Organic Matter Stable Isotope (δ13C, δ15N) Response to Historical Eutrophication of Lake Taihu, China

Abstract

We explored the use of carbon and nitrogen isotope ratios (δ13C, δ15N) in sediment organic matter as proxy indicators of historical changes in the trophic state of Lake Taihu, the third largest freshwater lake in China. Stable isotope signatures in four sediment cores spanning the 20th century were compared with instrumental records of lake-water trophic state. The comparative study shows that, between ∼ ∼1950 and 1990 AD, the δ13C and δ15N of sediment organic matter throughout Lake Taihu increased along the trophic gradient from oligotrophy to eutrophy due to biological isotopic fractionation. However, in the 1990s, the trophic state of Lake Taihu diverged into two different trophic systems, a hypereutrophic western Lake Taihu dominated by blue-green algae and a mesoeutrophic eastern Lake Taihu dominated by vascular aquatic plants. During the post-1990 AD shift from mesoeutrophic to hypereutrophic state in western Lake Taihu, organic matter δ13C and δ15N decreased sharply in response to pronounced shifts in the aquatic ecosystem. The results indicate that 13C-depleted phytoplankton replaced macrophytes in western Lake Taihu. δ15N values in western Lake Taihu also decreased because of N2 fixation by cyanobacteria in this highly productive ecosystem. By contrast, in eastern Lake Taihu, organic matter δ13C and δ15N values show a post-1990 AD trend towards slightly lower values, but they remain higher than the long-term average. This recent 13C–enrichment of organic matter indicates that periods of high productivity in the restricted eastern sub-basin of Lake Taihu limited aqueous CO2 availability, causing a decrease in isotopic discrimination during photosynthesis. After ∼ ∼1990 AD, organic matter δ15N values for eastern Lake Taihu only dropped slightly, suggesting that the contribution of phytoplankton to the sediment organic matter increased slightly. Taken together, the results indicate that nitrogen-fixing cyanobacteria probably played a much smaller role in primary productivity in this part of eastern Lake Taihu, compared with western Lake Taihu. Despite the complexity of carbon and nitrogen cycles in lakes, the agreement between the stable isotope signatures and instrumental records for Lake Taihu suggests that δ13C and δ15N in sediment organic matter are capable of recording important shifts in the spatial and temporal evolution of lake-water trophic state.