Tracking eutrophication in Taihu Lake using the diatom record: potential and problems

Taihu Lake is the third largest freshwater lake in China and has been experiencing eutrophication problems for several decades. Diatoms in short sediment cores from three bays in northern Taihu Lake were studied in addition to 1-year of seasonal phytoplankton samples in order to evaluate the rate and magnitude of nutrient enrichment. The dominant species found in the phytoplankton samples appeared in high percentages in the surface sediment samples, suggesting that the latter faithfully record the modern diatom flora. The diatom preservation status varied among the three cores, while in all cores the preservation deteriorated with sediment depth. Due to the superior diatom preservation in the core from Mashan Bay, the fossil diatom record of this core and an established diatom total phosphorus (TP) transfer function were used to reconstruct the nutrient history of Taihu Lake. Diatom assemblages changed from Aulacoseira-dominated to other eutrophic planktonic species, such as Stephanodiscus minutulus, Cyclostephanos tholiformis, Cyclotella atomus, C. meneghiniana and S. hantzschii in ca. 1980. Diatom-inferred TP concentrations exhibited little change prior to 1980, with values around 50 μg/l. However, after 1980 TP concentrations increased significantly and remained in excess of 100 μg/l, reflecting eutrophication of Taihu Lake. Comparison with TP measurements in the water column from 1988 to 2004, as well as the analogue analysis among fossil and modern samples, demonstrates that the diatom-TP inference model can reliably hindcast past TP concentrations. Therefore, the baseline TP value of about 50 μg/l, can be used as a restoration target for Taihu Lake. However, due to the complexity of this very large, shallow aquatic ecosystem, caution should be exercised when employing the diatom record to track eutrophication. Further studies on the mechanism of diatom distribution, evolution and preservation are recommended for Taihu Lake.