Responses of toad tadpoles to ammonium nitrate fertilizer and predatory stress: differences between populations on a local scale

Sublethal, Behavioral, and Developmental Effects of the Neonicotinoid Pesticide Imidacloprid on Larval Wood Frogs (Rana sylvatica)

Imidacloprid, a neonicotinoid pesticide, is used to prevent the spread of the hemlock woolly adelgid, currently affecting Eastern Hemlock trees across North America. When the pesticide is sprayed directly onto soil around infested trees (soil drenching), it can run off into aquatic systems, with potential negative effects on biota. Simultaneously, climate change may lead to faster pool drying, which acts as an additional stressor for sensitive species such as amphibians. We evaluated the sublethal effects of imidacloprid (10 ppb), and interaction with shorter hydroperiods on the larval behavior, growth, and survival of a model organism, the wood frog (Rana sylvatica). We performed 3 behavioral experiments evaluating swimming speed, time spent swimming, and distance the larvae swam. We found that larvae raised in 10 ppb imidacloprid or shorter hydroperiod did not differ in their swimming time, distance, and speed from nonexposed larvae. Naïve larvae exposed for 20 min to 10- to 500-ppb concentrations also showed similar performance to nonexposed larvae. However, when we applied a stimulus halfway through each experiment, we found that larvae exposed to 10 ppb imidacloprid (short and long term) swam shorter distances and spent less time swimming, suggesting that imidacloprid exposure may slow reaction time, potentially increasing the risk of predation. To minimize impacts on pool-breeding amphibians, imidacloprid application to combat the invasive hemlock woolly adelgid should use trunk injection and avoid soil drenching. Environ Toxicol Chem 2021;40:1840-1849. © 2021 SETAC.

Living in polluted waters: A meta-analysis of the effects of nitrate and interactions with other environmental stressors on freshwater taxa

Nutrient effluents from urban and agricultural inputs have resulted in high concentrations of nitrate in freshwater ecosystems. Exposure to nitrate can be particularly threatening to aquatic organisms, but a quantitative synthesis of the overall effects on amphibians, amphipods and fish is currently unavailable. Moreover, in disturbed ecosystems, organisms are unlikely to face a single stressor in isolation, and interactions among environmental stressors can enhance the negative effects of nitrate on organisms. Here, the effects of elevated nitrate on activity level, deformity rates, hatching success, growth and survival of three taxonomic groups of aquatically respiring organisms are documented. Effect sizes were extracted from 68 studies and analysed using meta-analytical techniques. The influence of nitrate on life-stages was also assessed. A factorial meta-analysis was conducted to examine the effect of nitrate and its interaction with other ecological stressors on organismal survival. Overall, the impacts of nitrate are biased towards amphibians (46 studies) and fish (13 studies), and less is known about amphipods (five studies). We found that exposure to nitrate translates to a 79% decrease in activity, a 29% decrease in growth, and reduces survival by 62%. Nitrate exposure also increases developmental deformities but does not affect hatching success. Nitrate exposure was found to influence all life-stages except embryos. Differences in the sensitivity of nitrate among taxonomic groups tended to be negligible. The factorial meta-analysis (14 amphibians and two amphipod studies) showed that nitrate in combination with other stressors affects survival in a non-additive manner. Our results indicate that nitrate can have strong effects on aquatic organisms and can interact with other environmental stressors which compound the negative effects on survival. Overall, the impacts of nitrate and additional stressors are complex requiring a holistic approach to better conserve freshwater biodiversity in the face of ongoing global change.

Influence of dissolved oxygen conditions on toxicity of ammonium nitrate to larval natterjack toads

Temporary ponds, where many amphibians from temperate regions breed, show an annual cycle with a maximum water volume in spring followed by a progressive desiccation throughout late spring and summer. This desiccation leads to a decrease in dissolved oxygen and an increase in nitrogen levels, which can additionally increase because of anthropogenic sources such as chemical fertilizers. We analyzed the toxicity posed by environmentally relevant levels of a common nitrogenous fertilizer, ammonium nitrate, at different conditions of oxygen availability to Bufo calamita tadpoles, which typically develop in ephemeral ponds. Ammonium nitrate (90.3 mg N-NO3NH4/l) and hypoxic conditions (initial dissolved oxygen 4.53 ± 0.40 mg/l) caused significant lethal effects after 7 and 12 days of exposure, respectively. At the end of experiment (16 days), mortality rates were 32.5 % in individuals exposed to the fertilizer and 15 % in those growing under hypoxic conditions. When both stressors were combined, they showed an additive effect on tadpole survival. Malformations, such as oedemas and spinal curvatures, and locomotory abnormalities, were detected after 12 days of experiment in >90 % of individuals exposed to 45.2 mg N-NO3NH4/l under hypoxic conditions, whereas none of these stressors by separate related to abnormality rates >35 %. Delayed development was also observed in tadpoles exposed to ammonium nitrate with hypoxia affecting developmental rate only after 12 days of exposure. The results are discussed in terms of potential mechanisms linking negative effects of both factors as well as in terms of potential alterations of the ecological plasticity that often allows amphibians to survive in unpredictable environments.

Density effects on ammonium nitrate toxicity on amphibians. Survival, growth and cannibalism

Temporary ponds where many amphibians breed experience a gradual desiccation that leads to growing larval densities, which can reduce tadpole survival rates and increase cannibalistic interactions among carnivorous salamander larvae. Concentrations of many agrochemicals, including nitrogenous fertilizers, can also increase as the water volume decreases. We analyzed the effects of ammonium nitrate fertilizer at two larval densities on growth and survival of Rana dalmatina tadpoles, and on cannibalistic behaviors of Salamandra salamandra larvae. We observed synergistic lethal effects until day 17 of exposure, when ammonium nitrate-related mortality of tadpoles was five times greater at high than at low density. The fertilizer impaired tadpole growth. This effect was stronger at low density than at high density. In the absence of ammonium nitrate, the incidence of cannibalism was higher at high than at low larval density. However, fertilizer exposure removed this density-dependent effect, which, under field conditions, would negate some potentially advantageous effects of cannibalism such as reduced intraspecific competition and the stimulation of defensive behaviors.