A study compared a wetland purifying stream water with and without a litter layer on its surface. The wetland had reeds and was built in Korea in 2001. In 2007, a 12.2cm litter layer was formed on its surface, which was removed in 2008. The wetland with the litter layer removed more nitrogen from the water compared to the one without it. Specifically, the wetland with the litter layer removed 64.76% of total nitrogen and 60.83% of ammonia nitrogen, while the wetland without the litter layer removed 54.69% of total nitrogen and 50.61% of ammonia nitrogen. This shows that having a litter layer on the wetland's surface makes it more efficient at removing nitrogen from the water.