Spatial-seasonal dynamics of chromophoric dissolved organic matter in Lake Taihu, a large eutrophic, shallow lake in China

Abstract

From 2005 to 2009, the spatial distribution and the seasonal dynamics of chromophoric dissolved organic matter (CDOM) were explored in Lake Taihu in eastern China. The spatial-seasonal dynamics of CDOM absorption and three CDOM composition variables, including spectral slope ( S), spectral slope ratio (S R) and the M value, defined as the ratio of absorption at 250 nm/365 nm, were analyzed and discussed. Furthermore, river input processes and degradation of phytoplankton were studied to assess their impact on CDOM composition and the factors involved in the spatial-seasonal variability of CDOM. The CDOM absorption coefficient at a wavelength of 350 nm, a(350), ranged from 1.37–9.55 m −1 with a mean of 3.33 ± 1.32 m −1. Spatially, higher a(350) values, but lower spectral slope, spectral slope ratio and M values, were recorded in the northern algae dominated bays while lower values were recorded in southeastern macrophyte dominated bays. The a(350) was significantly higher in inshore waters than in offshore waters. Values of S, S R and M decreased during the flood and algal bloom season in spring and summer whereas a(350) increased. In the three river profiles, the gradual decrease of a(350) along the trajectory from the river mouth into the lake during the flood season showed the contribution of allochthonous CDOM. A laboratory phytoplankton degradation experiment was conducted to determine the contribution of CDOM production from phytoplankton. The significant increase of a(350) with time in the CDOM production experiment underlines the importance of autochthonous CDOM production during the algal bloom season. In summary, the significant increase of a(350) in spring and summer (algal bloom season) may be due to both the allochthonous CDOM input from the surrounding rivers and the autochthonous production of CDOM from degrading phytoplankton.