Eutrophication of the Three Gorges Reservoir After Its First Filling

Abstract

Trophic status of water, including that of the main river channel, backwaters of tributaries, and thrity-five reservoirs on tributaries of the Three Gorges Reservoir basin was surveyed. In the main channel, concentrations of dissolved total nitrogen (DTN) and phosphorus (DTP) exceeded the low limit for eutrophication assessment, and the ratio of DTN to DTP was over 16:1. Chlorophyll a ranged from 1.58 to 7.53 mg/m³ at the water surface. A trophic gradient from oligotrophic to mesotrophic was spatially observed. Eutrophy factors could be divided into three clusters in terms of flow direction. In twelve tributary bays of the Three Gorges Reservoir, the integrated trophic state index was 33.3–66.1. Of these tributaries, five were eutrophic in May, eight were eutrophic in June, and the rest were mesotrophic. The eutrophication is currently worse than before impoundment. In the thirty-five reservoirs studied on the tributaries, finally, it was found that nutrients and chlorophyll a were high while organic pollution and SD were low. Twenty-two reservoirs were eutrophic, one was oligotrophic, and the rest were mesotrophic. Our investigation found that water storage already had a strong impact on the eutrophication of the Three Gorges Reservoir after the first filling. Chinese scientists and water managers are therefore facing a highly unsatisfactory and challenging situation.