Coastal sand and mud sediments have different microbial communities and chemical processes due to waves and tides. The disturbance from these forces affects oxygen levels, leading to changes in microbial populations and activities. By studying sediment samples from different sites, researchers found that more disturbed areas had more active ammonia-oxidizing archaea, while less disturbed areas had bacteria involved in sulfate reduction and other processes. This shows that the level of hydrodynamic disturbance plays a crucial role in shaping microbial communities and biogeochemical processes in coastal sediments.