The researchers studied how submerged plants affect nutrients and oxygen in a shallow reservoir in China. By combining two models, they found that the plants can improve water quality by reducing nutrient levels and increasing oxygen. Even though the plants release some nutrients when they die, it's not as significant as nutrient-rich water entering the reservoir. They also discovered that a critical level of phosphorus loading can cause the reservoir to switch from clear to turbid water. This switch is likely to happen when phosphorus levels reach around 6.1 mg per square meter per day, especially if the initial phosphorus concentration is high. This study suggests that clear water in the reservoir is at risk of becoming turbid if nutrient levels get too high.