This study investigated the long-term degradation behavior of nylon 6 and poly(butylene succinate) (PBS) filaments in artificial and natural seawater. FE-SEM analysis showed no significant surface erosion in nylon 6, whereas PBS exhibited clear surface degradation under both conditions. Molecular weight and mechanical properties of PBS progressively decreased during immersion, with a faster initial degradation observed in natural seawater. This difference was attributed to indigenous microbial communities actively participating in PBS degradation from the beginning, while in artificial seawater, hydrolysis dominated initially and microbial activity accelerated only after colonization. These results demonstrate that differences in initial microbial distribution strongly influence the rate and mechanism of polymer degradation, providing valuable insights into the environmental fate of biodegradable polymers in marine ecosystems.