A ~ 40-year paleoenvironmental record from the Swan Oxbow, Yangtze River, China, inferred from testate amoebae and sedimentary pigments

Abstract

We present a ~ 40–year record of environmental change in the Swan Oxbow, Yangtze River, China, inferred from testate amoeba and sedimentary pigment data, combined with remote sensing and analysis of local socio–economic growth. These data indicate there were several distinct phases of aquatic conditions linked to human activities in the region: (1) Between ca. AD 1970 and 1984, there may have been some exchange of water and organic matter between the Swan Oxbow and the main river channel, following initial hydrologic disconnection in 1972. The lake area was relatively large in the early phase after the oxbow first formed, and the trophic state generally increased during that time frame, (2) From ca. AD 1984 to 1992, the lake area was about a third smaller in size, while the human population and GDP increased about 10% and 10x, respectively, compared to values in 1975. The nutrient status (inferred from testate amoeba and pigment data) increased, owing to the greater discharge of nutrients and separation of the Swan Oxbow from the main Yangtze River, which reduced water supply and increased sedimentation in the oxbow, (3) From ca. AD 1992 to 2003, the lake continued to diminish in size, to an area < 20 km2, except in 1998, when a major flood occurred. The testate amoeba and pigment data suggest that water quality had improved by that time, which probably reflects efforts to control agricultural and industrial activities, including establishment of two national reserves in 1992 and 1993, created to protect the rare Pere David’s deer (Elaphurus davidianus) and the freshwater Baiji dolphin (Neophocaena phocaenoides asiaeorientalis), (4) From ca. AD 2003 to 2005, the water area remained relatively unchanged, owing to construction of a dam following the 1998 flood. Occurrence of testate amoeba species Difflugia biwae and D. tuberspinifera, however, indicates that water pollution and eutrophication had been controlled in Swan Oxbow. The Swan Oxbow yielded important information about the effects of environmental protection and restoration in the middle reaches of the Yangtze River.