Outdoor mesocosm experiment provides novel insights into the relative importance of water and food as sources of microplastics to aquatic amphibians.

Most studies examining microplastic uptake by aquatic amphibians have only examined uptake from water, disregarding other potentially important exposure pathways. Here, we compare the relative importance of water and food as sources of microplastics to the American bullfrog (Lithobates catesbeianus), as well as investigate how exposure pathway affects the concentration and size of microplastics in tadpoles. Following a 2 × 2 factorial design, 60 wild-caught tadpoles (Gosner stage 25;15 tadpoles per treatment) were exposed in outdoor mesocosms to one of four treatments, in which microplastics were present or absent in water and food. To assess water exposure, white polyethylene (PE) fragments were added to the water column (888 ± 514 particles/L), whereas to assess food exposure, grey PE fragments were added to food (12.3 ± 3.46 particles/g agar). Over the 10-d experiment, tadpoles were collected every 48 h to analyze the microplastics in their gastrointestinal tract (GIT) and carcass (body with the GIT removed). Here, we report evidence that tadpoles acquired microplastics from both water and food, with tadpoles that were simultaneously exposed through both pathways containing an average of 666 ± 699 particles/g waterborne microplastics and 120 ± 118 particles/g foodborne microplastics in their GIT. The waterborne pathway contributed significantly to total microplastic concentrations in both the GIT and carcass, whereas the foodborne pathway only contributed significantly to total microplastics concentrations in the GIT. On average, 71 ± 23% of microplastics in tadpoles were in their GIT, suggesting that ingestion was the dominant-but not exclusive-route of exposure. Microplastic size did not differ between exposure routes, however, microplastics were on average smaller in tadpoles (0.088 ± 0.099 mm) than in water (0.231 ± 0.094 mm) or food (0.139 ± 0.078 mm). Our study emphasizes the importance of considering multiple exposure pathways when assessing risks of plastic pollution to aquatic biota.