ECOLOGICAL ENVIRONMENT CHANGES INFERRED FROM SUBFOSSIL CHIRONOMID RECORD OF SHAHU LAKE IN WUHAN CITY

This study aimed to reconstruct total phosphorus (TP) concentration in the water column and to reveal the history of environmental changes since the 1970s in Shahu Lake based on subfossil chironomid assemblages analysis of a 41-cm length sediment core, combined with ²¹⁰Pb dating, chironomid-inferred total phosphorus (CI-TP) transfer function in the middle and lower reaches of the Yangtze River, detrending correspondence analysis (DCA) and documentary data of Wuhan City. Three distinct stages were identified in chironomid assemblages. From 1973 to 1989, the dominance of macrophyte-related taxa (e.g. Cricotopus sylvestris and Dicrotendipes nervosus-type) indicated that the development of macrophytes in the lake, with CI-TP fluctuating from 47 to 55 μg/L. Meanwhile, sedimentary TP maintained around 700 mg/kg. From 1989 to 2000, sedimentary TP doubled its concentration and CI-TP increased to above 100 μg/L, concurrent with a marked decrease of the relative abundance in macrophyte-related taxa. These lines of evidence revealed that the lake had shifted from a mesotrophic status to a eutrophic condition. After 2002, sedimentary TP increased continuously to more than 2000 mg/kg. Concurrently, the chironomid assemblages were dominated by eutrophic species (i.e., Tanypus and Propsilocerus akamusi-type), with the CI-TP concentration above 150 μg/L. The first DCA axis captured 62.1% of total variance in the chironomid data and was significantly correlated with sedimentary TP, suggesting that variations in chironomid assemblages were mainly the consequence of nutrient enrichment. Lake eutrophication was in association with both lake area shrinkage and sewage inputs resulted from the urbanization in Wuhan City. This study revealed that the baseline TP concentration of Shahu Lake was around 50 μg/L and reducing external nutrient inputs is crucial for water environment protection.