<p>Microbial biofilms are important components in macrophyte decomposition, and their composition depends on the decomposition stage and host plant quality. Here, we investigated how macrophyte tissue quality (i.e., C:N:P stoichiometry and phenolic contents) influences epiphytic microbial biofilms during litter decomposition. Consecutive experiments were conducted to (1) modify the C:N:P stoichiometry and phenolic content of the freshwater macrophyte <i>Elodea nuttallii</i> by manipulating light and nutrient availability and (2) test how the modified tissue quality affected epiphytic microbial biofilm diversity and community composition before and during macrophyte decomposition. Our results showed that shading led to lower C:N ratios (28.6 to 12.6) and higher phenolic content (10.8 to 19.2&#xa0;µg/mg dry weight). Simultaneously, shading affected the epiphytic bacterial and fungal community composition, and these shifts correlated with the macrophyte C:N ratio. While no effects of macrophyte tissue quality on decomposition rates were observed, the epiphytic bacterial community composition on the litter was significantly affected by light treatment, time, and their interaction. Bacterial community composition shifted from a high abundance of Comamonadaceae to a more diverse community over time. Overall bacterial diversity was lower on the litter grown in the shaded mesocosms. Fungal diversity and community composition during litter decomposition were not affected by litter quality. Overall, our results reveal a structuring role of macrophyte tissue quality on its associated microbial biofilm and uniquely show a continuation of light-driven changes in epiphytic bacterial community composition after exposure. We conclude that light-driven changes in C:N stoichiometry are a crucial factor in shaping epiphytic microbial communities during macrophyte decomposition.</p>