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BackgroundGlyphosate-based herbicides are the most widely used pesticides in agriculture, horticulture, municipalities and private gardens that can potentially contaminate nearby water bodies inhabited by amphibians and algae. Moreover, the development and diversity of these aquatic organisms could also be affected by human-induced climate change that might lead to more periods with extreme temperatures. However, to what extent non-target effects of these herbicides on amphibians or algae are altered by varying temperature is not well known.MethodsWe studied effects of five concentrations of the glyphosate-based herbicide formulation Roundup PowerFlex (0, 1.5, 3, 4 mg acid equivalent glyphosate L−1as a one time addition and a pulse treatment of totally 4 mg a.e. glyphosate L−1) on larval development of Common toads (Bufo bufo, L.; Amphibia: Anura) and associated algae communities under two temperature regimes (15 vs. 20 °C).ResultsHerbicide contamination reduced tail growth (−8%), induced the occurrence of tail deformations (i.e. lacerated or crooked tails) and reduced algae diversity (−6%). Higher water temperature increased tadpole growth (tail and body length (tl/bl) +66%, length-to-width ratio +4%) and decreased algae diversity (−21%). No clear relation between herbicide concentrations and tadpole growth or algae density or diversity was observed. Interactive effects of herbicides and temperature affected growth parameters, tail deformation and tadpole mortality indicating that the herbicide effects are temperature-dependent. Remarkably, herbicide-temperature interactions resulted in deformed tails in 34% of all herbicide treated tadpoles at 15 °C whereas no tail deformations were observed for the herbicide-free control at 15 °C or any tadpole at 20 °C; herbicide-induced mortality was higher at 15 °C but lower at 20 °C.DiscussionThese herbicide- and temperature-induced changes may have decided effects on ecological interactions in freshwater ecosystems. Although no clear dose-response effect was seen, the presence of glyphosate was decisive for an effect, suggesting that the lowest observed effect concentration (LOEC) in our study was 1.5 mg a.e. glyphosate L−1water. Overall, our findings also question the relevance of pesticide risk assessments conducted at standard temperatures.

Glyphosate-based herbicide formulations are broadly used in agriculture, silviculture, horticulture as well as in private gardens all over the world, thus posing the risk of potential contamination of nearby aquatic bodies inhabited by amphibians. Concurrently, climate change can be expected to alter the temperature of amphibian breeding sites. However, while either glyphosate-based herbicides or temperature have been shown to separately affect the development of amphibians, very little is known on possible interactive effects. We studied the impact of herbicide concentrations and temperature on growth and development of eggs and tadpoles of the Common toad (Bufo bufo L.). We hypothesized that (i) eggs would be better protected against herbicides than tadpoles because of their jelly coating, (ii) that higher temperatures would reduce potential herbicide effects because of an accelerated growth and a lower sensitivity of larger specimens. We conducted one experiment starting with eggs (Gosner stage, GS 8) and another experiment starting with tadpoles (GS 21–24) using a full factorial design with 5 concentrations of the herbicide formulation Roundup® LB Plus (0.0 mg acid equivalent L−1, 0.5, 1.0, or 1.5 mg a.e. L−1 and a pulse treatment with 3 times (egg experiment) or 5 times (tadpole experiment) addition of 0.5 a.e. mg L−1 over the course of several weeks) and two temperature levels (15 and 20°C). Contrary to our expectation, our results showed that toad eggs are more sensitive to herbicides than tadpoles leading to an averaged 31% increase in total length, tail length, and body length compared to the herbicide-free control. Tadpole morphology, development, or mortality was not influenced by herbicides. There was no correlation between herbicide concentration and the effect strength on eggs or tadpoles. Higher temperature accelerated growth of both eggs and tadpoles. As one of the first we also observed interactive effects between herbicide concentrations and temperature especially for egg development resulting in more pronounced herbicide effects at lower temperatures than at higher temperatures. This is quite remarkable as ecotoxicologial risk assessment studies are usually conducted at a constant temperature, thereby perhaps not adequately examining non-target effects at natural conditions. more pronounced herbicide effects at lower temperatures than at higher temperatures. This is quite remarkable as ecotoxicologial risk assessment studies are usually conducted at a constant temperature, thereby perhaps not adequately examining non-target effects at natural conditions.