We tested whether the presence of fungi coming in with the leaves (also known as phyllosphere fungi) on decomposing aquatic leaf litter inhibited or facilitated the growth of fungi colonizing from the river. In soils, phyllosphere fungi dominate early stages of decomposition and dictate later fungal community structure, but few have studied this effect in aquatic habitats, and none have examined taxon-specific changes in activity. Understanding priority effects of fungi entering the river with the leaf is key to predicting how microbial communities will behave. To test the impact of phyllosphere community on colonizer activity, we autoclaved leaves to remove phyllosphere fungi, and then compared growth rates of fungi on autoclaved leaves and non-autoclaved leaves incubated in Beaver Creek, AZ for 6 and 35 days. We used quantitative stable isotope probing (qSIP) to quantify the growth rate of individual taxa in situ. At day 6, most colonizing taxa were growing at similar rates on both treatments. By day 35, growth rates of colonizing fungi were significantly higher on the autoclaved leaves. On the non-autoclaved leaves, most taxa were no longer growing at all. There was also higher diversity on the autoclaved leaves than non-autoclaved leaves at 35 days. We also found that colonizing fungi differed in their response to the presence of phyllosphere fungi. Lemoniera, for example, was dramatically inhibited by phyllosphere fungi, while Amniculicola was not. This research furthers our understanding of priority effects on aquatic litter, showing that initial community dictates the relative abundance and taxon specific growth rates of fungal taxa.