Long-Term Variations of TN and TP in Four Typical Chinese Lakes

Abstract:

Few studies have examined long-term variations in the nutrients that cause lake eutrophication, namely total nitrogen (TN) and total phosphorous (TP). As a result, little is known about the peaks/troughs and trends in TN and TP in lakes known to be vulnerable to eutrophication such as Lake Dongting, Lake Poyang, Lake Chao, and Lake Tai, all of which are located in the Yangtze River basin of China. The objectives of this study were therefore to: 1) detect possible step changes in nutrient time series data; and 2) elucidate TN and TP temporal trends at multiple (annual, seasonal, and monthly) time scales, as influenced by factors such as river-lake connectivity and lake hydrologic conditions (water depth and inflow source). The distribution-free cumulative sum technique and modified Mann-Kendall approach were applied to the long-term nutrient data measured for these four typical lakes and revealed that both TN and TP in Lake Dongting and Lake Poyang exhibited an upward trend at annual, seasonal, and monthly time scales, although they are oligotrophic lakes. The nutrient concentrations in Lake Dongting underwent a step change around 2003, which can be attributed to point and non-point source pollution, overexploitation, and dam construction. The TN values for Lake Chao and Lake Tai experienced step changes in 1999 and 2002, respectively. Before the step-change years, a significant (p-value < 0.05) upward trend in both TN and TP was detected for Lake Chao at the annual scale as well as in summer and May, but for Lake Tai this was manifested at the annual scale only. In contrast, after the step-change years, a significant downward trend was detected for both lakes at the corresponding time scales. As expected, the TN and TP concentrations in Lake Chao and Lake Tai, which are located in the lower basin of the Yangtze River, were noticeably higher than those in Lake Dongting and Lake Poyang, which are located in the middle basin.