Salinity swings can make a common herbicide far deadlier in estuaries

Estuaries are among the most dynamic environments on Earth, places where fresh river water collides with the open sea and salinity can swing dramatically with every tide, storm, and drought. These same ecosystems are also the first stop for pesticide-laden runoff from farmland. A new open-access study published in Discover Oceans has now shown that these two stressors do not act independently: the toxicity of the widely used herbicide atrazine to a common estuarine worm changes enormously depending on how salty the water is. The lower the salinity, the deadlier the herbicide becomes, a finding with major implications for how scientists assess ecological risk in transitional waters.

The research team, led by Bruno Yuri Francolino of the Federal University of Pernambuco in Brazil together with colleagues in Brazil, Belgium, and New Zealand, focused on the marine nematode Litoditis marina, a microscopic worm that lives in estuarine sediments. Nematodes are ideal bioindicators because they are easy to culture in the laboratory, enormously abundant in sediments, and have short generation times. The species complex L. marina contains at least ten cryptic species, and the team used the PmII strain, which is known for its proven osmoregulatory ability, meaning it can actively adjust its internal salt balance when external salinity changes. This makes it a particularly relevant model for estuarine conditions.

Atrazine itself is one of the most extensively applied triazine herbicides in the world, sprayed on crops such as corn, sorghum, coffee, cocoa, and sugarcane to control weeds. Its chemical properties make it a formidable pollutant: it has a relatively low soil adsorption coefficient, with a mean Koc of 126.9 L/kg, which facilitates its movement into groundwater through leaching. It also has low solubility in water, 33 mg/L at 25 degrees Celsius, which allows it to be transported into estuaries and bays via runoff. Crucially, atrazine is highly persistent, particularly in sediments, and is still commonly detected in water bodies even in regions where its use has been banned for years. Organisms living in sediments may therefore experience prolonged exposure to varying concentrations of the herbicide.

To test how salinity modulates atrazine toxicity, the researchers designed a fully crossed laboratory experiment. Adult nematodes were exposed to atrazine concentrations of 0.2, 1.0, 2.0, 10.0, and 20.0 mg/L at four salinity levels: 15, 20, 25, and 30, reported as dimensionless values on the Practical Salinity Scale of 1978. The worms were placed on agar plates that simulate exposure via sediment pore water and ingestion, with Escherichia coli K12 added as food to ensure oral uptake would take place, mirroring the natural situation in which food resources are available alongside the toxin. Mortality was quantified every 24 hours for 120 hours, a duration chosen because it represents the maximum experimental time before the F1 generation reaches adulthood, which would confound the mortality count of the initial cohort. Four replicates were prepared for each treatment combination.

The results were striking. Salinity variation alone did not cause significant mortality in adult nematodes, confirming that the salinity levels tested were within the worm’s tolerance range. But when atrazine entered the picture, salinity became a decisive modulator of toxicity. At the lowest atrazine concentration of 0.2 mg/L, mortality reached 38.5 percent at the lowest salinity of 15 after 120 hours, whereas mortality at the same concentration was less than 24 percent at all other salinities. At the highest concentration of 20 mg/L, mortality was 100 percent at salinity 15, 94.5 percent at salinity 20, 88.5 percent at salinity 25, and 71.5 percent at salinity 30. In other words, mortality values decreased consistently with increasing salinity.

The quantitative toxicity thresholds told the same story. The 120-hour LC50, the concentration that kills 50 percent of the population, was 0.66 plus or minus 0.01 mg/L at salinity 15, which is 10.4 times lower, and therefore more toxic, than the LC50 of 6.88 plus or minus 0.64 mg/L at salinity 30. A similar pattern was observed for the LC20, the concentration causing 20 percent mortality, which was 9.2 times lower at salinity 15 compared with salinity 30. The lethal concentrations were significantly dependent on exposure time, on salinity, and on the interaction of these two factors, as confirmed by a non-parametric permutational analysis of variance based on 9,999 permutations. LC20 and LC50 values decreased with increasing exposure time at all salinity levels; at salinity 15, the LC20 dropped from 2.00 plus or minus 0.27 mg/L after 24 hours to more than 13-fold lower after 120 hours.

