Undersökning av utvecklingen hos vanlig groda (Rana temporaria) och större dammsnäcka (Lymnaea stagnalis) i dagvattendammar i Stockholmsområdet 2025

On behalf of Stockholm municipality, SLU studied if chemical contaminants in stormwater ponds may affect the development of the common frog (Rana temporaria) and the great pond snail (Lymnaea stagnalis). The study was conducted in two parts: one focusing on frogs, in which embryonic and tadpole development was followed from egg to metamorphosis, and one focusing on snails, in which embryonic development was monitored until hatching. Water, sediment and plant material were collected from stormwater ponds representing different levels of estimated contamination, and the biological results were linked to chemical analyses of metals, nutrients and organic pollutants in both water and sediment. In the frog study, the tadpoles developed normally in all examined ponds. Neither survival, the occurrence of malformations, nor the time to metamorphosis differed from the reference site in a manner suggesting negative effects. Minor differences in growth were observed between ponds—for example, tadpoles from Mörtviksdammen were slightly smaller than those from Bergslagsdammen—but these differences did not appear biologically problematic and showed no clear connection to specific chemical substances. In contrast, gene expression analyses revealed a clear induction of the biomarker Cyp1a in tadpoles exposed to water and sediment from Flatendammen, consistent with exposure to polycyclic aromatic hydrocarbons (PAHs) or similar organic contaminants. Flatendammen also contained the highest sediment concentrations of PAHs and oil-related compounds. No changes were detected in the expression of the metal-binding protein metallothionein, indicating that metal exposure in these ponds was not sufficient to trigger a biological response in the tadpoles. The snail study similarly showed that embryonic development proceeded normally in all stormwater ponds. Neither mortality, malformations, hatching success, embryo size, nor heart rate displayed any apparent effects. These results suggest that the chemical concentrations measured in the water did not induce developmental impacts on this species under the given exposure conditions. Chemical analyses showed that several metals occurred at levels exceeding relevant reference values in both the frog and snail studies—particularly copper, zinc and, in some cases, arsenic and nickel. In sediment from Flatendammen, concentrations of several metals also exceeded threshold values for contaminated soil. Despite this, no direct effects on survival or developmental endpoints were observed in the animals. Chloride concentrations were moderate and below levels generally associated with negative impacts on amphibians, and nutrient levels overall were not considered problematic for either frogs or snails. Overall, the results show that the survival or normal development of tadpoles or snail embryos was not significantly affected by water and/or bottom material from the investigated stormwater ponds. However, the elevated Cyp1a expression in larvae exposed to water and bottom material from the Flatendammen pond indicates that there may be organic pollutants capable of triggering biological responses before more obvious effects on survival or growth become apparent. This should be regarded as an early warning signal. The study thus shows that stormwater ponds in Stockholm can function as reproductive habitats for both common frogs and pond snails from a contamination perspective, but that monitoring of both water and sediment should be carried out to ensure that this continues in the future. It is appropriate to apply the precautionary principle when exposing amphibians to ponds with high levels of contaminants in the sediments to avoid the risk of adverse effects.