A recent nationwide survey found that insecticides and fungicides may adversely affect aquatic ecological communities. However, data on the effects of fungicides and fungicide mixtures on aquatic communities are limited, making it difficult to link observed concentrations in streams to the effects on organisms causally. To help narrow this data gap, we conducted an experiment to document the response of stream basal resources (natural biofilm communities) to exposure to different environmentally relevant fungicide concentrations. The experiment occurred over 10-days in July 2024 at the U.S. Geological Survey (USGS) Montana Experimental Stream Observatory (MESO) in Helena, Montana. Fungicide manipulations were carried out by randomly distributing 21 experimental “stream” units among four Living Stream™ bunks holding the units at 20°C using chillers and illuminating them on a 16:8 light: dark cycle. The experiment design included the following in triplicate: a control, a solvent control (acetone was used to solubilize pyraclostrobin), and fungicide mixtures (azoxystrobin + propiconazole + pyraclostrobin) at 5 concentrations ranging from 0.86 µg/L (sum of each fungicide at near No Observable Effect Concentrations) to 539 µg/L (active ingredient, ~100x observed concentrations in the USGS Regional Stream Quality Assessment 2013-2017). We collected data by BenthoTorch to characterize algal community composition, samples of periphyton for chlorophyll-a (Chl-a) concentration, ash free dry mass (AFDM), and assessed biofilm nutrient uptake by measuring free residual nutrient concentrations (total phosphorous, total nitrogen, nitrate-plus-nitrite, soluble reactive phosphorus, silica). Chl-a concentrations initially increased with the addition of acetone, but then decreased as fungicide concentrations increased, while AFDM remained similar across treatments. The autotrophic index indicated an initial spike in autotrophic activity, but heterotrophic activity dominated at higher fungicide mixture concentrations by day 10. Results indicate fungicides inhibited diatom growth/photosynthesis; releasing the biofilm demand for dissolved nutrients and permitting heterotrophic bacterial communities to dominate the biofilm community.