The researchers studied how the ecosystem in the Pearl River Estuary changed from 1981 to 1998. They used models to look at the food web and found that the ecosystem became less mature over time. There were more lower-level organisms and fewer top predators in 1998 compared to 1981. The way energy moved through the ecosystem also changed, with less going to higher trophic levels in 1998. They found no clear keystone species in 1998, but identified two in 1981. Overall, organisms like phytoplankton and zooplankton played important roles in the ecosystem.