Response of Sediment Characteristics and Zooplankton Subfossil to Trophic State Change in Wuliangsuhai Lake

The remains of zooplankton subfossil and sediment characteristics were examined from a macrophyte dominated shallow lake Wuliangsuhai in Inner Mongolia, China, in order to study past limnological changes. The lake began to switch to eutrophication gradually in the middle 1980s, creating a Transitional Sediment Layer (TSL, about 17 cm depth) that can be visually used to separate sediments derived from plankton and macrophyte. Ratio of total nitrogen and total phosphorus (N/P) and organic matter that are associated with eutrophic species, such as Potamogeton pectinatus and Phragmites australis, decreased with increase of the depth after the TSL was formed. Metals (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn and V) concentration began to increase with depth above the TSL, indicating a transformation of external source. The changes in composition of the zooplankton fragments were rather gradual up the core, whereas the total numbers of fragments show the most marked changes from the TSL to present. Data from this study suggested that Wuliangsuhai Lake has experienced accelerated eutrophication after the TSL was formed.