphate pesticide residues in water and fish from two Ethiopian highland lakes

Article Title: Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia

Article References: Yimer, T. F., Kassa, Y., Mulugeta, M., Akele, M. L., Alemu, A. K., Yenealem, D., Amare, A., Moges, Z., Birhan, T. A., & Tibebe, D. (2026). Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia. Heliyon, 12(15), Article e45551. https://doi.org/10.1016/j.heliyon.2026.e45551

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45551

Keywords: organophosphate pesticides, Lake Tana, Lake Hayqe, Ethiopia, chlorpyrifos-methyl, Daphnia, ecological risk assessment, bioconcentration, Nile tilapia, water contamination, fish consumption, risk quotient