Plastic pollution has been increasingly reported in both marine environment and inland waters, but their fate is not well understood. Several studies have showed that the surface of plastic debris can be colonized by microbes, leading to the sinking of floating plastic debris in marine environment. In this work, development of biofilm on polypropylene sheet (squares with a side length of 5 and 10 mm) and their buoyancy changes were studied in a freshwater lake in four seasons. Results showed that biofilm development have different growth rate and distinct algae composition in different seasons, which are mainly related to the difference in temperature, nutrient levels, and suspend solids in lake water. Biofilm development was much quicker on small plastics in all seasons. At the end of the experiment, all plastics lost buoyancy in summer while only a small portion lost buoyance in other seasons. Sinking of the floating plastics can be attributed to the development of biofilm and the trapped minerals. Our results demonstrated that biofilm development can cause the sinking of floating plastics in fresh lakes but the time required to lose buoyance can differ seasonally. Floating plastics will remain in water for a longer time in cold season but sink in a short time in warm season. Future research is required to determine the influence of plastic types and shapes, and quantitative relation between environmental variables and the sinking behavior of the fouled plastics should be established for a better prediction of their fate in the freshwater environment.