Algae provide labile carbon to microbial heterotrophs that can increase or decrease rates of decomposition (i.e., positive and negative priming, respectively). Light availability is a main driver of algal growth on organic matter. However, other environmental factors such as nutrients in the water column and substrate are likely to play roles. In particular, metals are increasingly recognized as important nutrients for algal growth and photosynthesis, yet their interactive effects on decomposition have not been explored. We performed a stream-side mesocosm experiment near the Cuyahoga River in Kent, OH (USA) to assess the effects of light and metal availability on decomposition rates using 32 flow-through channels. Cotton strips and leaf packs (Populus deltoides) were incubated within mesocosms that received treatments of light (ambient and shaded) and metals (ambient and elevated) in a fully crossed design. After 14 days, organic-matter samples were collected to analyze decomposition rate, beta-glucosidase (GLU) activity, ergosterol content, and chlorophyll (chla) concentration. Results show that decomposition rates only differed between light treatments and on on cotton strips (multiway ANOVA, p = 0.029) where ambient treatments had lower rates of decomposition than shaded treatments. Additionally, differences in chla concentration between light treatments were only present on cotton strips (Kruskal-Wallis, p<0.001) and concentrations were 4-6x greater in the unshaded treatments. Additionally, while marginally significant (Conover-Iman test, p = 0.086), the unshaded treatment without metal additions had a chla concentration 2.8x less than when metals were added. There were no differences between treatments for ergosterol content or GLU activity. These results show that while light availability can decrease decay for the breakdown of organic matter in the Cuyahoga River, metals may only play a secondary role through increasing algal production.