Biofilms play a vital role in aquatic environments as a key form of microbial life and as fundamental chemical modulators. Beyond naturally occurring biofilms, increasing attention is being directed toward a novel form of “artificial” biofilm that develops on plastic surfaces, known as the plastisphere. The widespread presence of plastic debris in aquatic ecosystems provides abundant surfaces for microbial colonization; however, the ecological significance of this emerging habitat remains poorly understood. In this study, we investigated the impact of the plastisphere on the metabolism of aquatic ecosystems. We conducted in situ experiments along a geographical gradient, selecting lakes with diverse hydro-morphological features and trophic statuses. Plastic substrates of different polymer compositions were incubated in three lakes (Iseo, Italy; Tahoe and Castle, USA) to allow biofilm development. After 30 days, chlorophyll-a concentration was measured as a proxy for photosynthetic biomass. Additionally, a subset of plastic pieces was incubated in light/dark bottles to quantify changes in nutrient concentrations and dissolved oxygen. From these data, we estimated net ecosystem production (NEP), gross primary productivity (GPP), and community respiration (R). Our results showed that the magnitude of plastisphere effects on lake metabolism varies by lake type, with more pronounced changes in NEP observed in more eutrophic systems. The direction of change varied across lakes, indicating a shift toward more autotrophic conditions in samples from the meso-eutrophic Iseo and the oligotrophic Tahoe systems, while more heterotrophic conditions were observed in the alpine, pristine Castle Lake, although differences compared to controls were small in both Tahoe and Castle. Changes in nutrient dynamics were also evident, highlighting the potential for these microbial communities to influence multiple ecosystem processes. Overall, our findings suggest that plastic debris may contribute to primary productivity, especially promoting the ‘greening’ of littoral zones, although the extent of this effect varies on specific system characteristics.