A wetland was studied for 2 years to see how well it could clean up polluted river water. They used plants to help remove nitrogen and phosphorus from the water. In the second year, the plants did a better job at cleaning up the water compared to the first year. This was because the plants grew more and absorbed more nutrients. One type of plant, Phragmites australis, was better at cleaning up the water than another type, Typha orientalis. The wetland became more efficient at removing nitrogen because of the increased plant growth.