Effects of microplastics and phenanthrene on the stable isotope and ecological stoichiometry of juvenile discus fish (Symphysodon haraldi)

To investigate the effects of microplastics and phenanthrene on the growth, energy storage, stable isotope composition and ecological stoichiometry of the juvenile discus fish (Symphysodon haraldi), we set three concentrations of microplastics (0, 100 and 1000 μg/L ) and two levels of phenanthrene (0 and 50 μg/L ), forming a total of six exposure combinations for eight weeks. The results showed that the final body weight of fish was not influenced by exposure to microplastics and phenanthrene, but exposure to phenanthrene reduced the condition factor. The hepatosomatic index (HSI) was influenced by the interaction of microplastics and phenanthrene. Exposure to 100 μg/L microplastics increased HSI under 50 μg/L phenanthrene, and exposure to phenanthrene increased HSI under 100 μg/L microplastics. The protein content increased with increased concentrations of phenanthrene. Microplastics and phenanthrene had an interaction effect on carbohydrate and crude fat contents. Increased concentrations of microplastics increased crude fat content but reduced carbohydrate level under 0 μg/L phenanthrene, and exposure to phenanthrene decreased crude fat content but increased carbohydrate level under 1000 μg/L microplastics. The δ¹³C of juveniles decreased with increase concentrations of microplastics. The δ¹⁵N was influenced by the interaction of microplastics with phenanthrene, but no significant differences between exposure combinations were observed. Exposure to microplastics increased body C content and exposure to phenanthrene increased N level. Moreover, microplastics and phenanthrene had an interaction effect on P content. Exposure to 100 μg/L microplastics reduced P content under 0 μg/L phenanthrene but exposure to 1000 μg/L microplastics reduced P level under 50 μg/L phenanthrene. Exposure to phenanthrene increased P content under 100 μg/L microplastics but reduced P level under 1000 μg/L microplastics. The C/N ratio was influenced by exposure to microplastics or phenanthrene alone, but ratios of N/P and C/P were also affected by the interaction of microplastics and phenanthrene. The results of this study suggest that exposure to microplastics and phenanthrene showed no significant effect on the growth and survival of juvenile discus fish, but it could change the energy reserves of fish body, which might account for the changes in their stable isotope composition and C, N and P stoichiometry characteristics. The results of this study can provide scientific basis for the ecological risk assessment of microplastics and phenanthrene in aquaculture waters.