The team also fitted exponential regression models to predict lethal concentrations across a continuous salinity range from 8 to 35. The models showed that at salinity levels up to 35, the LC20 for atrazine exceeded 50 mg/L after 24 hours of exposure, and the LC50 exceeded 140 mg/L after 24 hours at salinity 35. The slope of the fitting curves decreased with increasing time, indicating that the toxic impact of atrazine becomes progressively more significant as exposure duration increases. These regressions provided good predictions of the lethal concentrations in the marine range, with very high R-squared values.

Why does lower salinity make atrazine more dangerous? The researchers propose several mechanisms. One possibility is a physicochemical salting-out effect, in which the lipophilic atrazine becomes less soluble at higher salinities, potentially increasing its partitioning into the lipids of the nematodes. However, previous experiments demonstrated no significant loss of atrazine after 128 days at different concentrations and salinities, suggesting that the observed differences in toxicity are more likely influenced by molecular associations or physiological absorption rather than by changes in the pesticide’s bioavailability. Perhaps more importantly, lower salinity imposes osmotic stress on estuarine organisms, forcing them to expend energy on osmoregulation. This diversion of metabolic resources could compromise the efficiency of cellular detoxification pathways, such as those mediated by cytochrome P450 monooxygenases and glutathione S-transferases, which are essential for metabolizing xenobiotics like atrazine. A reduced capacity for detoxification would lead to higher intracellular accumulation of the parent compound and its toxic metabolites, thereby exacerbating cellular damage and increasing mortality.

The findings align with earlier work on other species. A study using nauplii of the copepod Eurytemora affinis showed that after 96 hours, the LC50 values were 0.5, 2.6, and 13.2 mg/L at salinities of 5, 15, and 25, respectively. Increased atrazine mortality at low salinity has also been documented in adult shrimp of the species Metapenaeus affinis and in juveniles of the jellyfish Cassiopea sp. Interestingly, L. marina PmII in the present study was more sensitive to atrazine during the first 48 hours of exposure than the related cryptic species L. marina PmIII, which had a 48-hour LC50 of 72.30 mg/L at salinity 25. This was unexpected because copepods are generally considered more sensitive to pollution than nematodes, yet the nematode’s LC20 values compared unfavorably with those of harpacticoid copepods such as Robertsonia propinqua and Quinquelaophonte sp.

The authors caution that their acute LC50 data represent worst-case, short-term scenarios, such as those following intense agricultural runoff events, and that the lethal concentrations observed are higher than the chronic average environmental concentrations of atrazine typically found in aquatic systems. However, peak concentrations in runoff following application events can be orders of magnitude higher, occasionally entering the low mg/L range during acute discharge events. Crucially, the results support the idea that the safety margin between environmental and lethal concentrations is not static but dynamically narrows under low-salinity conditions. Given that salinity fluctuations in estuarine environments are intensifying owing to climate change, and that estuaries function as sinks for toxic agents where pollutants accumulate due to particle sedimentation and high ionic strength, the team argues that salinity fluctuation must be incorporated as a key modifying factor in ecological risk assessments for transitional water bodies. Future research, they suggest, should investigate the transgenerational effects of sub-lethal atrazine concentrations under fluctuating salinity, focusing on reproduction, growth, and the underlying molecular mechanisms of osmoregulation and detoxification.

Subject of Research: Salinity modulation of atrazine toxicity in the estuarine nematode Litoditis marina

Article Title: The modulating role of salinity in herbicide toxicity to the marine nematode Litoditis marina

Article References: Francolino, B. Y., Valdes, Y., de França, F. J. L., Moens, T., Ingels, J., & dos Santos, G. A. P. (2026). The modulating role of salinity in herbicide toxicity to the marine nematode Litoditis marina. Discover Oceans, 3(1), Article 3. https://doi.org/10.1007/s44289-026-00115-7

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00115-7

Keywords: atrazine, salinity, Litoditis marina, nematodes, ecotoxicology, estuarine pollution, LC50, osmoregulation, triazine herbicides, marine benthos, modulating, role