Microbial communities have an important role in river ecosystems, but we still do not have a good understanding of how biofilm microbial communities are formed and how they function (especially their ability to degrade organic matter) in different environments. Field studies can provide insight into the natural patterns and processes of microbial communities in the ecosystem. Here we show the preliminary results of an ongoing 1-year field study (commenced in June 2023, ending in May 2024) that assesses the temporal variation in function of biofilm microbial communities in a riverine environment with large seasonal changes. This study aims to assess the relationship between the community metabolic profile (catabolic activity of carbon sources) of freshwater epilithic biofilm and several seasonally variable variables, namely water parameters (i.e., depth, velocity, flow, conductivity, pH, temperature, dissolved oxygen, chemical oxygen demand, and biochemical oxygen demand), dissolved nutrients (i.e., ammonium, nitrite, nitrate, and phosphate), and macroinvertebrate community diversity. Sampling is conducted once a month in a small semi-natural waterway (width between banks = 1.6 m) with low environmental heterogeneity. Each month cobbles are sampled from the streambed (below the water surface) at two shaded sites 10 m apart (6 cobbles at each site for a total of 12 cobbles each month). The biofilm is scrubbed from 108 cm² of cobbles each month. Water parameters and dissolved nutrients are measured using a flow meter, Laqua probe, and Kyoritsu packtest testing kits. Macroinvertebrates are sampled from the substrate and riparian vegetation using a kicknet then sorted and identified using a stereomicroscope. The metabolic activity of the biofilm community is profiled by measuring the utilization of 31 carbon substrates with Biolog “EcoPlates”, thus examining the catabolic activity of heterotrophs. In the poster session, we will present a mixed effects model analysis to determine the extent to which patterns of biofilm community metabolic activity are explained by the physical environment. The implications of seasonal changes in streambed epilithic biofilm communities for stream ecosystems will also be discussed.